Search

The deep scattering layer (DSL), a stratum of the marine diel vertical migration (DVM) organisms inhabiting the mesopelagic ocean, plays a crucial role in transporting carbon and nutrients from the surface to depth through the migration of its organisms. Using 18 months of in-situ observations and altimeter sea level data, we reveal for the first time the intraseasonal variations and underlying mechanisms of the DSL and the DVM to the east of the Taiwan Island. Substantial vertical speeds acquired from the Acoustic Doppler Current Profiler were used to examine the distribution and variation of the DVM. Innovatively, the results for the power spectrum analysis of the scattering intensity demonstrated a significant intraseasonal variability (ISV) with an 80- day period in the DSL. Furthermore, the variation in the DVM was closely linked to the DSL and showed an 80-day ISV during the observation. A dynamic relationship between the ISV of the DSL east of Taiwan Island and the westward-propagating mesoscale eddies was established. Anticyclonic (cyclonic) eddy movement toward Taiwan Island triggers downward (upward) bending of the local isotherms, resulting in a layer of DSL warming (cooling) and subsequent upper boundary layer deepening (rising). These findings underscore the substantial influence of mesoscale eddies on biological activity in the mesopelagic ocean, establishing a novel understanding of ISV dynamics in the DSL and their links to eddy-induced processes. [ABSTRACT FROM AUTHOR]

Abstract Background The diel vertical migration (DVM) of fish provides an active transport of labile dissolved organic matter (DOM) to the deep ocean, fueling the metabolism of heterotrophic bacteria and archaea. We studied the impact of DVM on the mesopelagic prokaryotic diversity of the Red Sea focusing on the mesopelagic deep scattering layer (DSL) between 450–600 m. Results Despite the general consensus of homogeneous conditions in the mesopelagic layer, we observed variability in physico-chemical variables (oxygen, inorganic nutrients, DOC) in the depth profiles. We also identified distinct seasonal indicator prokaryotes inhabiting the DSL, representing between 2% (in spring) to over 10% (in winter) of total 16S rRNA gene sequences. The dominant indicator groups were Alteromonadales in winter, Vibrionales in spring and Microtrichales in summer. Using multidimensional scaling analysis, the DSL samples showed divergence from the surrounding mesopelagic layers and were distributed according to depth (47% of variance explained). We identified the sources of diversity that contribute to the DSL by analyzing the detailed profiles of spring, where 3 depths were sampled in the mesopelagic. On average, 7% was related to the epipelagic, 34% was common among the other mesopelagic waters and 38% was attributable to the DSL, with 21% of species being unique to this layer. Conclusions We conclude that the mesopelagic physico-chemical properties shape a rather uniform prokaryotic community, but that the 200 m deep DSL contributes uniquely and in a high proportion to the diversity of the Red Sea mesopelagic.

Background: The diel vertical migration (DVM) of fish provides an active transport of labile dissolved organic matter (DOM) to the deep ocean, fueling the metabolism of heterotrophic bacteria and archaea. We studied the impact of DVM on the mesopelagic prokaryotic diversity of the Red Sea focusing on the mesopelagic deep scattering layer (DSL) between 450–600 m. Results: Despite the general consensus of homogeneous conditions in the mesopelagic layer, we observed variability in physico-chemical variables (oxygen, inorganic nutrients, DOC) in the depth profiles. We also identified distinct seasonal indicator prokaryotes inhabiting the DSL, representing between 2% (in spring) to over 10% (in winter) of total 16S rRNA gene sequences. The dominant indicator groups were Alteromonadales in winter, Vibrionales in spring and Microtrichales in summer. Using multidimensional scaling analysis, the DSL samples showed divergence from the surrounding mesopelagic layers and were distributed according to depth (47% of variance explained). We identified the sources of diversity that contribute to the DSL by analyzing the detailed profiles of spring, where 3 depths were sampled in the mesopelagic. On average, 7% was related to the epipelagic, 34% was common among the other mesopelagic waters and 38% was attributable to the DSL, with 21% of species being unique to this layer. Conclusions: We conclude that the mesopelagic physico-chemical properties shape a rather uniform prokaryotic community, but that the 200 m deep DSL contributes uniquely and in a high proportion to the diversity of the Red Sea mesopelagic. [ABSTRACT FROM AUTHOR]

The use of underwater recording is widely implemented across different marine ecology studies as a substitute for more invasive techniques. This is the case of the Deep Scattering Layer (DSL), a biomass-rich layer in the ocean located between 400 and 600 m deep. The data processing of underwater videos has usually been carried out manually or targets organisms above a certain size. Marine snow, or macroscopic amorphous aggregates, plays a major role in nutrient cycles and in the supply of organic material for organisms living in the deeper layers of the ocean. Marine snow, therefore, should be taken into account when estimating biomass abundance in the water column. The main objective of this project is to develop a new software application for the automatic detection and analysis of biomass abundance relative to time in underwater videos, taking into consideration small items. The application software is based on a pipeline and client-server architecture, developed in Python and using open source libraries. The software was trained with underwater videos of the DSL recorded with low-cost equipment. A usability study carried out with end-users shows satisfaction with the user-friendly interface and the expected results. The software application developed is capable of automatically detecting small items captured by underwater videos. In addition, it can be easily adapted to a web application. [ABSTRACT FROM AUTHOR]

The use of underwater recording is widely implemented across different marine ecology studies as a substitute for more invasive techniques. This is the case of the Deep Scattering Layer (DSL), a biomass-rich layer in the ocean located between 400 and 600 m deep. The data processing of underwater videos has usually been carried out manually or targets organisms above a certain size. Marine snow, or macroscopic amorphous aggregates, plays a major role in nutrient cycles and in the supply of organic material for organisms living in the deeper layers of the ocean. Marine snow, therefore, should be taken into account when estimating biomass abundance in the water column. The main objective of this project is to develop a new software application for the automatic detection and analysis of biomass abundance relative to time in underwater videos, taking into consideration small items. The application software is based on a pipeline and client-server architecture, developed in Python and using open source libraries. The software was trained with underwater videos of the DSL recorded with low-cost equipment. A usability study carried out with end-users shows satisfaction with the user-friendly interface and the expected results. The software application developed is capable of automatically detecting small items captured by underwater videos. In addition, it can be easily adapted to a web application.

Diurnal vertical migration (DVM) of mesozooplankton in the Deep Scattering Layer (DSL) of the Indian seas is poorly studied. This cyclical vertical migration substantially controls the carbon sequestration in the ocean. The present research is a comprehensive examination to analyse the factors affecting the DVM pattern of the zooplankton community in the Arabian Sea (AS) and the Bay of Bengal (BoB). Echo sounder profiling was conducted at shallow depths (∼10–400m) of the AS (January 2023) and BoB (March 2023) with a period of 24 h to monitor the DVM pattern of the DSL. Vertical migration in both basins showcased the notable influence of the spatio-temporal contrast in the occurrence of daybreak, with the day (descend) and night (ascend) cycle of the DSL. Delayed descent was observed in the AS contrary to BoB, owing to the delayed day break in the AS relative to BoB. Intensity and temporal pattern of the incoming solar radiation were correlated with the DVM whereas diurnal variation of sea surface temperature was observed to be contrasting. The preliminary analysis is indicative of the diversified community structure of the zooplankton community in these basins resulting from the vertical migration. Furthermore, it is conclusive that the surface residence time of the zooplankton is distinct and is affirmed based on daybreak and light intensity particular for each basin. Since daybreak vary with the geolocation, sole dependence on a particular time for migration study can be erroneous, which is highlighted in the present study. [Display omitted] • Diurnal vertical migration of the DSL in the Arabian Sea and the Bay of Bengal. • Factors determining the ascending and descending of the organisms. • In situ and remote sensing technologies to monitor the diurnal migration. • Light determines the surface residence time of the organisms. • Geolocation plays a major role in navigating the temporal pattern of migration. [ABSTRACT FROM AUTHOR]

The ecological characteristics of mesopelagic community are crucial to understand the pelagic food web, replenishment of pelagic fishery resources, and building models of the biological pump. The deep scattering layers (DSLs) and diel vertical migration (DVM) are typical characteristics of mesopelagic communities, which have been widely observed in global oceans. There is a strong longitudinal environmental gradient across the tropical Pacific Ocean. Nevertheless, the longitudinal variation of DSLs along this gradient was still largely unclear until now. We investigated the DSLs across the tropical Pacific Ocean using data of shipboard acoustic Doppler current profiler at 38 kHz from July to December 2019. The study area was divided into three sub-regions by cluster analysis of environmental variables: the western part (WP), the transition part (TP), and the eastern part (EP). The result confirmed that the longitudinal variation of DSLs and DVM: the weight migrating depth of mesopelagic organisms was reduced from 571.2 ± 85.5 m in the WP to 422.6 ± 80.8 m in the EP; while the migrating proportion was minimum in the TP (35.2 ± 12.8%), and increased to 86.7 ± 16.2% in the EP. Multiple regressions analysis showed that both the mesopelagic average oxygen and chlorophyll a concentration were significant factors which influenced the upper boundary depth and weight migrating depth, while the center mass depth was only influenced by the chlorophyll a. Since higher demand of most predators of mesopelagic animals for dissolved oxygen and light intensity, the limitations of predator behavior by environmental conditions might explain the observed spatial heterogeneity of DSLs. Combining the previous results and the findings of this study, it implied that declined biomass, shallower habituating depths, and lower migration proportion of mesopelagic animals under more extremely oligotrophic conditions with global change in future, would reduce the active carbon flux and hinder food supply to deep-sea biological communities in the tropical Pacific Ocean.

The deep sea (i.e., >200 m depth) is a highly dynamic environment where benthic ecosystems are functionally and ecologically connected with the overlying water column and the surface. In the aphotic deep sea, organisms rely on external signals to synchronize their biological clocks. Apart from responding to cyclic hydrodynamic patterns and periodic fluctuations of variables such as temperature, salinity, phytopigments, and oxygen concentration, the arrival of migrators at depth on a 24-h basis (described as Diel Vertical Migrations; DVMs), and from well-lit surface and shallower waters, could represent a major response to a solar-based synchronization between the photic and aphotic realms. In addition to triggering the rhythmic behavioral responses of benthic species, DVMs supply food to deep seafloor communities through the active downward transport of carbon and nutrients. Bioluminescent species of the migrating deep scattering layers play a not yet quantified (but likely important) role in the benthopelagic coupling, raising the need to integrate the efficient detection and quantification of bioluminescence into large-scale monitoring programs. Here, we provide evidence in support of the benefits for quantifying and continuously monitoring bioluminescence in the deep sea. In particular, we recommend the integration of bioluminescence studies into long-term monitoring programs facilitated by deep-sea neutrino telescopes, which offer photon counting capability. Their Photo-Multiplier Tubes and other advanced optical sensors installed in neutrino telescope infrastructures can boost the study of bioluminescent DVMs in concert with acoustic backscatter and video imagery from ultra-low-light cameras. Such integration will enhance our ability to monitor proxies for the mass and energy transfer from the upper ocean into the deep-sea Benthic Boundary Layer (BBL), a key feature of the ocean biological pump and crucial for monitoring the effects of climate-change. In addition, it will allow for investigating the role of deep scattering DVMs in the behavioral responses, abundance and structure of deep-sea benthic communities. The proposed approach may represent a new frontier for the study and discovery of new, taxon-specific bioluminescence capabilities. It will thus help to expand our knowledge of poorly described deep-sea biodiversity inventories and further elucidate the connectivity between pelagic and benthic compartments in the deep-sea.

Records of backscatter and vertical velocity obtained from moored Acoustic Doppler Current Profilers (ADCP) enabled new insights into the dynamics of deep scattering layers (DSLs) and diel vertical migration (DVM) of mesopelagic biomass between these deep layers and the near-surface photic zone in the southern Norwegian Sea. The DSL exhibits characteristic vertical movement on inter-monthly time scales, which is associated with undulations of the main pycnocline between the warm Atlantic water and the underlying colder water masses. Timing of the DVM is closely linked to the day-night light cycle—decent from the photic zone just before sunrise and ascent immediately after sunset. Seasonal variations are also evident, with the highest DVM activity and lowest depth averaged mean volume backscatter strength (MVBS) during spring. This suggests that both oceanographic and optical conditions are driving the complex dynamics of pelagic and mesopelagic activity in this region. We hypothesize that the increased abundance of calanoid copepods in the near-surface layer during spring increases the motivation for vertical migration of pelagic and mesopelagic species, which therefore can explain the increased DVM activity during this season.

By definition, the mesopelagic twilight zone extends from 200 to 1000 m depth. Rather than confining the twilight zone to a certain depth interval, we here propose a definition that covers absolute light intensities ranging from 10−9 to 10−1 μmol quanta m−2 s−1. The lowest intensity of this twilight habitat corresponds to the visual threshold of lanternfishes (Myctophidae). The highest intensity corresponds to the upper light exposure of pearlsides (Maurolicus spp.), which have a unique eye adapted to higher light intensities than the lanternfishes. By this definition, the daytime twilight habitat extends deeper than 1000 m in very clear oceanic water, while may even be largely located above 200 m in very murky coastal waters. During moonlit nights in clear water, the twilight habitat would still extend deep into the mesopelagic depth zone, while becoming compressed toward the surface in dark nights. Large variation in night light, from 10−3 μmol quanta m−2 s−1 during moonlit nights to 10−8 μmol quanta m−2 s−1 in dark overcast nights, implies that division of light into night- and daylight is insufficient to characterize the habitats and distributional patterns of twilight organisms. Future research will benefit from in situ light measurements, during night- as well as daytime, and habitat classification based on optical properties in addition to depth. We suggest some pertinent research questions for future exploration of the twilight zone. [ABSTRACT FROM AUTHOR]

Abstract The main prey of the Atlantic mackerel undertake diel vertical migrations, however diel and vertical patterns in the feeding by mackerel remains poorly known thus limiting our understanding and quantification of ecosystem trophodynamics and potentially influencing stock assessment and ecosystem based management. By intensive monitoring of adult Atlantic mackerel and its prey during summer in the Irminger Sea, it was found that the diel feeding cycle is of high quantitative importance. Fresh stomach content analysis indicated a marked diel pattern with copepods as the main prey between 16 and 24 (solar time), and euphausiids/myctophids between 22 and 08. Compared with the zooplankton distribution and composition, there were no indications of feeding below the mixed layer. Mackerel selected for the largest and most energy rich prey, namely copepods with prosome lengths >2 mm, and then switched from copepods to euphausiids/myctophids around midnight, despite that the copepods were also aggregated during night. Stomach fullness increased significantly with length and condition, suggesting that individuals that were larger were more effective feeders. The stomach content by weight primarily consisted of copepods (65%, exclusively Calanus finmarchus), euphausiids (18%), and to a lesser extent teleosts (5%, mainly myctophids), hyperiids (4%) and cephalopods (2%). The results are likely widely applicable in the main feeding season and areas (the oceanic parts of the Nordic Seas during summer), because of similarities in the diet of the adult mackerel and in the prey's diel vertical migration. In conclusion, it is imperative to account for the diel dynamics when quantifying the mackerel diet and its impact on prey populations. [ABSTRACT FROM AUTHOR]

Prey and predators continuously react to each other and to their environment, adjusting their behavior to maximize their fitness. In a pelagic environment, organisms can optimize their fitness by performing diel vertical migrations. We applied a game-theoretic approach to investigate the emergent patterns of optimal habitat selection strategies in a multiple-habitat arena. Our setup allows both players to choose their position at day and at night in the water column. The model reproduces features of vertical migrations observed in nature, including residency at depth or at the surface, vertical migrations, mixed strategies, and bimodal distributions within a population. The mixed strategies appear as a consequence of frequency-dependent processes and not of any intraspecies difference between individuals. The model also reveals a curious feature where natural selection on individuals can provoke distinct regime shifts and precipitate an irreversible collapse in fitness. In the case presented here, the increasing voracity of the predator triggers a behavioral shift in the prey, reducing the fitness of all members of the predator population. [ABSTRACT FROM AUTHOR]

The deep scattering layer is an ubiquitous aggregation of zooplankton and forage fish in the ocean. It features the striking phenomenon of diel vertical migration, where animals remain in deep, dark water in daylight hours and migrate upwards at dusk. The common explanation for this is that prey avoid vulnerability to visual predators. Here, we develop a game-theoretic explanation for the deep scattering layer and its diel vertical migrations, focusing on one generic predator species and one generic prey species. The model is formulated in continuous time and space, and by neglecting the cost of locomotion, it allows fine-grained predictions of vertical distributions. The Nash equilibrium features a distinct deep scattering layer which undertakes diel vertical migrations, the range of which increases with predator abundance. Maximum feeding rates are predicted to occur at dawn and dusk. Predator interference emerges from the game dynamics in the form of a complicated functional dependency of gross encounter rates on predator abundance. In turn, the growth rate of the prey decreases monotonically with predator abundance. In addition to providing a mechanistic explanation for the striking phenomenon of diel vertical migrations, the model yields quantitative predictions of vertical distributions and diel patterns in feeding intake, which may be compared with acoustic data, observed individual behavior, or stomach data. [ABSTRACT FROM AUTHOR]

The west coast of Hawaiʻi Island hosts elevated primary production compared to offshore waters and an abundance of large pelagic animals for reasons that are not entirely understood. Here we show that the nearshore environment off the west coast of Hawaiʻi exhibits an increased biomass and abundance of mesopelagic micronekton. Acoustic surveys from this study and prior work show a higher nautical area scattering coefficient (as a proxy for biomass) in a deep non-migratory layer in nearshore sites compared to offshore sites that is persistent over five years. Cobb trawl samples taken at the depths of the deep scattering layer (∼450 to ∼550 m) in 2016 and 2017 showed 1.3 to 2.2 times higher biomass and 2.7 times higher abundance nearshore (∼4 km from shore) compared to offshore (∼24 km from shore). Fishes dominated the trawl catches and a large fraction of the nearshore enhancements were due to Sternoptychidae and Serrivomeridae across both years. In contrast, Melamphaeidae consistently were more abundant and had greater biomass offshore. This deep scattering assemblage contrasts with the mesopelagic boundary layer assemblage by being nonmigratory and taxonomically different in composition. These nearshore mesopelagic enhancements could occur as the result of increased nearshore food supplies deriving from Island Mass Effect enhanced primary production or from advection and concentration in a complex flow environment in the lee of the island. Regardless of mechanism, this temporally persistent, high biomass, largely nonmigratory layer of mesopelagic micronekton is different from the better known migratory mesopelagic boundary layer assemblage and may provide food to deeper diving marine mammals and pelagic fishes possibly explaining the aggregation of large pelagic animals in this region. • Acoustic surveys show a deep non-migratory layer of micronekton in nearshore waters. • Trawls found ∼2-fold higher abundance and biomass compared to offshore sites. • The aggregation may arise from the Island Mass Effect or the complex flow environment. • The aggregation may explain aggregations of large pelagic animals in the area. [ABSTRACT FROM AUTHOR]

Three hundred and twenty-nine species of micronektonic fishes were identified in 1040 midwater trawl collections taken between 1963 and 1974 from the North and South Atlantic, the Mediterranean, the Caribbean and the Gulf of Mexico. The target of most tows was the deep scattering layer, and consequently the dominant species in the material were those that were concentrated in the layer. The results only generally confirm the 11 Atlantic mesopelagic ecoregions previously recognized. The geometric mean of the proportion of joint occurrences (GMPJO) of species with tows within each ecoregion was used to characterize the faunas of the ecoregions. The ecoregion affinities of fishes were compared to those of decapod shrimp in the same collections. The fish and shrimp faunas of ecoregions could be distinguished by GMPJO values, but the ranges of species favoring each ecoregion varied widely in extent and did not conform well to ecoregion boundaries or features of circulation. This suggests that co-occurring species respond differently to the physical properties and resulting biological factors defining mesopelagic ecoregions. [ABSTRACT FROM AUTHOR]

While sound scattering layers (SSLs) have been described previously from ice-covered waters in the Arctic, the existence of a viable mesopelagic community that also includes mesopelagic fishes in the Arctic has been questioned. In addition, it has been hypothesized that vertical migration would hardly exist in these areas. We wanted to check if deep scattering layers (DSLs) was found to the west and north of Svalbard (79°30′N−82°10′N) during autumn 2015, and if present; whether organisms in such DSLs undertook vertical migrations. Our null hypothesis was that there would be no evidence of diel vertical migration. Multi-frequency acoustic observations by hull mounted echo sounder (18, 38, and 120 kHz) revealed a DSL at depths ~210–510 m in areas with bottom depths exceeding ~600 m. Investigating eight geographical locations that differed with respect to time periods, light cycle and sea ice conditions, we show that the deeper layer of DSL displayed a clear ascending movement during night time and a descending movement during daytime. The high-light weighted mean depth (WMD) (343–514 m) with respect to backscattered energy was statistically deeper than the low-light WMD (179–437 m) for the locations studied. This behavior of the DSL was found to be consistent both when the sun was continuously above the horizon and after it started to set on 1 September, and both in open water and sea ice covered waters. The WMD showed an increasing trend, while the nautical area backscattering strength from the DSL showed a decreasing trend from south to north among the studied locations. Hydrographic observations revealed that the diel migration was found in the lower part of the north-flowing Atlantic Water, and was disconnected from the surface water masses above the Atlantic Water during day and night. The organisms conducting vertical migrations were studied by vertical and oblique hauls with zooplankton nets and pelagic trawls. These data suggest that these organisms were mainly various mesopelagic fishes, some few larger fishes, large zooplankton like krill and amphipods, and various gelatinous forms.

Biodiversity is well studied in ecology and the concept has been developed to include traits of species, rather than solely taxonomy, to better reflect the functional diversity of a system. The deep sea provides a natural environmental gradient within which to study changes in different diversity metrics, but traits of deep-sea fish are not widely known, hampering the application of functional diversity to this globally important system. We used morphological traits to determine the functional richness and functional divergence of demersal fish assemblages along the continental slope in the Northeast Atlantic, at depths of 300–2,000 m. We compared these metrics to size diversity based on individual body size and species richness. Functional richness and size diversity showed similar patterns, with the highest diversity at intermediate depths; functional divergence showed the opposite pattern, with the highest values at the shallowest and deepest parts of the study site. Species richness increased with depth. The functional implications of these patterns were deduced by examining depth-related changes in morphological traits and the dominance of feeding guilds as illustrated by stable isotope analyses. The patterns in diversity and the variation in certain morphological traits can potentially be explained by changes in the relative dominance of pelagic and benthic feeding guilds. All measures of diversity examined here suggest that the deep areas of the continental slope may be equally or more diverse than assemblages just beyond the continental shelf.

Foraging drives many fundamental aspects of ecology, and an understanding of foraging behavior aids in the conservation of threatened species by identifying critical habitats and spatial patterns relevant to management. The world’s largest ray, the oceanic manta ( Manta birostris ) is poorly studied and threatened globally by targeted fisheries and incidental capture. Very little information is available on the natural history, ecology and behavior of the species, complicating management efforts. This study provides the first data on the diving behavior of the species based on data returned from six tagged individuals, and an opportunistic observation from a submersible of a manta foraging at depth. Pop-off archival satellite tags deployed on mantas at the Revillagigedo Archipelago, Mexico recorded seasonal shifts in diving behavior, likely related to changes in the location and availability of zooplankton prey. Across seasons, mantas spent a large proportion of their time centered around the upper limit of the thermocline, where zooplankton often aggregate. Tag data reveal a gradual activity shift from surface waters to 100–150 m across the tagging period, possibly indicating a change in foraging behavior from targeting surface-associated zooplankton to vertical migrators. The depth ranges accessed by mantas in this study carry variable bycatch risks from different fishing gear types. Consequently, region-specific data on diving behavior can help inform local management strategies that reduce or mitigate bycatch of this vulnerable species. [ABSTRACT FROM AUTHOR]

Diel vertical migration (DVM) facilitates biogeochemical exchanges between shallow waters and the deep ocean. An effective way of monitoring the migrant biota is by acoustic observations although the interpretation of the scattering layers poses challenges. Here we combine results from acoustic observations at 18 and 38 kHz with limited net sampling in order to unveil the origin of acoustic phenomena around the Canary Islands, subtropical northeast Atlantic Ocean. Trawling data revealed a high diversity of fishes, decapods and cephalopods (152 species), although few dominant species likely were responsible for most of the sound scattering in the region. We identified four different acoustic scattering layers in the mesopelagic realm: (1) at 400–500 m depth, a swimbladder resonance phenomenon at 18 kHz produced by gas-bearing migrant fish such as Vinciguerria spp. and Lobianchia dofleini , (2) at 500–600 m depth, a dense 38 kHz layer resulting primarily from the gas-bearing and non-migrant fish Cyclothone braueri , and to a lesser extent, from fluid-like migrant fauna also inhabiting these depths, (3) between 600 and 800 m depth, a weak signal at both 18 and 38 kHz ascribed either to migrant fish or decapods, and (4) below 800 m depth, a weak non-migrant layer at 18 kHz which was not sampled. All the dielly migrating layers reached the epipelagic zone at night, with the shorter-range migrations moving at 4.6 ± 2.6 cm s − 1 and the long-range ones at 11.5 ± 3.8 cm s − 1 . This work reduces uncertainties interpreting standard frequencies in mesopelagic studies, while enhances the potential of acoustics for future research and monitoring of the deep pelagic fauna in the Canary Islands. [ABSTRACT FROM AUTHOR]

The distribution and density of Deep Scattering Layers (DSLs) were quantified along North Atlantic transits from Ireland to the Grand Banks of Newfoundland in the springs of 2012, 2013 and 2014 employing a calibrated Simrad EK60 echo sounder at 38 kHz. Concurrently, Sippican T5 XBTs (eXpendable Bathy Thermographs) were used to profile temperatures to 1800 m. In each year the scattering layers spanned the deep basin at depths ranging from near surface to approximately 900 m, but annual mean densities differed significantly. Higher DSL densities were recorded during years that exhibited higher sea temperatures at the depths of major DSL concentration (400–600 m), higher sea level anomalies and stronger eastward geostrophic currents. The highest concentration of the DSLs in each year was found in the area east of the Grand Banks that corresponded with areas of anticyclonic eddies. In this region DSL densities in 2014 were among the highest recorded worldwide (>7000 m 2 nautical mile – 2 ). Midwater fishing indicated DSLs were dominated by Myctophids and Sternoptychids. Anticyclonic eddy formation is discussed as a possible means of transport and aggregation of the DSLs in that region, where oceanographic influences may play a dominant role in the distribution and density of the DSLs and upper trophic level fishes. [ABSTRACT FROM AUTHOR]

Climate change-induced ocean deoxygenation is expected to exacerbate hypoxic conditions in mesopelagic waters off the coast of southern California, with potentially deleterious effects for the resident fauna. In order to understand the possible impacts that the oxygen minimum zone expansion will have on these animals, we investigated the response of the depth of the deep scattering layer (i.e., upper and lower boundaries) to natural variations in midwater oxygen concentrations, light levels, and temperature over time and space in the southern California Current Ecosystem. We found that the depth of the lower boundary of the deep scattering layer (DSL) is most strongly correlated with dissolved oxygen concentration, and irradiance and oxygen concentration are the key variables determining the upper boundary. Based on our correlations and published estimates of annual rates of change to irradiance level and hypoxic boundary, we estimated the corresponding annual rate of change of DSL depths. If past trends continue, the upper boundary is expected to shoal at a faster rate than the lower boundary, effectively widening the DSL under climate change scenarios. These results have important implications for the future of pelagic ecosystems, as a change to the distribution of mesopelagic animals could affect pelagic food webs as well as biogeochemical cycles. [ABSTRACT FROM AUTHOR]

Abstract: A 30-month time series of mean volume backscattering strength (MVBS) data obtained from moored acoustic Doppler current profilers (ADCPs) is used to analyze the evolution of vertically migrating scattering layers and their seasonal and annual variability in the Arabian and Oman Seas. Substantial diel vertical migration (DVM) is observed almost every day at all three mooring sites. Two daytime layers (Layers D1 and D2) and one nighttime layer (Layer E1) are typically present. The greatest biomass is observed near the surface during the night in Layer E1 and at depth between 250 and 450m during the daytime in Layer D2. All layers are deepest during the spring inter-monsoon and shallowest during the summer/fall southwest monsoon (SWM). Seasonal modulation of the D2 biomass change is evident in our high-resolution data. The lowest biomass in D2 is measured in the early summer (May or June) followed by a rapid biomass increase during the SWM (June–November) until the biomass reaches a maximum at the end of the SWM season. Short-period oscillations in D2 biomass are often seen with periods ranging from days to one month. Occasionally, a lower nighttime layer E2 is formed between 180 and 270m, mostly near the time of full moons. The upper daytime layer D1 is centered at 200m and densely concentrated. It is only formed during the winter northeast monsoon (NEM) and the spring inter-monsoon. The influence of physical processes on layer distribution is also investigated. Interestingly, the two daytime layers are found to be formed at the two boundaries of the Persian Gulf outflow water (PGW) and follow the seasonal depth change of the PGW. The timing of the DVM and the formation, persistence, decay and reformation of the deep scattering layers seem to be governed by light, both solar and lunar. The scattering strength, the layer depth and the layer thickness are likewise closely related to the Moon phase at night. Cloud coverage, the isotherm and the isohaline also appear to affect the distribution and depth of the scattering layers. The continuous multiple-year acoustic data from ADCPs allow us, for the first time, to study the seasonal and annual variations of scattering layers in this region. [Copyright &y& Elsevier]

Climate-driven range shifts are ongoing in pelagic marine environments, and ecosystems must respond to combined effects of altered species distributions and environmental drivers. Hypoxic oxygen minimum zones ( OMZs) in midwater environments are shoaling globally; this can affect distributions of species both geographically and vertically along with predator-prey dynamics. Humboldt (jumbo) squid ( Dosidicus gigas) are highly migratory predators adapted to hypoxic conditions that may be deleterious to their competitors and predators. Consequently, OMZ shoaling may preferentially facilitate foraging opportunities for Humboldt squid. With two separate modeling approaches using unique, long-term data based on in situ observations of predator, prey, and environmental variables, our analyses suggest that Humboldt squid are indirectly affected by OMZ shoaling through effects on a primary food source, myctophid fishes. Our results suggest that this indirect linkage between hypoxia and foraging is an important driver of the ongoing range expansion of Humboldt squid in the northeastern Pacific Ocean. [ABSTRACT FROM AUTHOR]

Abstract: Habitat utilization, behavior and food habits of the jumbo or Humboldt squid, Dosidicus gigas, were compared between an area recently inhabited in the northern California Current System (CCS) and a historically established area of residence in the Gulf of California (GOC). Low dissolved oxygen concentrations at midwater depths define the oxygen minimum zone (OMZ), an important environmental feature in both areas. We analyzed vertical diving behavior and diet of D. gigas and hydrographic properties of the water column to ascertain the extent to which squid utilized the OMZ in the two areas. The upper boundary of the OMZ has been shoaling in recent decades in the CCS, and this phenomenon has been proposed to vertically compress the pelagic environment inhabited by aerobic predators. A shoaling OMZ will also bring mesopelagic communities into a depth range with more illumination during daytime, making these organisms more vulnerable to predation by visual predators (i.e. jumbo squid). Because the OMZ in the GOC is considerably shallower than in the CCS, our study provides insight into the behavioral plasticity of jumbo squid and how they may respond to a shoaling OMZ in the CCS. We propose that shoaling OMZs are likely to be favorable to jumbo squid and could be a key indirect factor behind the recent range expansion of this highly migratory predator. [Copyright &y& Elsevier]

Abstract: Changes in the composition and biomass distribution of deep-living zooplankton over wide gradients of depth (400–2300m) and longitude (~180km) have been analyzed in the Balearic Basin (western Mediterranean), seeking the environmental variables responsible for these changes. Zooplankton tends to aggregate at different levels of the water column (forming Deep Scattering Layers, DSL) and in the Benthic Boundary Layer (BBL). Macrozooplankton biomass and composition were analyzed along a transect performed in July 2010 in midwater (between ~350 and 450m) and near the bottom (at ~5–200mab), over soundings of 450–2263m, including the top of Valencia Seamount (at ~40° 25′ N–02° 42′ E, 1076m). Zooplankton changed significantly in composition at the mesoscale (~180km) in both the DSL and the BBL. Siphonophores and calanoid copepods were the most dominant deep zooplankton taxa, calanoids reaching higher abundance in the BBL (1761–5177 individuals/1000m3) than in the DSL (1568–1743 individuals/1000m3). There was a significant increase in near-bottom zooplankton biomass over the middle slope, at 1000–1300m, linked to an increase in scyphozoans and siphonophores (Lensia spp. and Abylopsis tetragona) with peaks of 1.5–2.0gWW/1000m3. The peak of near-bottom zooplankton at 1000–1300m coincided with the lowest temperatures (13.08°C) and maximum O2 concentration (4.40ml/l) near the bottom and below 1000m with higher records in near-bottom turbidity. Gelatinous zooplankton are the main prey in the diet of the demersal fish Alepocephalus rostratus in the western Mediterranean, fish responsible for the peak of megafauna biomass reported at around 1200–1400m in the deep Mediterranean and at similar depths in other oceanic areas (e.g. the NW Atlantic). We suggest that deep-sea environmental conditions can govern peaks of near-bottom zooplankton, as well as influence the structure of the demersal fish community. [Copyright &y& Elsevier]

The deep scattering layers (DSL) in the central equatorial Pacific form an important prey resource in a relatively oligotrophic habitat. In March of 2006, we used a calibrated 38-kHz SIMRAD EK60 scientific sonar to assess the spatial distribution of the deep scattering layer relative to broad-scale oceanographic features and fine-scale physical and biological measurements. We conducted a single continuous transect from approximately 10°S to 20°N at 170°W while measuring acoustic backscatter, current velocity and direction, temperature, salinity, oxygen, and fluorescence with depth, coincident with marine mammal occurrence. These data were combined with remotely sensed sea surface height, chlorophyll, and sea surface temperature data to examine patterns in DSL distribution. To analyze DSL density with depth, acoustic backscatter was binned into surface (<200 m), mid (200-550 m) and deep (550-1000 m) layers. Backscatter was highly correlated with chlorophyll- a and low sea surface height anomalies and was greatest near the equator. We found high diel variability in DSL depth and scattering intensity between the mid and surface layers as well as a shallowing of the deep layer moving northward across the equator. Marine mammal sightings consisted primarily of odontocetes with their distribution coincident with higher acoustic densities of their forage base. Shifts in DSL distribution and scattering intensity are an important component towards understanding the behavior and distribution of highly migratory predator species. [ABSTRACT FROM AUTHOR]

Abstract: Twelve ocean sunfish, Mola mola, were tagged with pop-up satellite archival tags off the coast of Kamogawa, Japan during the spring of 2001 and 2003–2006. Transmitted data were obtained from seven tags. An eighth tag was recovered and provided a highly detailed four-month dataset from which several recognizable diving patterns emerged. This M. mola spent considerable time at the surface, possibly warming itself. These apparent basking events were punctuated by regular deep dives below the thermocline to depths as great as 600m and temperatures as low as 2°C. The vast majority of dives occurred during the day, with relatively little vertical movement at night. Geolocation estimates were possible between tag and release for five individuals. No large, basin-scale movements were apparent with most molas remaining relatively close to their initial tagging location over a six to nine month period. Seasonal movements were apparent for some molas and corresponded to regional shifts in oceanography. Northward movements from the Kuroshio Current into the Kuroshio–Oyashio transition zone and the Oyashio Current during the summer months coincided with an increase in temperature and reduction in chlorophyll a concentrations in waters near central Japan. In the fall, most molas returned inshore to the coast of Japan. Molas are targeted and incidentally captured by fishermen in these waters, and the information presented here is vital for effective fisheries management and to forecast changes in mola behaviors associated with environmental variability. [ABSTRACT FROM AUTHOR]