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The superhydrophobicity of submerged surfaces typically pertains to the trapped air film at the liquid–solid interface, subject to wettability transitions from a Cassie–Baxter state to more unstable states that gradually collapse to high retention regimes, which are energetically more favorable. In this work, the dynamic evolution of those transient metastable states is correlated to the underwater acoustic performance of laser textured superhydrophobic surfaces, resolving the dependence of the ultrasound spectral response with the immersion time to capture the genuine contribution of the hierarchical subwavelength morphology, regardless of the air layer effects. Acoustic wave attenuation of the incident ultrasound energy is extensively quantified in transmission, accounting for instantaneous broadband sound blocking (>30 dB) within the spectral range 0.5–1.5 MHz. As a result of the air layer detachment with the immersion time, transmission coefficients increase accordingly, while acoustic fields in reflection unexpectedly evolve toward stealthiness and naïve acoustic camouflage, mostly ascribable to dissipative mechanisms at air layer interfaces. The intrinsic decay of the air layer effect is tentatively determined at different frequencies, since quantitative understanding of the transient lifetime governing underwater surface wettability is critical to design stable superhydrophobic character of laser induced subwavelength metastructures on the most promising acoustic materials – from eco‐friendly natural to artificial. [ABSTRACT FROM AUTHOR]

Bidirectional controllable propagation of waterborne sound holds significant importance in developing multifunctional underwater acoustic and mechanical devices. However, the existing waterborne acoustic metasurfaces have rarely explored the bidirectional sound modulations. Here, a class of Janus waterborne acoustic metasurface, enabling two‐faced arbitrarily asymmetric wavefront manipulations is reported. A three‐degree‐of‐freedom mechanical system facilitated by acoustic‐structure interaction underwater is proposed to introduce bianisotropic responses of unit cells. Monolayer ultrathin Janus metasurface is inversely designed by utilizing a function‐structure integrated topology optimization framework. Distinct underwater acoustic functionalities, including axial and oblique focusing, beam splitting, and sound diffusion, are successfully demonstrated. Underwater experiments are further conducted to validate the concept of Janus metasurface. The good consistency between experimental and simulated results confirms the excellent two‐faced asymmetric acoustic focusing performance. The proposed Janus metasurface opens up a new dimension for designing advanced waterborne acoustic devices with two‐faced multifunctional wavefront manipulations. [ABSTRACT FROM AUTHOR]

In the marine environment, many animals use sound to interact and communicate with their conspecifics or other species. Over the last decades, the development of sound recording systems has allowed a significant advance in our knowledge of sound production in marine animals. We deployed miniature sound and movement tags on Antarctic fur seals (AFS), a small otariid foraging on mesopelagic fish, to uncover potential underwater vocalizations in this species. Tags recorded underwater sounds synchronously with high‐resolution AFS movements and diving behavior, allowing us to investigate the behavioral context of vocalizations in the natural environment. We provide evidence of underwater vocalizations in foraging female AFS in a context of foraging at sea. AFS produced stereotyped calls made of low‐frequency pulses produced in series, exclusively during foraging dives. We hypothesized that these acoustic pulse series could be used as an acoustic lure to confuse or attract fish prey, however, a larger sample size is needed to study the adaptive significance of these underwater vocalizations. [ABSTRACT FROM AUTHOR]

Optical properties of afterglow luminescent particles (ALPs) in mechanoluminescence (ML) and mechanical quenching (MQ) have attracted great attention for diverse technological applications. However, these unique phenomena need to be more clearly explained for the specific photonic application. Here, ALPs are designed for the development of a wearable and rewritable photonic display system as a communication toolbox under dark conditions or underwater environments with limited communication. This display system demonstrates long‐lasting MQ after short ML along the handwritten trajectories with mechanical pressure and the written content can be easily erased by short UV light irradiation, preserving the system integrity with the high reproducibility of ML and MQ responses. The effect of trapped electrons and the recharging process on the ML and MQ is assessed, which provides insights into their underlying mechanisms. In addition, this display system exhibits remarkable resistance to humidity and retains its rewritable and photonic capabilities underwater for a long‐term period. Furthermore, the rewritable property of display system on human skin is demonstrated, confirming their effectiveness as a wearable photonic display system. Taken together, this research will pave a big new avenue to develop biophotonic materials for various biomedical applications with mechano‐optical conversions. [ABSTRACT FROM AUTHOR]

Multibeam imaging SONARs have been used for a range of measurement applications, such as measurements of fish lengths. This study aimed to quantify the accuracy of imaging SONAR systems, that varied in frequency and beam geometry, to measure the length of synthetic targets positioned perpendicularly.Blueprint Oculus imaging SONAR systems, with four different (centre) frequencies (750 kHz, 1.2 MHz, 2.1 MHz and 3 MHz), were used to measure the length of three targets of nominal lengths: 10 cm, 20 cm and at ranges between 1 and 15.5 m. The effect of beam geometry on measurement error was then examined using regression analysis.This study found that there was an overestimation of the actual length of the target for all measurements that was inversely proportional to the horizontal beamwidth. The measurement error can be reduced by normalising for beamwidth. However, the variation of measurements (i.e. the precision) was found to also increase with range, which was attributed to the increasing beam separation.It is important that the effect of beamwidth on the accuracy of target length measurements, by imaging SONARs is acknowledged in future studies. One approach to mitigating this problem, is to limit the range at which length measurements are made to a beamwidth that produces an estimated level of error that is acceptable for that study. [ABSTRACT FROM AUTHOR]

We examine the dependence of the penetration depth and fractional surface area (e.g., whitecap coverage) of bubble plumes generated by breaking surface waves on various wind and wave parameters over a wide range of sea state conditions in the North Pacific Ocean, including storms with sustained winds up to 22 m s−1 and significant wave heights up to 10 m. Our observations include arrays of freely drifting SWIFT buoys together with shipboard systems, which enabled concurrent high‐resolution measurements of wind, waves, bubble plumes, and turbulence. We estimate bubble plume penetration depth from echograms extending to depths of more than 30 m in a surface‐following reference frame collected by downward‐looking echosounders integrated onboard the buoys. Our observations indicate that mean and maximum bubble plume penetration depths exceed 10 and 30 m beneath the surface during high winds, respectively, with plume residence times of many wave periods. They also establish strong correlations between bubble plume depths and wind speeds, spectral wave steepness, and whitecap coverage. Interestingly, we observe a robust linear correlation between plume depths, when scaled by the total significant wave height, and the inverse of wave age. However, scaled plume depths exhibit non‐monotonic variations with increasing wind speeds. Additionally, we explore the dependencies of the combined observations on various non‐dimensional predictors used for whitecap coverage estimation. This study provides the first field evidence of a direct relation between bubble plume penetration depth and whitecap coverage, suggesting that the volume of bubble plumes could be estimated by remote sensing. Plain Language Summary: Quantifying the statistics of bubble plumes generated during ocean surface wave breaking is essential to understanding the exchange between the ocean and the atmosphere. Bubble plumes also cause important variations in underwater acoustics and optics. Recent studies also suggest that the statistics of bubble plumes are skillful predictors for total energy loss during wave breaking, which is an essential quantity for accurate wave forecasting. In this study, we examine how these bubble plume statistics change with different wind and wave conditions, including during storms. We used echosounders on drifting buoys to detect the bubbles and estimate how deep they go in the ocean. We also used shipboard camera systems to measure the surface area of the bubble plumes. We successfully develop multiple empirical relationships that allow us to predict how bubble plume depth and surface area change as a function of simple wind and wave statistics. These statistics are readily available from existing forecast models or typical ocean buoys. Our findings reveal that bubble plume depth is correlated with its visible surface area. This intriguing correlation suggests that we might estimate the volume of bubble plumes simply by observing the ocean surface from above. Key Points: Bubble plumes generated during ocean surface wave breaking are observed with echosounders on drifting buoysBubble plume depths are well correlated with whitecap coverage, wind speed, and spectral wave steepnessBubble plumes persist for many wave periods and exceed the persistence of visible surface foam [ABSTRACT FROM AUTHOR]

A hydrophone is a core component of sonar and plays a significant role in underwater detection. High sensitivity and wide‐frequency responsiveness are crucial parameters for underwater mechanosensors. This study introduces an innovative wood‐based hydrophone for precise detection of water wave spectra and a wide range of underwater sounds. The wood hydrophone consists of a conductive wood electrode with deposited polypyrrole (PPy) microbranches (polypyrrole wood (PW)) and a cellulosic micropatterned anti‐swelling hydrogel (MPASH) serving as the dielectric layer. As pressure increases within the range of 6.4 Pa to 1.2 kPa, the densely distributed PPy microbranches within the wood fiber cell lumina contact with more tips of the self‐wrinkled micropatterns of MPASH. This interaction results in a four‐order‐of‐magnitude increase in capacitance, leading to an unprecedented sensitivity of 1153 kPa−1 toward microstresses. Due to the presence of multisized configurations in both PW and MPASH, the sensitivity of the wood hydrophone decreases gradually (268 kPa−1 up to 37 kPa). The receiving sensitivity of the wood hydrophone for acoustic waves is greater than ‐174 dB within the frequency range of 250–2300 Hz. This work presents a promising avenue for the development of underwater detection devices utilizing graded wood channels and micropatterned cellulosic hydrogels. [ABSTRACT FROM AUTHOR]

Harbor porpoises (Phocoena phocoena) are the only cetaceans routinely sighted in Hood Canal, a narrow fjord that comprises the western edge of Puget Sound, Washington, USA. Harbor porpoises are sensitive to anthropogenic sounds, including noise from recreational and commercial vessel traffic, and the United States Navy, which conducts military training and testing within Hood Canal that can include underwater sound sources. This study was funded as part of the Navy monitoring program to assess potential impacts of naval activities on cetaceans. We conducted vessel-based line-transect surveys for harbor porpoises in Hood Canal in 2022-2023 to derive seasonal estimates of abundance and density. We carried out surveys over 37 days and surveyed the entire canal twice per season totaling 2,176 km of on-effort track line. We recorded 809 oneffort harbor porpoise groups and 1,385 individuals. Seasonal abundance estimates were lowest in winter (308 animals, 95% CI = 189-503) and gradually increased through spring and summer to a peak of 1,336 animals (95% CI = 826-2,160) in fall. Overall porpoise density was highest in central Hood Canal, an area that includes a designated United States Navy training range, though porpoise sightings were notably absent in a 21-km² area adjacent to the naval submarine base within this otherwise high-density region. Though we collected only a single year of data, these results suggest that harbor porpoise abundance in Hood Canal increased significantly since it was last estimated (2013-2015). The notable seasonal fluctuation of harbor porpoise abundance suggests Hood Canal may host a larger percentage of the overall Washington Inland Waters stock during the fall season, raising important management considerations. [ABSTRACT FROM AUTHOR]

Freshwater fishes exhibit a wide range of auditory adaptations and capabilities, which are assumed to help them navigate their environment, avoid predators, and find potential mates. Yet, we know very little about how freshwater environments sound to fish, or how fish with different auditory adaptations respond to different soundscapes. We first compiled data on fish hearing acuity and adaptations and provided a portrait of how anthropogenic sounds compare to natural sounds in different freshwater soundscapes. We then conducted a sound‐enrichment field experiment at Lake Saint Pierre, a large fluvial lake in Canada, to evaluate the effect of motorboat sound exposure on the fish community by looking at the extent to which changes in species abundances were linked to auditory adaptations. Data compilation showed that the hearing acuity of most species overlaps with a wide range of ambient and anthropogenic underwater sounds while the field experiment showed that species with more specialized auditory structures were captured less often in sound‐enriched traps, indicating avoidance behavior. Our findings highlight the importance of considering species' sensorial adaptations when evaluating the community‐scale effects of anthropogenic sounds on the fish community, especially at low levels of anthropogenic activity. [ABSTRACT FROM AUTHOR]

Organisms, such as fishes and invertebrates and including their larval stages, listen to underwater soundscapes to detect information about nearby habitats. Such soundscapes may be influenced by habitat degradation or enhancement, which can lead to acoustically mediated feedback loops affecting the overall ecosystem.Despite the importance of underwater sounds on ecological functioning, there have been limited studies documenting soundscapes of intertidal ecosystems and few, if any, of living shoreline soundscapes. Living shorelines would especially benefit from acoustically mediated effects for objectives like encouraging fish and invertebrate settlement.This case study used a Before‐After‐Control‐Impact design to sample soundscapes and nekton (i.e., fishes and mobile macroinvertebrates) at a living shoreline construction and a nearby hardened shoreline in Cedar Key, FL (USA). Diel soundscape patterns and acoustic attenuation at the two sites were also described a year following the living shoreline construction.In the acoustic sampling, the high frequency bands of both shorelines were dominated by invertebrate sounds that were influenced by season, site and time of day, while the low frequency band of the living shoreline was often dominated by a loud anthropogenic sound. About a year after the living shoreline installation—despite similar measured acoustic attenuation at both sites—the living shoreline featured louder sound pressure levels compared to the hardened shoreline, which may be particularly beneficial for promoting foundational species and other organism settlement.These results demonstrate that Gulf of Mexico intertidal habitats may have soundscape differences even within close proximity and that living shorelines may enhance acoustic characteristics in ways beneficial to continued shoreline development. This represents an important step in better understanding the relationships between habitat structures, nekton communities, and their associated soundscapes as well as the application of passive acoustic monitoring to improve coastal management and conservation. [ABSTRACT FROM AUTHOR]

The rarest seal and the world's most endangered pinniped species, the Mediterranean monk seal (Monachus monachus), has a small and isolated population in the Madeira Archipelago (Portugal). This species tends to be extremely wary of humans and, therefore, very difficult to approach and study.Passive acoustic monitoring (PAM) is a non‐invasive, cost‐effective tool that can be a valuable complement for the traditional monitoring methods, providing insight for effective conservation of the seal in the Madeira Archipelago.In this pilot study, custom‐designed autonomous underwater recorders were deployed in two marine protected areas (Garajau Partial Nature Reserve and the Desertas Islands Nature Reserve) to assess the potential of PAM to detect and monitor this elusive and endangered species in the Madeira Archipelago.Two call types putatively produced by M. monachus were detected in a subsample of audio files recorded over a 3‐month acoustic deployment; these call types share similarities with the /growl/ and /hiccup/ recently described for M. monachus in a Mediterranean population. The most common sound type detected was the low‐frequency growl. No obvious pattern was found in the abundance of sounds according to sampling date, and no significant difference was found in the abundance of sounds in different periods of the day.The ability to detect the species' underwater vocalizations with PAM opens the possibility of future monitoring plans based on data obtained from audio recordings. These data can provide relevant information for conservation, namely, on the presence and abundance of the seals. [ABSTRACT FROM AUTHOR]

Summary: Internet of underwater things (IoUT) for underwater monitoring is known worldwide for smart interlinked underwater things that exhibit the capacity to monitor the vast unexplored waters of the oceans. Concept of IoUT has been derived from Internet of Things (IoT) in order to acquire the exquisite benefits of networking in underwater environment. IOT standards and technologies do not work well in underwater environment, such as infrared, Wi‐Fi, and radio frequency (RF) due to high channel errors and limited range up to few meters. Acoustic waves, however, can be used to communicate both in shallow and deep oceans due to their low frequency (kHZ) signal. In context of IoUT, communication based on acoustic links enables different applications such as underwater exploration, environmental monitoring, and disaster prevention even without availability of GPS facility like free space environment. In unpredictable and changing underwater environment, energy efficiency becomes a major challenge during data routing along multiple devices. Batteries of the sensor nodes, autonomous underwater vehicle (AUV), and remotely operated vehicle (ROV) cannot be removed with easiness and difficult to recharge, and the only way out is efficient sensor node selection for relaying to save massive amount of energy. Energy aware channel routing protocol (ECARP) does not consider the depth of the node while selecting the relay nodes to forward the data. Relay node selection in underwater Internet of things (IOUT) is a primary problem addressed in this research based on channel state information (CSI) for establishing best path to relay information among IOUT devices. Our major focus was to develop better technique for the relay node selection using a CSI and select relay node by looking at its depth from ocean surface and residual energy in the proposed ED‐CARP. Simulation results validate that proposed ED‐CARP can decrease the communication cost and increase the network lifetime. [ABSTRACT FROM AUTHOR]

Capturing pseudonoise (PN) synchronous signals is an important issue in spreading spectrum underwater acoustic communication. In this letter, local waveform domain information is combined with multi‐strategy to enhance the Doppler tolerance of conventional Partial Matched Filtering‐Fast Fourier Transform (PMF‐FFT) acquisition algorithm under low signal‐to‐noise ratio (SNR). First, an optimized algorithm based on partial reconstruction in waveform domain was proposed, such that the Doppler tolerance increased by 38.9% when the SNR = −15 dB. Second, a local baseband multi‐waveform compression algorithm was proposed to further enhance the Doppler parallel search ability multiply. At last, frequency offset estimation was completed after searching the multi‐waveform serially. The effectiveness of the proposed algorithm was verified through the experiment at sea. [ABSTRACT FROM AUTHOR]

Passive acoustic monitoring has been used for decades as a non‐invasive tool for quantifying biodiversity in terrestrial and marine ecosystems. Recently, there has been increased interest in the potential for the method to survey freshwater biodiversity. Fundamental aspects of freshwater soundscape phenology, however, often remain poorly understood, despite their importance for suitable survey design.To gain a greater understanding of daily acoustic variation in aquatic insect‐dominated temperate pond soundscapes, 840 hr of underwater sound recordings were collected from five ponds in the southwest of the U.K. We calculated six commonly used acoustic indices to investigate diel trends and evaluated the suitability of each acoustic index to identify biologically complex pond soundscapes. In addition, macroinvertebrates were collected from each pond to investigate potential drivers of diel soundscape variation.The ponds studied possessed clear patterns of daily acoustic variation, with acoustic activity typically peaking between 02:00 and 04:00 and around the solar noon. Acoustic Entropy showed the greatest variation between day and night soundscapes and was best suited for detecting overall daily acoustic variation in the study ponds. However, the Normalised Difference Soundscape Index and the Bioacoustic Index captured strong diel variation in aquatic insect‐dominated soundscapes. Furthermore, we calculated that a minimum hydrophone deployment time of 24 hr is required to ensure that soundscape variation is adequately captured.This study provides an increased understanding of daily acoustic variation in insect‐dominated temperate pond soundscapes, enabling us to provide guidelines for the design and implementation of future passive acoustic monitoring surveys. We suggest that a minimum of 24 hr is required to adequately capture pond soundscape variation. This will increase the chance of detecting key soniferous species in the soundscape and enable more accurate assessments of temperate pond soundscapes. [ABSTRACT FROM AUTHOR]

Based on the recent development of underwater sound absorption coatings, this paper analyzes the evolution of underwater sound absorption coatings from two aspects: traditional structures and metamaterials. The traditional structures are divided into three types: cavity structures, composite structures, and impedance‐matching structures. First, the development history of traditional structures, from cavities to hybrid structures, is presented, and the optimization and recent improvements in sound absorption fundamentals are explained. For different types of structures, the sound absorption effects of their respective structures are presented. At the same time, recent development progress of underwater coating metamaterials is summarized, and the sound absorption fundamentals and effects of local and nonlocal resonance types are compared. In addition, typical underwater sound absorption metamaterials are described in detail, and the application prospect of the acoustic bubble structure in practice is summarized. Moreover, this paper compares and discusses various structural types of underwater sound absorption coatings and points out the shortcomings of recent related research. Finally, it points out current open issues in the field and suggests future development directions of underwater sound absorption coatings, defining the possible development focus of underwater sound absorption coatings. [ABSTRACT FROM AUTHOR]

Underwater communication (UWC) is widely used in coastal surveillance and early warning systems. Precise channel estimation is vital for efficient and reliable UWC. The sparse direct‐adaptive filtering algorithms have become popular in UWC. Herein, we present an improved adaptive convex‐combination method for the identification of sparse structures using a reweighted normalized least‐mean‐square (RNLMS) algorithm. Moreover, to make RNLMS algorithm independent of the reweighted l1‐norm parameter, a modified sparsity‐aware adaptive zero‐attracting RNLMS (AZA‐RNLMS) algorithm is introduced to ensure accurate modeling. In addition, we present a quantitative analysis of this algorithm to evaluate the convergence speed and accuracy. Furthermore, we derive an excess mean‐square‐error expression that proves that the AZA‐RNLMS algorithm performs better for the harsh underwater channel. The measured data from the experimental channel of SPACE08 is used for simulation, and results are presented to verify the performance of the proposed algorithm. The simulation results confirm that the proposed algorithm for underwater channel estimation performs better than the earlier schemes. [ABSTRACT FROM AUTHOR]

Icing phenomenon that occurs universally in nature and industry gets a great impact on human life. Over the past decades, extensive efforts have been made for a wide range of anti‐icing/deicing surfaces, but the preparation of anti‐icing/deicing interfaces that combine stability, rapid self‐healing and excellent anti‐icing/deicing performance remains a challenge. In this study, a photothermal solid slippery surface with excellent comprehensive performance is prepared by integrating cellulose acetate film, carbon nanotubes with paraffin wax (CCP). Apart from the excellent anti‐icing and deicing properties at −17 ± 1.0 °C under 1 sun illumination, the surface can further achieve deicing at temperatures as low as −22 ± 1.0 °C under infrared light. The fabricated surface also exhibits great stability when placed in harsh conditions such as underwater or ultra‐low temperature environments for over 30 days. Even when suffering from physical damage, the prepared surface can rapidly self‐repair under 1 sun illumination or near‐infrared (NIR) illumination within 16.0 ± 1.5 s. Due to the rapid and repeatable self‐healing performance, the lubricating properties of the interface material do not deteriorate even after 50 repeated abrasing‐repairing cycles. The photothermal solid slippery surface possesses wide‐ranging applications and commercial value at high latitude and altitude regions. [ABSTRACT FROM AUTHOR]

Metamaterials have offered unprecedented potentials for wave manipulations. However, their applications in underwater acoustic wave control have remained largely unexplored. This is because of the limited material choices and the lack of reliable fabrication techniques for the complicated structures. Herein, a metamaterial with microlattice structures as the building blocks is proposed for underwater operations. By designing the building blocks of the metamaterial and assembling them in a layered fashion, anisotropy is embedded in the structure, which results along different effective sound speeds in orthogonal directions. The designed metamaterial is fabricated by metal additive manufacturing using aluminum and steel. Experiments are performed using a resonator tube to evaluate its performance in water. An anisotropy ratio of around 2 is achieved, which is in good agreement with numerical simulations. The proposed metamaterial provides an effective means for underwater sound control with reduced fabrication difficulties and increased service life. [ABSTRACT FROM AUTHOR]

Due to the absorption and scattering of underwater light, underwater images often have some phenomena such as colorcast, low contrast, and blurred details. Therefore, an underwater image enhancement method based on entropy weight fusion is proposed. First, white balance processing can effectively correct the blue (green) color appearance of the image. Then the white‐balanced images are converted from RGB space to LAB space, and L channel is processed with improved adaptive gamma correction, and then converted back to RGB space. CLAHE and bilateral filtering are performed in RGB space. The RGB space is converted to HSV space, the V channel is processed by single‐scale retinex algorithm combined with guided filtering, stretching the R channel with s‐cosine curve,and then converted back to RGB space. Finally, the three results are fused by entropy weight to obtain the final enhanced image. Experimental results show that the proposed algorithm can improve the contrast and clarity of underwater images, and effectively remove colorcast. [ABSTRACT FROM AUTHOR]

Cation–π interactions are considered to be among the most important strong noncovalent interactions in aqueous solutions and as regulators of various biological phenomena, including biological adhesion, signal transduction, and liquid–liquid phase separation (LLPS). Despite their significant roles, cation–π interactions in aqueous environments are not systematically and experimentally understood. Here, we directly experimentally examined cation–π interactions between cationic and aromatic moieties common to biological systems using a surface forces apparatus (SFA) augmented by computational methods. Specifically, we chose cationic and aromatic moieties with different levels of hydration and observed how the molecular hydration of these moieties affects the formation and strengths of cation–π bonds. To date, the charge densities of the interacting moieties are primarily considered to modulate the interaction strength; however, it is experimentally and computationally revealed that the hydration strengths (or hydrophobic properties) of the interacting moieties are more important for controlling in underwater interactions. [ABSTRACT FROM AUTHOR]

We assessed the effectiveness of herding techniques on adult Silver Carp Hypophthalmichthys molitrix in a tributary to the Missouri River. Sites (600 m) were contained using block nets and treated with one of five herding techniques: (1) a method commonly used by commercial fishers in the United States (commercial technique), (2) pulsed‐DC electrofishing (electric technique), (3) broadband sound administered with underwater speakers (sound technique), (4) both sound and electric in combination (combination technique), and (5) solely the boat, with no added stimulus (control). Treatments were administered at sites with repeated 20‐min runs (3×) in a slow, bank‐to‐bank pattern downstream through a 4.5‐m opening between the block net and shoreline. Herding effectiveness was quantified by run category (cumulative for the second and third runs) using adaptive resolution imaging sonar. Mean herding effectiveness of the combination technique was three to four times greater than that of the other techniques regardless of the number of runs included. The effectiveness of the combination technique was significantly greater than the sound technique and control with inclusion of the second run and the commercial technique with inclusion of the third run. All herding techniques, excluding the combination technique, were statistically similar to the control. Inclusion of the second run was advantageous for all techniques, as herding effectiveness increased 54–103%, whereas inclusion of the third run was less beneficial (9–17% increase). Repeated runs allowed fish additional time to vacate the sample reach, but behavioral responses (i.e., acclimation or sensitization) were likely technique dependent. This study describes which techniques are most effective for herding Silver Carp and provides insight for improving mass removal efforts. [ABSTRACT FROM AUTHOR]

Noise produced by human activities has increased in the oceans over the last decades. Whereas most studies have focused on the impact of anthropogenic noise on marine mammals and fishes, those focusing on marine invertebrates are rarer and more recent, especially when considering peri-metamorphic benthic stages, highly sensitive to anthropogenic perturbations. A careful review of the literature reveals a simplistic characterization of the acoustics within the containers used to quantify larval and juvenile responses to noise, thus weakening the conclusions of such works. To address this problem, we developed the Larvosonic system, a laboratory tank equipped with acoustic assets to assess the impacts of noise on young stages of marine invertebrates. We first provide a careful analysis of the tank sound field using different sound types, and we assess the effects of expanded polystyrene units on the sounds emitted by a professional audio system in order to dampen reverberation and resonance. Then, we apply this acoustic calibration to the effects of both pile driving and drilling noises on postlarvae of the scallop bivalve Pecten maximus. Acoustic recordings highlight that diffuser and bass trap components constitute effective underwater sound absorbents, reducing the reflection of the whole frequency bandwidth. Scallop experiments reveal that both type and level of the tested noise influenced postlarval growth, with interactive effects between trophic environment and noise level/spectra. The Larvosonic system thus constitutes an efficient tool for bioacoustics research on bentho-planktonic invertebrate species. [ABSTRACT FROM AUTHOR]

Elevated levels of anthropogenic noise, especially those observed through boating activity, can negatively impact fish species, but it remains unclear which species are most affected and which behavioural metrics are best used in assessing fish responses to underwater noise. The effects of boat sounds on freshwater species are of particular interest because freshwater environments are less studied than the marine realm despite comparably high levels of biodiversity.In the current study, we examine the behavioural responses to boat noise in two freshwater species that differ in their hypothesised response to sound inputs: the spottail shiner (Notropis hudsonius), a species with known hearing specialisations, and the bluegill sunfish (Lepomis macrochirus), a species with more generalised hearing capabilities. Fish were presented with boat noise in a laboratory setting, and their swimming, escape and foraging behaviours were assessed to examine differential responses in relation to hypothesised hearing abilities.Both species showed a decrease in general swimming behaviours but an increase in erratic movements in response to boat noise, indicative of stress responses for both species. Despite the similarities in response based on swimming behaviours however, only spottail shiners exhibited true escape responses to the onset of the noise stimulus, suggesting a more extreme reaction in the species with a more refined hearing ability.Taken together, these results show that freshwater fish can respond to increased levels of anthropogenic noise, but that the severity of the response may differ based on auditory structures and therefore presumed hearing ability. The differences seen between behavioural metrics used (swimming vs. escape responses) also demonstrate how care must be taken in choosing a metric when developing exposure guidelines for underwater sound exposures, as different metrics could lead to differential impact assessments. [ABSTRACT FROM AUTHOR]

Inspired by recent theoretical demonstrations of nonlocal waterborne acoustic metasurfaces (WAMs), herein experimental evidence of a WAM with highly nonlocal features for high‐efficient waterborne sound control is proposed. An easy‐to‐implement nonlocal metasurface with enhanced long‐range interaction is designed and the effects of fluid–solid interaction (FSI) are considered. Results show that the nonlocal WAMs are observed to manipulate underwater sound with enhanced efficiency, as high as 97% was experimentally achieved, compared with a maximum efficiency of 71% of the conventional local devices. Besides, the robustness against frequency fluctuations and coarse discretization of the nonlocal WAMs are observed. The proposed device is potentially applicable for underwater signal communication and high‐resolution ultrasonic imaging. [ABSTRACT FROM AUTHOR]

Underwater disturbances are the largest error source in Global Navigation Satellite System - Acoustic ranging combination technique (GNSS-A) seafloor geodetic observation. In particular, the gradient of sound speed structure (SSS) directly affects the horizontal accuracy and needs to be examined. Previous studies have not investigated its temporal change component. In this paper, we verified the assumption that the underwater gradient structure does not change significantly during GNSS-A observation for several hours through applying a modified version of an analysis software called GARPOS to actual data of SGO-A (provided by Japan Coast Guard). Obtained results suggested that this assumption holds at many observation data, and the positioning accuracy becomes better. Some non-improved observation epochs were speculated to be accompanied by structure changes for which this assumption was not valid. It is suggested that the SSS change during observation will be an important research topic in GNSS-A. [ABSTRACT FROM AUTHOR]

Using data from Argo floats, satellite altimetry, and satellite sea surface temperature (SST), we investigate the merging of two anti‐cyclonic eddies in the Tasman Sea. The eddies are of different size and different density. Once the distance between the eddies falls below a critical separation distance, water from the smaller, denser eddy is observed to flow around the larger, lighter eddy, sink to greater than 400 m depth, and slowly spiral toward the eddy‐center. The merging event is characterized by filaments around the eddy perimeter, evident in high‐resolution, satellite SST data. The merged eddy develops multiple well‐mixed layers, each almost 400 m thick, with large anomalies in temperature and salinity that penetrate to about 1,000 m depth. After the merging is complete, water from the lighter eddy is stacked on top of water from the denser eddy. Using a simple conceptual model, we show that the merged profiles can be simply explained by subduction of surface waters from the denser eddy, and mixing with subsurface waters from the lighter eddy. The stacked eddy has a distinct signature in TS‐space. Using this TS‐signature, together with altimetry for context, we identify over 20 other examples when Argo floats sample stacked eddies in the Tasman Sea. We speculate that merged eddies may be more common than previously thought, with implications for ocean productivity, underwater acoustics, and the transfer of energy across scales. Plain Language Summary: Eddies are rotating features that exist everywhere in the ocean. They could be large or small and they often bump into each other. Eddies are easily seen by satellite measurements, but their properties below the surface aren't always known. Thousands of Argo floats drift around the ocean measuring ocean properties between the surface and 2,000 m depth. These Argo floats often sample eddies. In this study, we report observations from Argo floats that sample two nearby eddies as they join together. As they merge together, water from the smaller, denser eddy wraps around the larger, lighter eddy, and spirals downwards towards the eddy center. The resulting eddy has a distinct subsurface profile that resembles the light eddy stacked on top of the dense eddy. Having identified the key signature of these stacked eddies, we hunt for more, and find plenty. We think these stacked eddies are quite common, and may be important for many applications. Key Points: Observations from Argo measured the properties of two warm‐core eddies in the Tasman Sea that merge over a period of about a monthWater from smaller, denser eddy wraps around the larger, lighter eddy and sinks, with the merged eddy developing multiple well‐mixed layersAnalysis of other Argo profiles suggests that the stacked eddies, with similar features, may be more common than previously thought [ABSTRACT FROM AUTHOR]

Summary: Deepwater utilization and underwater sound communication with inadequate bandwidth, elongated propagation interruption, signal failure issues, and sensor node malfunction due to ecological circumstances are the limitations of the underwater wireless sensor network. Further, the challenges in underground wireless sensor network (WSN) include superior bandwidth, high power expenses, information handling, and cross‐layer design. On the other hand, cooperative communications facilitate competent employment of communication resources, by permitting nodes in a network to work together in information communication. As wireless sensor networks adapt to multi‐hop transmission in high data traffic, there is a high demand for achieving energy efficiency. Moreover, in the course of proximity nodes to the sink, an energy hole is started for the extended network lifetime. This paper proposes the design of multi‐hoped cooperative communication‐based wireless underground sensor networks that guarantees energy efficiency in a hostile environment. In addition, network coverage and connectivity improvement are also improved by this proposed design. [ABSTRACT FROM AUTHOR]

Passive acoustic monitoring (PAM) involves recording the sounds of animals and environments for research and conservation. PAM is used in a range of contexts across terrestrial, marine and freshwater environments. However, financial constraints limit applications within aquatic environments; these costs include the high cost of submersible acoustic recorders. We quantify this financial constraint using a systematic literature review of all ecoacoustic studies published in 2020, demonstrating that commercially available autonomous underwater recording units are, on average, five times more expensive than their terrestrial equivalents. This pattern is more extreme at the low end of the price range; the cheapest available aquatic autonomous units are over 40 times more expensive than their terrestrial counterparts. Following this, we test a prototype low‐cost, low‐specification aquatic recorder called the 'HydroMoth': this device is a modified version of a widely used terrestrial recorder (AudioMoth), altered to include a waterproof case and customisable gain settings suitable for a range of aquatic applications. We test the performance of the HydroMoth in both aquaria and field conditions, recording artificial and natural sounds, and comparing outputs with identical recordings taken with commercially available hydrophones. Although the signal‐to‐noise ratio and the recording quality of HydroMoths are lower than commercially available hydrophones, the recordings with HydroMoths still allow for the identification of different fish and marine mammal species, as well as the calculation of ecoacoustic indices for ecosystem monitoring. Finally, we outline the potential applications of low‐cost, low‐specification underwater sound recorders for bioacoustic studies, discuss their likely limitations, and present important considerations of which users should be aware. Several performance limitations and a lack of professional technical support mean that low‐cost devices cannot meet the requirements of all PAM applications. Despite these limitations, however, HydroMoth facilitates underwater recording at a fraction of the price of existing hydrophones, creating exciting potential for diverse involvement in aquatic bioacoustics worldwide. [ABSTRACT FROM AUTHOR]

Climate change is a relevant threat on a global scale, leading to impacts on ecosystems and ocean biodiversity. A considerable fraction of marine life depends on sound. Marine mammals, in particular, exploit sound in all aspects of their life, including feeding and mating. This work explores the impact of climate change in sound propagation by computing the three‐dimensional global field of underwater sound speed. The computation was performed based on present conditions (2006–2016) and a "business‐as‐usual" future climate scenario (Representative Concentration Pathway 8.5), identifying two "acoustic hotspots" where larger sound speed variations are expected. Our results indicate that the identified acoustic hotspots will present substantial climate‐change‐induced sound speed variations toward the end of the century, potentially affecting the vital activities of species in the areas. Evidence is provided of the impact of such variation on underwater sound transmission. As an example of a species impacted by underwater transmission, we considered one marine mammal endangered species, the North Atlantic right whale (Eubalaena glacialis), in the northwestern Atlantic Ocean. To the best of our knowledge, this is the first global‐scale data set of climate‐induced sound speed changes expected under a future scenario. This study provides a starting point for policies oriented research to promote the conservation of marine ecosystems and, in particular, endangered marine mammals. Plain Language Summary: Marine fauna, especially cetaceans, depend on sound for all biological functions. Climate change is probably the greatest anthropogenic challenge that humankind faces and it is damaging biodiversity and ecosystems on a global scale. In this work, we coupled these two issues by analyzing future climate change consequences on underwater sound propagation. We based our computations on a high‐emissions scenario, which represents a likely outcome if society does not act on reducing greenhouse gas emissions. We calculated global changes in sound speed and identified the areas that will be most affected at the end of the century. We showed the expected changes on sound propagation using as example one endangered cetacean species whose typical calls will undergo changes in future. We conclude that these future sound speed variations will affect marine mammals' communication and other vital activities that rely on sound propagation. To the best of our knowledge, this is the first global‐scale assessment of sound speed and its expected future changes. It provides a starting point for policies that promote the conservation of marine ecosystems and, in particular, endangered marine mammals. In addition, it paves the way to investigate the possible combination of such changes with other anthropogenic pressures (e.g., vessel traffic) which might endanger multiple species in the future. Key Points: New data set of global sound speed at the end of XXI century with associated uncertainty: global sound speed generally increasesClimate‐induced sound speed variations are substantial in selected "acoustic hotspots"Underwater sound transmission loss is expected to decrease in acoustic hotspots potentially changing anthropogenic and natural soundscapes [ABSTRACT FROM AUTHOR]

Quantifying air‐water gas exchange is critical for estimating greenhouse gas fluxes and metabolism in aquatic ecosystems. In high‐energy streams, the gas exchange rate k is poorly constrained, due to an incomplete understanding of turbulence and bubble contributions to k. We performed a flume experiment with air bubble additions to evaluate the combined effects of turbulence and bubbles on k for helium, argon, xenon, and methane. We created contrasting hydraulic conditions by varying channel slope, bed roughness, water discharge, and bubble flux. We found that k increased from 1–4 to 17–66 m d−1 with increases in turbulence and bubble flux metrics. Mechanistic models that explicitly account for these metrics, as well as gas diffusivity and solubility, agreed well with the data and indicated that bubble‐mediated gas exchange accounted for 64–93% of k. Bubble contributions increased with bubble flux but were independent of gas type, as bubbles did not equilibrate with the water. This was evident through modeled bubble life and equilibration times inferred from bubble size distributions obtained from underwater sound spectra. Sound spectral properties correlated well with turbulence and bubble flux metrics. Our results demonstrate that (a) mechanistic models can be applied to separate free surface‐ and bubble‐mediated gas exchange in running waters, (b) bubble life and equilibration times are critical for accurate scaling of k between different gases, and (c) ambient sound spectra can be used to approximate contributions of turbulence and bubbles. Plain Language Summary: Aquatic systems exchange gases with the atmosphere and this exchange is important for many fundamental ecosystem processes and the global greenhouse gas cycle. How fast gases exchange with the atmosphere is, however, difficult to determine, especially in streams and rivers where bubbles can speed up the exchange of certain gases. Here, we used experimental stream channels to create a wide range of flow conditions, and test how these conditions affect the rate at which different gases in the water exchange with the atmosphere. We found that irregular water motions and bubbles generally enhance gas exchange and that these effects can be described by physical equations of flow and bubble characteristics in similar ways as previously shown in oceans. The equations can be used to quantify the specific contribution of bubbles to gas exchange and this can be important when comparing exchange rates of different gases such as carbon dioxide and oxygen. We also find that important drivers of gas exchange can be derived from flow and bubble sound recorded by microphones. In essence, we provide new equations and field methods that will improve our understanding and ability to quantify gas exchange processes in streams and rivers. Key Points: Air–water gas exchange rates in bubbly running waters follow the same mechanistic scaling laws as in oceansBubble life and equilibration times are critical for accurate scaling of gas exchange ratesKey drivers of gas exchange correlate with ambient sound spectral signatures [ABSTRACT FROM AUTHOR]

Underwater sound absorption materials with broadband and resistance to high pressure are crucial for noise control in the ocean. Herein a new method is presented to synergistically improve material's resistance to high pressure with broadband by hybridizing carbon nanotubes (CNTs) with graphene nanoplatelets (GNPs) in polydimethylsiloxane (PDMS) matrix. The micro‐structure shows the geometrical synergy between the nanoplatelets (GNPs) and nanotubes (MWCNT‐COOH). Besides, GNPs also form junctions with air micro‐voids. The underwater acoustic tests reveal that, compared with equivalent nanocomposites containing only CNTs or GNPs, the hybridized materials show significantly better sound absorption performance with a high hydrostatic pressure increased from 0.4 MPa to 1.0 MPa at the frequency range of 1500 Hz to 7000 Hz. Therefore, the hybridization of one‐dimensional (1D) and two‐dimensional (2D) nano‐carbon materials provides a new route for increasing the underwater sound absorption performance at high hydrostatic pressure. [ABSTRACT FROM AUTHOR]

Ocean‐generated seismic waves are omnipresent in passive seismic records around the world and present both a challenge for earthquake observations and an input signal for interferometric methods for characterization of the Earth's interior. Understanding of these waves requires the knowledge of the depth dependence of the oceanic noise at the transition into the continent. To this end, we examine 80 days of continuous acquisition with distributed acoustic sensor (DAS) system deployed in two deep boreholes near the south‐eastern coast of Australia. The iDASv3™ system deployed in a deep borehole at the CO2CRC Otway Project site provides sufficiently high sensitivity and low instrumentation noise for frequencies from 100 mHz to 20 Hz. Analysis of the seismograms and correlation with wave climate allows decomposing the DAS response into microseisms generated by swell from remote storms (∼ $\sim $0.15 Hz) and local winds (between 0.2 and 2 Hz), and strong body wave energy from large surf breaks at the coast (from 2 to 20 Hz). The depth dependence of the microseisms provides useful insights into the energy partition between the Rayleigh wave modes and may augment conventional kinematic analysis of the sparse surface seismological arrays. Overall, ocean‐generated signals at each channel along the borehole are strongly related to the wave climate, so that—with sufficient amount of training data—the passive seismic records on several downhole DAS sensors has a potential for high‐precision monitoring of formations surrounding the borehole as well as remote storms in the ocean. Plain Language Summary: This study attempts to decipher the complex ambient seismic wavefield in coastal regions. The microseisms have been under scrutiny for a long time: first, as a noise in earthquake monitoring systems and, later, as a target signal used for characterization of the upper Earth crust. The study relies on a unique data set acquired in a deep borehole using the most advanced distributed acoustic sensing (DAS) system deployed at the coast of Victoria (Australia). Compared with the conventional seismological networks, this new measurement technology adds a new dimension to the data–depth variation in the broad frequency range of the parameters of the seismic waves. Using just one borehole, we identified and quantitatively characterized the multitude of ambient seismic sources at the Victorian coast: remote storms, local winds, and surf breaks. Our measurements show a reasonable agreement with the established nonlinear models that predicts the statistical characteristics of seismic wavefield ocean waves. We found that the link between the amplitudes on DAS along the borehole and wave climate is so stable that with sufficient amount of training data, the passive seismic records may be used for high‐precision monitoring of both—formations surrounding the borehole and remote storms in the ocean. Key Points: A DAS system in a single borehole to identify and quantitatively characterize the multitude of ambient seismic signals at the Australian coastSeismic measurements are in a good agreement with theoretical predictions from the ocean waves spectraThe link between the DAS response and wave climate enables monitoring both the formations surrounding the borehole and remote storms in the ocean [ABSTRACT FROM AUTHOR]

The U.S. National Oceanic and Atmospheric Administration's Office of National Marine Sanctuaries manages a system of marine protected areas encompassing more than 2,000,000 km2. U.S. National Marine Sanctuaries (NMS) have been designated to provide protection for their conservation, recreational, ecological, historical, scientific, cultural, archaeological, educational, or aesthetic qualities. Due to the large variability of attributes among NMS, designing coordinated system‐wide monitoring to support diverse resource protection goals can be challenging. Underwater sound monitoring is seeing increasing application to marine protected area management because it is able to support this wide variety of information needs. Passive acoustics are providing invaluable autonomous information regarding habitat associations, identifying species spatial and temporal use, and highlighting patterns in conditions that are otherwise difficult to survey. Using standardized equipment and analysis methods this study collected ambient underwater sound data and derived measurements to investigate temporal changes in sound pressure levels and power spectral density, identify presence of select species of importance and support within and among site comparison of ambient underwater sound among eight sites within four U.S. NMS. Broadband sound pressure levels of ambient sound (10–24,000 Hz) varied as much as 24 dB re 1 µPa (max difference 100–124 dB re 1 µPa) among the recording sites, sanctuaries, and seasons. Biotic signals, such as snapping shrimp snaps and vocalizations of fishes, exhibited distinct diel and seasonal patterns and showed variation among sites. Presence of anthropogenic signals, such as vessel passage, also varied substantially among sites, ranging from on average 1.6–21.8 h/d. The study identified measurements that effectively summarized baseline soundscape attributes and prioritized future opportunities for integrating non‐acoustic and acoustic variables in order to inform area‐specific management questions within four ecologically varying U.S. National Marine Sanctuaries. [ABSTRACT FROM AUTHOR]

Localization is considered as an important research concept for underwater acoustic sensor networks (UASNs). It performs significant role in diverse routing methods, estimating the node position and node recovery. In UASNs, localization methods have different characteristics compared with the terrestrial networks. The challenges involved in UASNs are varying water temperature and pressure, time synchronization of beacon nodes, complicated ocean currents, and positioning of nodes. To overcome these challenges, a virtual node is deployed using the Nelder–Mead algorithm with the static localization method. In this study, two types of localization methods namely static and dynamic methods are considered and a virtual node is deployed in a static localization manner. Since anchor nodes cannot communicate to the entire network for localization additionally, virtual nodes are deployed to measure the received signal strength indicator and error ratio for effective transmission. In addition "GPS node" is equipped with a ship for easy deployment without communication overhead. The simulation result justifies that static localization for an autonomous underwater sensor networks perform with better coverage rate without time synchronization and acoustic transmission overhead. [ABSTRACT FROM AUTHOR]

Exploration of the marine environment using underwater remote‐sensing methods is the most reasonable method for investigating submerged archaeological heritage sites, preserving their current condition and maintaining them for future generations. While non‐invasive recognition of heritage sites is one of the main objectives of underwater archaeology, the nearshore environment of the small Polish town of Puck hides one of the biggest medieval harbours in the Baltic Sea. The following research objectives were met in this study: (a) exploration of underwater archaeological heritage harbour in Puck using high‐resolution hydroacoustics, 3‐D shallow seismics and underwater photogrammetry; (b) reconstruction of the submerged Puck medieval port by the combining of hydroacoustics, 3D shallow seismics, underwater photogrammetry and archival documentation; (c) estimation of the rate at which the bottom sediments containing the archaeological objects are being destroyed. The underwater archaeological site of Puck harbour was explored using multiple surveying methods, including multibeam echosounder, parametric sub‐bottom profiler, underwater photogrammetry, aerial photography and comparison with archival documentation. Very high‐resolution bathymetry from a multibeam echosounder allowed seabed features such as piles, stones and horizontal structural elements to be identified, along with other archaeological artefacts, boat wrecks and boat‐building artefacts. The 3‐D shallow seismic datasets allowed us to distinguish, for example, several outcropping structures, a previously unknown buried shipwreck, past excavation trenches, the harbour boundary and a palaeochannel of the Płutnica River. Fusing datasets from multiple sources in the geographic information system (GIS) environment allowed for a combined visualization of the heritage sites, providing a broad archaeological demonstration of the underwater structure. By comparing with archival documentation, we calculated that 43% of biogenic layers containing archaeological objects eroded over 26 years. Our results demonstrated that without any doubt, underwater remote‐sensing methods are the most appropriate for exploration and investigation of underwater heritage sites while maintaining their original value. [ABSTRACT FROM AUTHOR]

One of the main challenges of underwater archaeology is to develop non‐invasive research of heritage sites in order to enable their further protection for future societies. This study explores, identifies and classifies archaeological objects in a shallow lake using underwater acoustics. We solved the aforementioned challenges by developing an innovative, object‐based, fuzzy‐logic classification of nine archaeological object categories based on multibeam echosounder bathymetry, 13 secondary features of bathymetry and 106 underwater diving prospections. We achieved an 86% correlation with ground‐truth samples, and 49% overall accuracy. The unique and repeatable workflow developed in this study can be applied to other case studies of underwater archaeology around the world. [ABSTRACT FROM AUTHOR]

Climate change is projected to cause the Arctic soundscape to become noisier due to sea ice loss and increased anthropogenic activity. Many studies on underwater sound levels have been conducted in the western North American Arctic and Fram Strait, but the rest of the Arctic is full of geographic gaps. Han et al. (2021, https://doi.org/10.1029/2021gl093097) published a study in Geophysical Research Letters on underwater sound levels in the East Siberian Sea, providing the first estimates of seasonal trends and the natural and anthropogenic drivers of underwater sound levels in this region. This is an excellent first step in filling geographic gaps in the Russian Arctic, and I call on other researchers to continue to fill these geographic gaps throughout the Arctic so that we can set a baseline and study changes to underwater sound levels being caused directly and indirectly by climate change. Plain Language Summary: The Arctic marine environment can be one of the quietest on Earth, particularly under solid sea ice. Climate change is causing sea ice to melt at an alarming rate and is also allowing increased human shipping activity. Both sea ice loss and increased shipping activity will likely lead to a noisier environment underwater in the Arctic. Many studies have examined how loud the underwater environment is in the Arctic of western North America and between East Greenland and Svalbard, but there are many areas throughout the Arctic with no studies. Han et al. (2021, https://doi.org/10.1029/2021gl093097) just published the first study on the loudness of the underwater environment in the East Siberian Sea. This study fills an important gap in our knowledge, and I call on other researchers to continue expanding studies into areas of the Arctic where we have no data on underwater sound levels. More studies will help us track the changes in underwater sound levels being caused by climate change. Key Points: Han et al. (2021) published a study in Geophysical Research Letters on underwater sound levels in the East Siberian SeaI place this study in the context of broader effort to establish baselines of underwater sound levels in the ArcticI call on researchers to continue filling geographic gaps in studies of underwater sound levels, particularly in the Russian Arctic [ABSTRACT FROM AUTHOR]

When estimating fish population abundance, it is important to recognize that differing habitat use may cause one gear type to be more effective and less biased than another. We generated and compared population abundance estimates (PE) for adult Yellow Perch Perca flavescens in Crystal Lake, Wisconsin using a spring mini‐fyke net mark–recapture survey and summer hydroacoustic surveys. Mean PE from the spring mark–recapture survey was 11,051 adult Yellow Perch (95% confidence limits of 9,878 and 12,541). This mean was 4.0–8.5 times greater than the range of mean summer hydroacoustic estimates (mean ± 95% CI = 1,291 ± 312 and 2,912 ± 703). Due to Yellow Perch spawning behavior, we assumed that the spring mark–recapture survey sampled the entire adult population, while summer hydroacoustics sampled the postspawn pelagic component. Using the mean of all hydroacoustic surveys (PE = 2,492; n = 5), we estimated that approximately 22% of adult Yellow Perch selected for pelagic habitats postspawn. Our study emphasizes the importance of evaluating gear bias and has implications for future assessments, particularly when the target species may exhibit multiple habitat preferences within a lake. [ABSTRACT FROM AUTHOR]

Ecoacoustics is increasingly being used to monitor species populations and to estimate biodiversity in marine ecosystems, but the underwater soundscapes of freshwater environments remain largely unexplored in this respect. Few studies exist concerning the acoustic diversity of ponds, but because aquatic plants and many arthropods such as Coleoptera and Hemiptera are known to produce sound, there is potential to use ecoacoustic techniques to monitor changes in biodiversity and conservation value.This pilot study compares the underwater soundscapes of recently restored open‐canopy ponds and unmanaged highly terrestrialized ponds situated in an arable agricultural landscape of North Norfolk, UK, in order to assess the benefits of farmland pond restoration.Daytime sound recordings were made for 10 min in each pond and analysed primarily for arthropod stridulations. In addition, six commonly used acoustic indices were calculated to assess the soundscape biodiversity between the unmanaged and the restored ponds. The stridulations of three diving beetle species (Dytiscidae) were recorded in tank studies to assess the potential for individual species recognition from underwater sound capture.Sound‐type richness and abundance, as estimated by visually and aurally identifying arthropod stridulation from spectrograms, were significantly higher in the restored open‐canopy ponds compared with the unmanaged terrestrialized ponds. In addition, the acoustic indices 'acoustic complexity' and 'biodiversity index' were significantly higher in restored open‐canopy ponds than in unmanaged terrestrialized ponds.The three dytiscid water beetle species recorded in a tank were found to produce distinctive and recognizable sounds, indicating potential to create an audio reference library that could be used for automatic acoustic monitoring of freshwater arthropods.Pond soundscapes are rich in biological information and this study suggests that, with further development, automated passive ecoacoustic monitoring could be an effective non‐invasive technique for assessing pond conservation value and pond restoration and management success. [ABSTRACT FROM AUTHOR]

Anthropogenic noise has increased underwater ambient sound levels in the range in which most fishes detect and produce acoustic signals. Although the impacts of increased background noise on fish development have been studied in a variety of species, there is a paucity of information on how noise affects parental care. Mouthbrooding is an energetically costly form of parental care in which the brooding fish carries developing larvae in the buccal cavity for the duration of development. In the African cichlid Astatotilapia burtoni, females carry their brood for ~2 weeks during which time they do not eat. To test the hypothesis that increased background noise impacts maternal care behaviors and brood development, we exposed brooding females to a 3‐h period of excess noise (~140 dB) played through an underwater speaker. Over half of noise‐exposed brooding females cannibalized or pre‐maturely released their brood, but 90% of control females exhibited normal brooding behaviors. RNA‐seq analysis revealed that transcripts related to feeding and parental care were differentially expressed in the brains of noise‐exposed females. Juveniles that were exposed to noise during their brood period within the mother's mouth had lower body condition factors, higher mortality and altered head transcriptomes compared with control broods. Furthermore, onset of adult‐typical coloration and behaviors was delayed compared with control fish. Together, these data indicate that noise has severe impacts on reproductive fitness in mouthbrooding females. Our results, combined with past studies, indicate that parental care stages are extremely susceptible to noise‐induced perturbations with detrimental effects on species persistence. [ABSTRACT FROM AUTHOR]

The soundscape is a crucial habitat feature for marine mammals. This study investigates the contribution of bearded seal vocalizations to the soundscape in the western Canadian Arctic, and also the vocal characteristics of bearded seals relative to sea ice conditions. Passive acoustic data were recorded near Sachs Harbour between August 2015 and July 2016. Sound pressure levels (SPL) in the 50–1,000 Hz and 1–10 kHz bands increased as the total duration of all bearded seal vocalizations increased, and this relationship was moderated by sea ice concentration. Bearded seals in this region had an overlapping vocal repertoire with bearded seals in other areas of the Arctic, and had seven additional vocalizations that have not been previously documented for this region. This study is the first detailed assessment of the influence of bearded seal calls on SPL, which shows the high potential of bearded seals to influence underwater sound levels during the mating season. Bearded seals live in a changing Arctic seascape, and their influence on the soundscape may shift as sea ice continues to diminish. It is imperative that acoustic monitoring continues within the Arctic, and this study provides a baseline for future monitoring as the Arctic continues to change. [ABSTRACT FROM AUTHOR]

The release of greenhouse gases from both natural and man‐made sites has been identified as a major cause of global climate change. Extensive work has addressed quantifying gas seeps in the terrestrial setting while little has been done to refine accurate methods for determining gas flux emerging through the seabed into the water column. This paper investigates large‐scale methane seepage from the Scanner Pockmark in the North Sea with a new methodology that integrates data from both multibeam and single‐beam acoustics, with single‐beam data covering a bandwidth (3.5 to 200 kHz) far wider than that used in previous studies, to quantify the rate of gas release from the seabed into the water column. The multibeam data imaged a distinct fork‐shaped methane plume in the water column, the upper arm of which was consistently visible in the single‐beam data, while the lower arm was only intermittently visible. Using a novel acoustic inversion method, we determine the depth‐dependent gas bubble size distribution and the gas flux for each plume arm. Our results show that the upper plume arm comprises bubbles with radii ranging from 1 to 15 mm, while the lower arm consists of smaller bubbles with radii ranging from 0.01 to 0.15 mm. We extrapolate from these estimates to calculate the gas flux from the Scanner Pockmark as between 1.6 and 2.7 × 106 kg/year (272 to 456 L/min). This range was calculated by considering uncertainties together with Monte Carlo simulation. Our improved methodology allows more accurate quantification of natural and anthropogenic gas plumes in the water column. Plain Language Summary: Understanding the rate of gas release from natural ebullition sites, such as pockmarks, into the water column is a major factor in understanding the input of greenhouse gases, such as methane and carbon dioxide, into the global ocean system. The detection and quantification of gas flux in the marine environment have relied upon acoustics. However, current active acoustic methods are mainly based on single‐frequency quantification, which can never unambiguously quantify the gas flux due to the bubble size distribution and the scattering across a range of frequencies, and lead to an ill‐conditioned inversion problem. This paper proposes a solution to this dilemma using two elements. First, we employ a wider range of frequencies than previously used, so that more of the bubble resonances are encompassed. Second, it assumes a form for the bubble size distribution, further constraining the solution and effectively regularizing the inversion. The broadband methodology enables us to quantify gas flux with frequencies spanning the resonances of all the bubbles in the plume, allowing more accurate quantification of natural and anthropogenic gas plumes in the water column. Key Points: A broadband active acoustic method for determining the bubble size distribution and gas flux of gas plumes in the water column is presentedImaging of the methane plume in the North Sea showed two distinct arms with larger bubbles and smaller bubbles, respectivelyTotal in situ methane flux from the pockmark into the water column is quantified as between 1.6 and 2.7 × 106 kg/year (272 to 456 L/min) [ABSTRACT FROM AUTHOR]

Assimilating oceanic observations into prediction systems is an advantageous approach for real-time ocean environment characterization. However, its benefits to underwater acoustic predictions are not trivial due to the nonlinearity and sensitivity of underwater acoustic propagation to small-scale oceanic features. In order to assess the potential of oceanic data assimilation, integrated ocean-acoustic Observing System Simulation Experiments are conducted. Synthetic altimetry and in situ data were assimilated through a variational oceanographic data assimilation system. The predicted sound speed fields are then ingested in a range-dependent acoustic model for transmission loss (TL) predictions. The predicted TLs are analyzed for the purpose of (i) evaluating the contributions of different sources to the uncertainties of oceanic and acoustic forecasts and (ii) comparing the impact of different oceanic analysis schemes on the TL prediction accuracy. Using ensemble member clustering techniques, the contributions of boundary conditions, ocean parameterizations, and geoacoustic characterization to acoustic prediction uncertainties are addressed. Subsequently, the impact of three-dimensional variational (3DVAR), 4DVAR, and hybrid ensemble-3DVAR data assimilation on acoustic TL prediction at two signal frequencies (75 and 2,500 Hz) and different ranges (30 and 60 km) are compared. 3DVAR significantly improves the predicted TL accuracy compared to the control run. Promisingly, 4DVAR and hybrid data assimilation further improve the TL forecasts, the hybrid scheme achieving the highest skill scores for all cases, while being the most computationally intensive scheme. The optimal scheme choice thus depends on requirements on the accuracy and computational constraints. These findings foster developments of coupled data assimilation for operational underwater acoustic propagation. [ABSTRACT FROM AUTHOR]

There is an increasing international focus to understand and quantify the potential ecological risks of low‐frequency underwater sounds produced from anthropogenic activities (e.g., commercial shipping, dredging, construction, and offshore energy production). For dredge operations, a risk‐based approach has been proposed for identifying, assessing, and managing risks; however, specific details of the framework and demonstration of the approach are lacking. Thus, the goal of this study was to provide a practical, concise, and reliable framework for assessing the effects of dredging sounds on aquatic life. The specific objectives were to 1) further specify a risk assessment approach for assessing underwater sounds from dredging operations, 2) demonstrate the utility of the approach in practice using a case study, and 3) document the strengths and challenges of the approach. The risk framework was adapted for underwater sounds to include a project formulation step, an analysis step to analyze and assess exposure and biological responses, a risk characterization process in which the preceding steps are integrated and uncertainty is addressed, and a risk management step. A key beneficial component of this framework is the use of a phased approach, whereby a screening step offers a process that utilizes existing or readily available information to evaluate risk. In general, a limitation of evaluating risks due to dredge operations is the degree of uncertainty surrounding effect thresholds for many marine species; however, this approach emphasizes the importance of documenting and communicating uncertainty to regulators, stakeholders, and practitioners in the decision‐making process. A case study example is included to illustrate how the framework can be applied in practice. The primary strength of this method is the intrinsic flexibility of the framework to adapt as the scientific understanding improves and new data become available in the rapidly evolving field of underwater acoustics. Integr Environ Assess Manag 2020;16:481–493. © 2020 SETAC KEY POINTS: Underwater sounds are an emerging concern to aquatic life, yet demonstration of a risk‐based approach to evaluate potential biological consequences of these complex and dynamic physical stressors is lacking.To address this need, a phased ecological risk assessment framework applied to underwater dredging sounds was developed to demonstrate the benefits of a transparent and repeatable process to estimate and communicate risks.Using a port expansion dredging project as a case study, a screening‐level assessment was used to illustrate the practicality of the approach using publicly available sound exposure modeling tools and published biological effects thresholds.A key beneficial component of the framework is the use of a phased approach, whereby a screening step offers a process that utilizes existing or readily available information to evaluate risk, reducing resources needed to conduct the assessment and for focusing more detailed evaluations. [ABSTRACT FROM AUTHOR]

Submarine groundwater discharge into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine ecology, the coastal sedimentary environment, and its potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical pore water and water column investigations at a well‐known groundwater seep site in Eckernförde Bay (Germany). We aim to better constrain the effects of shallow gas and submarine groundwater discharge on high‐frequency multibeam backscatter data and to present acoustic indications for submarine groundwater discharge. Our high‐quality hydroacoustic data reveal hitherto unknown internal structures within the pockmarks in Eckernförde Bay. Using precisely positioned sediment core samples, our hydroacoustic‐geochemical approach can differentiate intrapockmark regimes that were formerly assigned to pockmarks of a different nature. We demonstrate that high‐frequency multibeam data, in particular the backscatter signals, can be used to detect shallow free gas in areas of enhanced groundwater advection in muddy sediments. Intriguingly, our data reveal relatively small (typically <15 m across) pockmarks within the much larger, previously mapped pockmarks. The small pockmarks, which we refer to as "intrapockmarks," have formed due to the localized ascent of gas and groundwater; they manifest themselves as a new type of "eyed" pockmarks, revealed by their acoustic backscatter pattern. Our data suggest that, in organic‐rich muddy sediments, morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and subsequent advective groundwater flow. Plain Language Summary: Groundwater that seeps out of the ocean floor is a common global phenomenon. Marine ecosystems are highly dependent on nutrient supply from land, and recent studies have suggested that groundwater seeping out of the seafloor supplies even more nutrients to the world's oceans than rivers do. Also, nearshore freshwater springs have been used as a source of drinking water for decades and large offshore groundwater reserves in continental shelves can potentially prevent future freshwater shortages. We use echo sounding methods to search for indications for submarine groundwater. The methods enable us to carry out detailed investigations of intriguing seafloor depressions (i.e., "pockmarks"). Our accurate measurements give new insights into the morphology and characterization of the Eckernförde Bay pockmarks and reveal a new type of pockmark that is related to seeping groundwater. In the muddy sediment of Eckernförde Bay, methane gas forms in the sediments due to microbial decomposition of biomass. In areas of enhanced groundwater advection, this methane gas is brought closer to the seafloor, where pockmarks form and where we can detect the gas with our sonar system. Given the abundant global distribution of muddy gaseous sediments, our findings have important implications for the future detection of offshore groundwater systems. Key Points: We present a new form of eyed pockmarks caused by shallow gasWe establish acoustic indicators for submarine groundwater discharge associated with gas [ABSTRACT FROM AUTHOR]

Traditional physical screens designed to prevent fish entering dangerous areas (e.g., turbine intakes) can have negative impacts due to impingement or mechanical abrasion at high velocities. Behavioural deterrents may provide an alternative approach to screening. This study investigated the potential for a continuous broadband sound to modify the behaviour of two endangered species of anguilliform fish: European eel (Anguilla anguilla) and river lamprey (Lampetra fluviatilis). Experiments were conducted in an experimental channel. Eel and lamprey were, respectively, released upstream and downstream of an "acoustic maze". A single individual released per trial encountered two successive chambers that offered a choice of passage through either an ensonified or a control (ambient noise only) corridor with a speaker turned off. Two possible configurations were tested to control for any lateral bias with positions of the activated speakers reversed. The influence of treatment, chamber, and configuration on route selection, rejection, and time to pass were tested. No influence of any of the three factors on route selection was observed for eel. River lamprey tended to pass through the ensonified corridor more often under Configuration 2 but only in the first chamber. Both species were more likely to reject the ensonified corridors than the controls, and the time taken to pass these routes was greater for those that did so. For eel, the variation in time to pass was greater for the non‐migratory (yellow phase) life stage. Although the acoustic signal used in this study influenced fish behaviour, the response observed would likely be insufficient to induce a strong deterrent effect in the field if used in isolation. [ABSTRACT FROM AUTHOR]

Passive acoustic monitoring is gaining momentum as a viable alternative method to surveying freshwater ecosystems. As part of an emerging field, the spatio‐temporal replication levels of these sampling methods need to be standardised. However, in shallow waters, acoustic spatio‐temporal patchiness remains virtually unexplored.In this paper, we specifically investigate the spatial heterogeneity in underwater sounds observed within and between waterholes of an ephemeral river at different times of the day and how it could affect sampling in passive acoustic monitoring.We recorded in the Einasleigh River, Queensland in August 2016, using a linear transect of hydrophones mounted on frames. We recorded four times a day: at dawn, midday, dusk, and midnight. To measure different temporal and spectral attributes of the recorded sound, we investigated the mean frequency spectrum and computed acoustic indices.Both mean frequency spectrum and index analyses revealed that the site and diel activity patterns significantly influenced the sounds recorded, even for adjacent sites with similar characteristics along a single river. We found that most of the variation was due to temporal patterns, followed by between‐site differences, while within‐site differences had limited influence.This study demonstrates high spatio‐temporal acoustic variability in freshwater environments, linked to different species or species groups. Decisions about sampling design are vital to obtain adequate representation. This study thus emphasises the need to tailor spatio‐temporal settings of a sampling design to the aim of the study, the species and the habitat. [ABSTRACT FROM AUTHOR]

Applications in bioacoustics and its sister discipline ecoacoustics have increased exponentially over the last decade. However, despite knowledge about aquatic bioacoustics dating back to the times of Aristotle and a vast amount of background literature to draw upon, freshwater applications of ecoacoustics have been lagging to date.In this special issue, we present nine studies that deal with underwater acoustics, plus three acoustic studies on water‐dependent birds and frogs. Topics include automatic detection of freshwater organisms by their calls, quantifying habitat change by analysing entire soundscapes, and detecting change in behaviour when organisms are exposed to noise.We identify six major challenges and review progress through this special issue. Challenges include characterisation of sounds, accessibility of archived sounds as well as improving automated analysis methods. Study design considerations include characterisation analysis challenges of spatial and temporal variation. The final key challenge is the so far largely understudied link between ecological condition and underwater sound.We hope that this special issue will raise awareness about underwater soundscapes as a survey tool. With a diverse array of field and analysis tools, this issue can act as a manual for future monitoring applications that will hopefully foster further advances in the field. [ABSTRACT FROM AUTHOR]

Ecoacoustic methods are increasingly used to monitor the state of populations and ecosystems. In freshwater environments, they present the clear advantages of being non‐invasive, reducing bias, and providing continuous observations instead of only limited sampling snapshots in time. However, similar to standard bioassessment methods, temporal variation and choice of indicators can greatly influence ecoacoustic assessments, highlighting the importance of sampling and analysis design.In this study, we quantified diurnal variation in underwater sound and its effect on sampling regimes for two waterholes in the Einasleigh River, Northern Australia. Recording continuously for 6 days, and subsampling 5 s every 10 min, we found 22 distinct sounds that were emitted by fish, Hemiptera and Coleoptera as well as another 22 of abiotic or unknown origin.Through rarefaction analyses, we found that subsampling the data to 60% of the recorded sound events resulted in capture of most of the 44 identified sound types. Temporal heterogeneity—patchy sound events through time—needs to be considered when maximising detected sound events. Reducing the sampling interval from every 10 min to half‐hourly or hourly had a much greater effect on capturing all sound types compared to the number of days recorded or the length of the recording. Overall, only 10–20% of the sound events need to be annotated for most sound types to be described; for example, restricting analysis of the days recorded to only three and the recording interval to 0.5–1 s. Acoustic indices were dominated by three main event types—a diurnally flowing creek, a nocturnal chorus of Hemiptera, as well as a dawn chorus of terapontid fishes.We conclude with two key messages: First, a select group of informative signals can be monitored using very simple methods—namely, converting an audio stream into indices using freely available software. Second, however, to detect less acoustically dominant sound events, manual annotation or single call processing will still be needed. While these findings are encouraging, similar analysis will need to be conducted within other freshwater ecosystems before general conclusions about optimal sampling regimes can be drawn. [ABSTRACT FROM AUTHOR]

Sounds are more easily transmitted underwater than through air and many freshwater fish species can hear them, particularly over the low frequencies. Recent studies on freshwater fish evidenced that hearing sensitivities can be limited by the level of ambient noise, a phenomenon also known as acoustic masking. However, it is still unclear whether variations in ambient noises, such as those produced by human activities, may alter fish trophic interactions.The general objective of this study was to propose and evaluate a theoretical framework explicitly linking fish attack rates to species hearing sensitivities and ambient noise levels in freshwater ecosystems. The proposed model shows that the feeding activity of fish at an intermediate position in the food web is conditional on the probability of being acoustically detected by their predators, or of encountering an acoustically distressed resource.Model simulations and preliminary field results suggest that yellow perch (Perca flavescens) could feed more actively in the presence of augmented ambient noise levels. Yellow perch captures per unit effort were higher by a factor of 2.7 in noisy versus quiet days. We argue that fish exposed to intermediate levels of noise could maximise the probability of detecting food patches while minimising their predation risk.Acoustic monitoring programmes for freshwater ecosystems require a fundamental knowledge of underwater noise levels, species hearing sensitivities and features affecting sound propagation. The approach proposed in this paper is seminal in linking the above descriptors in a coherent mathematical framework to understand the effect of underwater sounds on trophic interactions. Such framework is needed to make testable predictions and generalise to other taxa and ecological contexts. [ABSTRACT FROM AUTHOR]