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Charrs (Salvelinus) reach their southernmost distribution in Japan, and are uniquely adapted to the short, steep streams of this island archipelago. Southern Asian Dolly Varden (Salvelinus curilus) occur only in Hokkaido Island, whereas white-spotted charr (Salvelinus leucomaenis) range to southern Honshu. Both species diverged from an ancestral lineage during the late Pliocene/early Pleistocene, when lowered sea levels created semi-enclosed water bodies in the seas of Japan and Okhotsk. Genetic analyses showed S. curilus represents the most ancient divergence from the Dolly Varden (Salvelinus malma) - Arctic charr (Salvelinus alpinus) group, and revealed five lineages of S. leucomaenis which align differently than traditional subspecies. Japanese charr display diverse and flexible life histories including anadromous fish with partial migration, and fluvial, adfluvial, and resident forms. In Hokkaido, Dolly Varden are distributed upstream and white-spotted charr downstream. They coexist in narrow sympatric zones through adaptive shifts by Dolly Varden in behavior and morphology that facilitate benthic foraging. Both species hybridize with native and nonnative salmonids, and are displaced from microhabitats and decline in abundance when rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) invade. Japan streams contain over 95,000 erosion control dams which create short stream fragments (medians ~200 m). This has increased extirpation of charr populations via lower genetic diversity and stochastic and demographic factors. Tributaries provide complex rearing habitats, afford refuges from floods, and supply recruits that sustain populations in mainstem fragments and create metapopulations in connected riverscapes. Charr play central roles in linked stream-riparian food webs, and cause direct and indirect effects that cascade to streambed algae and riparian predators when linkages are disrupted by anthropogenic effects or altered by native parasites. Many charr populations are threatened by habitat fragmentation and introgression or invasion by nonnative forms, but efforts to conserve charr are growing. These include restoring connectivity among pure populations above barriers that prevent invasions, protecting tributary nurseries, and instituting angling regulations to protect headwater populations. Key steps include inventorying pure populations, identifying conservation units, selecting appropriate management based on connectivity and biotic interactions, and engaging stakeholders and youth to engender an ethic for conserving irreplaceable charr lineages. [ABSTRACT FROM AUTHOR]

Due to rapid ocean warming at higher latitudes, northward habitat shifts of the dominant anadromous fish in the Northern Hemisphere, chum salmon (Oncorhynchus keta), have recently been reported. Physiological development is considered to be a trigger for habitat shifts during early life. However, the possibility of physiological adaptations to future ocean warming is not fully understood, because data on the physiological traits of juveniles, such as metabolic rate and swimming performance, are limited. Using swim tunnel respirometers, we identified the metabolic traits of different-sized chum salmon juveniles originating from the Sanriku coastal river, Japan, which is far from the northern feeding grounds. The weight gain of >2 g steeply increased the prolonged swimming performance, and the metabolic cost of swimming against the southward current in the Sanriku coast decreased with weight gain under the sea water temperature range of 8–12 °C. These findings indicate weight gain and inhabiting areas with temperatures < 13 °C might provide potential benefits for northward migration. These results will be valuable when assessing the increased costs associated with changing ocean conditions. [ABSTRACT FROM AUTHOR]

The Caspian kutum, Rutilus frisii, is an anadromous fish native to Iran and despite the development of its hatchery production, its larval development has been poorly described; hence describing its morphological growth pattern was the aim of this study. The Caspian kutum larval period in the early development was divided into three stages, namely eleuthero-embryo (0–5 dph), propterygio-larvae (5–13 dph), pterygio-larvae (13–20 dph) and after larval period early juvenile was initiated (> 20 dph). The eleuthero-embryo stage was characterized by the commencement of mix-feeding while the yolk-sac remains, while propterygio-larvae absorbed the yolk-sac and continued to develop feeding apparatus and fin-folds. In pterygio-larvae, the fins developed and squamation began to appear. During the eleuthero-embryo stage, allometry in the head, trunk and tail were nearly isometric, negative, and positive, respectively. In propterygio-larvae, the head and tail allometry were positive, while the trunk was still negative. By the pterygio-larval stage, the tail and trunk allometry were nearly isometric but the head was positive. After metamorphosis, the growth pattern changed as the head and trunk allometry became nearly isometric and positive, respectively, but the tail allometry remained almost isometric. The results confirmed that morphological development and growth patterns during early life stages in R. frisii closely match its immediate required function. [ABSTRACT FROM AUTHOR]

In recirculating aquaculture systems (RASs), ammonia excreted by fish must be converted to the less toxic nitrate before recirculation. Nitrifying microorganisms in biofilters used for this transformation can be sensitive to changes in salinity, which can present issues for systems that raise anadromous fish such as Atlantic salmon. Freshwater biofilters maintained at a low level of salinity (such as biofilters operated in coastal areas) may be better equipped to handle more drastic salinity shifts; therefore, experiments were performed on freshwater and low-salinity (3 ppt) biofilters to assess their ability to recover nitrification activity after an abrupt change in salinity (3, 20, and 33 ppt). Two-week tests showed full nitrification recovery in freshwater biofilters after a shift to 3 ppt but no ammonia oxidation in 20 or 33 ppt. Low-salinity-adapted filters (transitioned from 0 to 3 ppt) showed a small recovery (about 11%) after a shift to 20 ppt, and no activity when shifted to 33 ppt. Illumina sequencing revealed that, while nitrification was slowed or stopped with shifting salinities, the nitrifiers survived the salinity increases; conversely, the heterotrophic communities were more greatly affected and were reduced in proportion with increasing salinity. This work indicates that biofilters operated at low salinity may recover more quickly after large salinity changes, though this slight benefit may not outweigh the cost of low-level salinity maintenance. Further research into halotolerant heterotrophs in biofilms may increase the effectiveness of nitrifying biofilters under variable salinities. [ABSTRACT FROM AUTHOR]

Sea catfish (Genidens barbus) and whitemouth croaker (Micropogonias furnieri) were the most marine euryhaline species exploited by hunter-gatherers during the final Late Holocene along the coastal fringe of the eastern Pampa–Patagonia transition (Argentina). In the lower basin of the Colorado River, zooarchaeological evidence recovered from the San Antonio archaeological locality indicates prevalent fish consumption. However, no specific technology related to fishing has been recovered as part of the archaeological assemblages, hence strategies and techniques of fish capture remain unknown. The aim of this paper is to estimate the size of both species through archaeological otolith measurements to evaluate possible fish capture techniques. The analyzed sample is made up of 117 otoliths of whitemouth croaker and 863 of sea catfish. The estimated size of G. barbus individuals is between ca. 240 and 730 mm, which indicates the presence of juveniles and adults. In the case of M. furnieri, the estimated size range of individuals is ca. 350–750 mm, which indicates the representation of adult specimens. Based on the diversity of sizes represented in both species, their ethology, and the geomorphological characteristics of the study area, it is proposed that the fish could have been caught with either nets or bare hands through nonselective mass trapping strategies during the spring–summer season. [ABSTRACT FROM AUTHOR]

Antioxidants play a crucial role in maintaining health by scavenging free radicals that causes cellular damage and accelerates aging and disease. The present study focuses on the antioxidant levels in various organs of Hilsa fish, both male and female, with a weight range of 50 g to 780 g collected from the Hooghly River in the lower Gangetic plain. The study utilized commercially available free radicals such as, viz., 2,2'-azinobis-[3-ethyl-benzothiazoline-6-sulfonic acid] and ferric reducing antioxidant potential to measure the scavenging activity. The results indicated that female Hilsa fish possess higher levels of antioxidants (93.79 ± 0.26) compared to male counterparts (84.50 ± 0.65) in all organs, with levels increasing proportionally with weight. Interestingly, in males, the serum antioxidant profile was higher in the lower weight group (50-100 g), whereas, in females, the highest antioxidant activity was observed in the weight range of 300 g to 400 g. The present study was attempted for the first time to characterize the antioxidant distribution pattern in the different organs of Hilsa. These findings suggest that Hilsa fish organs, including serum, contain high levels of antioxidants that could significantly benefit human nutrition and potentially will contribute to anti-aging effects by incorporating Hilsa to their diet, which needs further clinical study. [ABSTRACT FROM AUTHOR]

Investigating the age at sea departure of returning Atlantic salmon is critical to better understand the role of emigration timing in survival. Among the methods available, the use of otolith chemistry can be challenging in anadromous fish, as the elements frequently used (i.e., magnesium, manganese, and zinc) can jointly reflect individual metabolism and environmental conditions. Here, we present a new unsupervised and robust method for successfully estimating the outmigration age of Atlantic salmon in the Bay of Biscay, based on otolith zinc concentrations. [ABSTRACT FROM AUTHOR]

Tenualosa ilisha is an anadromous fish from the Clupidae family that is distributed in several Asian countries. The relationship between T. ilisha from Indonesian waters and other waters in Asia is unknown, therefore this study was conducted to provide an overview of the relationship between T. ilisha from Indonesia and other waters in Asia based on the Cytochrome b gene (Cyt b) of mitochondrial DNA (mtDNA). The base sequence of T. ilisha from Labuhanbatu with a length of 568-572 base pairs (bp) was analyzed, then compared with Genbank data. All sequences compared showed that the composition of A+T (52.5%) was greater than G+C (47.5%). Genetic distance was calculated using Kimura two parameter (K2P); the lowest genetic distance of 0.000 was obtained between T. ilisha Labuhanbatu and Iraq with accession numbers LC619671.1; LC619673.1; LC619674.1. Meanwhile, the highest genetic distance is known to be 0.021 between T. ilisha Labuhanbatu and Bangladesh with the accession numbers MN748964.1, and MN748966.1. Neighbor-joining and maximum likelihood with 1000x repetition were the phylogenetic methods used to analyze the evolutionary relationships of T. ilisha. In general, both methods showed the same result that T. ilisha from Indonesia and other Asian waters are the same population. [ABSTRACT FROM AUTHOR]

Hydroelectric dams in the northeastern United States pose a challenge to the upstream spawning migrations of anadromous fishes, such as American Shad Alosa sapidissima, Alewife A. pseudoharengus, Blueback Herring A. aestivalis, Atlantic Salmon Salmo salar, and Sea Lamprey Petromyzon marinus. One response has been the construction and operation of fish lifts. A drawback of this solution is that fish lifts can be shut down. This study sought to determine how often fish lifts on major northeastern rivers (including lifts at the Cataract, Lockwood, and Milford dams in Maine; Holyoke Dam in Massachusetts; Scotland Dam in Connecticut; and Conowingo Dam in Maryland) shut down during their respective passage seasons and the causes for these shutdowns. We found that these lifts did not operate for 8–26% of their passage seasons, on average, with the mean season length lasting between 49 and 205 d. The causes for shutdowns were categorized as physical, mechanical, scheduling, other, or none given, with the primary cause varying among dams. Our results demonstrate an important weakness of fish lifts as a passage solution and, in turn, highlight the need for measures that limit disruptions to lifting where dam removal is not possible and strengthen the case for dam removal. [ABSTRACT FROM AUTHOR]

Restoration of movement corridors is a key management action used to address threats to migratory and other mobile species. Yet, we lack restoration effectiveness studies that allow for species to reestablish naturally (i.e. without supplementation) following habitat reconnection that capture all phases (dispersal, growth, and regulation) of recovery, and that takes an ecosystems approach. We investigated the natural recovery of migratory anadromous Coho salmon following habitat reconnection across a 5‐km section of Rock Creek, a forested tributary of the Cedar River, Washington, United States, 3 km upstream of Landsburg Dam. The dam blocked upstream fish movement for 102 years until the completion of a fish ladder in 2003. We also evaluated the response of non‐migratory trout, which are closely related to Coho salmon. Juvenile Coho salmon natal to the Cedar River dispersed into Rock Creek for rearing until spawning there in 2007. After restoration, juvenile Coho salmon density (fish/m2) increased 18‐fold, approaching an asymptote (i.e. regulation phase) a decade later. Coho salmon recovery in Rock Creek was spatially variable, however, slowing with distance from the site of restoration. Trout density was also higher after restoration relative to before, likely due to several mechanisms, including increased capacity resulting from the reestablishment of marine organic matter subsidies delivered by spawning anadromous fish. Our study demonstrates that migratory species can recover naturally after the restoration of habitat connectivity and associated movement corridors. Furthermore, our results suggest that such actions can also benefit nontarget species by reestablishing key ecosystem links driven by the target species. [ABSTRACT FROM AUTHOR]

Conservation planning requires understanding population sizes and trajectories. As with many imperiled species, green sturgeon (Acipenser medirostris) census data are limited, making it difficult to contextualize current threats and promote quantitative conservation goals. We combined spawning census, growth, and demographic data for the southern population of green sturgeon to build a population size and trajectory estimate. We generated a distribution of population size estimates and trajectories reflecting uncertainty from multiple sources. We then propagate these estimates through a demographic model to assess the potential impact of fishing bycatch. Our model suggests the population is below the recovery goal of 3000 adults. The most probable current total population estimate (including juveniles) is approximately 10 000 fish (5300–18 400 95% high density interval (HDI)), with 2400 adults (2197–2624 95% HDI). Simulated fishing bycatch pressure on the adults and subadults reduced abundance by a median value of 0.4% per year, which could be an impediment to recovery. Fisheries bycatch is one of many threats this population faces; this integrated framework may be used to assess how other threats may affect this population. [ABSTRACT FROM AUTHOR]

Striped Bass (Morone saxatilis) monitoring and research in California's Central Valley primarily occurs in the Sacramento River basin and the Sacramento-San Joaquin Delta. A relatively under-studied contingent of the Striped Bass population is present in the San Joaquin River and its tributaries. One such tributary, the Stanislaus River, is an important source of natural production of native anadromous fishes, including fall-run Chinook Salmon (Oncorhynchus tshawytscha), Steelhead (O. mykiss), and Pacific Lamprey (Entosphenus tridentatus). Because Striped Bass are a non-native piscivore, characterizing when and where their distribution overlaps with native fishes is a first step to assessing the potential for negative inter-specific interactions. We compiled incidental observations of Striped Bass made during long-term (1996 to 2021) salmonid monitoring programs performed at different times of the year, and found that Striped Bass were present at monitoring locations throughout most of the year, although not detected in every year. We also used data on Striped Bass captured during boat electrofishing surveys conducted during the 2019, 2020, and 2021 juvenile salmonid emigration season (February to June), for a more detailed evaluation of Striped Bass occupancy within the lower 65 kilometers of the Stanislaus River. Median fork length (FL) of Striped Bass captured from February to April was significantly larger than those captured in May and June (p < 0.001). Using dynamic occupancy models, we showed that Striped Bass larger than 300 mm FL were present, and their occupancy increases earlier in the year than individuals under 300 mm FL. In all 3 years, occupancy estimates were greater than 80% of the lower Stanislaus River by May. Our results suggest a high degree of spatial and temporal overlap with native migratory fishes, which may have important implications for understanding and managing how predation affects juvenile salmon and other native species. [ABSTRACT FROM AUTHOR]

Predation can play an important role in structuring ecological communities, and predator–prey dynamics can be altered following the introduction of new species. An unauthorized introduction of redside shiner (Richardsonius balteatus) into reservoirs in the Upper Skagit River, Washington, USA created concern that a consequent shift in predator–prey dynamics in the reservoirs could reduce recruitment and production of native salmonids in the basin. We estimated predation mortality in Ross Lake on nonnative redside shiner and juvenile native salmonids to evaluate the potential role of predation in regulating these populations and limiting survival of native species of concern. We used bioenergetics modelling and stable isotope analysis combined with directed field measurements of growth, seasonal diet and thermal experience of piscivorous salmonids to quantify their consumption demand on prey fishes to evaluate the relative magnitude of predation mortality on invasive redside shiners and native salmonids. While redside shiner are the dominant prey fish species in Ross Lake, the modest biomass of native salmonids consumed could translate into substantial mortality, the magnitude of which depended on the timing and size at which prey fishes were eaten. This information provides important context for how nonnative species may indirectly impact native species through shared predation (apparent competition) and can inform conservation decisions surrounding nonnative species control, sustainability of native salmonids and introductions of anadromous fishes. [ABSTRACT FROM AUTHOR]

Anthropogenic barriers are widely known to negatively impact the spawning migrations of anadromous fishes, by delaying or preventing passage upstream. Although the impacts of barriers on emigrating post‐spawned adults are less well studied, they could potentially impact the fitness and subsequent return rates of iteroparous species. In this study, passive acoustic telemetry was used to track the emigrations of 53 twaite shad Alosa fallax in the River Severn basin in their first spawning migration a year after being tagged, giving insights into their emigration movements and the impacts of anthropogenic weirs on these movements. A. fallax began their emigrations after spending varying amounts of time and migrating various distances within the river, with late‐emigrating individuals moving fastest and most directly. Emigrations became faster and more direct the further downstream individuals were from their furthest upstream extent. Downstream passage delays at weirs increased emigration times by a median of 61%, with environmental conditions (i.e., temperature, flow, and tidal influence on river level) having little influence on downstream passage at weirs with no modifications to facilitate fish passage. As weir‐induced emigration delays are suggested to deplete energy reserves (when energy levels are already depleted post‐spawning), limit spawning opportunities (by preventing access to downstream spawning habitat), and expose individuals to increased predation risk and suboptimal conditions (e.g., high temperatures), these delays can potentially diminish the benefits of iteroparity. The evidence presented here suggests that more consideration should be given to the potential impacts of anthropogenic barriers on the emigrations of iteroparous species when assessing river connectivity or undertaking barrier mitigation. [ABSTRACT FROM AUTHOR]

American shad (Alosa sapidissima) are an anadromous fish species native to North America that have an extensive range, but their populations are declining. Acoustic telemetry can play a vital role in better understanding the behavior and survival of this sensitive species, but successfully handling and tagging juvenile American shad can be challenging. We conducted several experiments to determine the best methods for collecting, transporting, holding, and tagging juvenile shad. Minimizing out-of-water handling and the use of a saltwater treatment during collection increased 24 h survival from 78 to 99% after transport. Saltwater was also fundamental in keeping tagged shad alive overnight. Shad as small as 50 mm, were implanted with a dummy acoustic transmitter using a pectoral incision method with no suture. In a 60 d holding evaluation, the tagged fish survived at a rate comparable to their non-tagged counterparts (81.5% for tagged, 70% for untagged). Tagged and untagged shad also had similar survival when exposed to a tank of predators. The results are important for improving conservation efforts for small, sensitive species of fish, like American shad. [ABSTRACT FROM AUTHOR]

Despite decades of restoration efforts in the U.S. Columbia River Basin (CRB), recovery of anadromous fish remains elusive. Regional stakeholder conflicts about objectives for fish restoration complicate efforts to collaboratively develop successful governance systems. While ecological research and solutions have been primary considerations for restoration initiatives, much of the literature recognizes social complexities as the root of the problem. This research employs a qualitative approach that illuminates how sociopolitical histories and lifeways underpin stakeholders' divergent objectives that challenge the formation of good governance in the CRB. We show how sociopolitical histories and lifeways reproduce a lack of representation of tribal members in leadership which inhibits the formation of good governance. This has significant implications for the process and outcomes of participatory governance and how governance processes can meaningfully integrate diverse voices in an intentional way to facilitate a just and equitable outcome for humans and ecosystems. [ABSTRACT FROM AUTHOR]

The field of fish microbiome research has rapidly been advancing, primarily focusing on farmed or laboratory fish species rather than natural or marine fish populations. This study sought to reveal the distinctive gut bacteriome composition and diversity within the anadromous fish species Tenualosa ilisha (hilsa), which holds the status of being the national fish of Bangladesh. We conducted an analysis on 15 gut samples obtained from 15 individual hilsa fishes collected from three primary habitats (e.g., freshwater = 5, brackish water = 5 and marine water = 5) in Bangladesh. The analysis utilized metagenomics based on 16S rRNA gene sequencing targeting the V3-V4 regions. Our comprehensive identification revealed a total of 258 operational taxonomic units (OTUs). The observed OTUs were represented by six phyla, nine classes, 19 orders, 26 families and 40 genera of bacteria. Our analysis unveiled considerable taxonomic differences among the habitats (freshwater, brackish water, and marine water) of hilsa fishes, as denoted by a higher level of shared microbiota (p = 0.007, Kruskal-Wallis test). Among the identified genera in the gut of hilsa fishes, including Vagococcus, Morganella, Enterobacter, Plesiomonas, Shigella, Clostridium, Klebsiella, Serratia, Aeromonas, Macrococcus, Staphylococcus, Proteus, and Hafnia, several are recognized as fish probiotics. Importantly, some bacterial genera such as Sinobaca, Synechococcus, Gemmata, Serinicoccus, Saccharopolyspora, and Paulinella identified in the gut of hilsa identified in this study have not been reported in any aquatic or marine fish species. Significantly, we observed that 67.50% (27/40) of bacterial genera were found to be common among hilsa fishes across all three habitats. Our findings offer compelling evidence for the presence of both exclusive and communal bacteriomes within the gut of hilsa fishes, exhibiting potential probiotic properties. These observations could be crucial for guiding future microbiome investigations in this economically significant fish species. [ABSTRACT FROM AUTHOR]

Diadromous fishes migrate between marine and fresh waters for reproduction. For anadromous species, which spawn in freshwater, improved access to freshwater spawning and nursery habitats and ability of juveniles to emigrate to the ocean may support population recovery. Despite the potentially enormous influence of early life stage survival on adult population size, managers and scientists have limited capacity to assess numbers of juvenile anadromous fishes leaving freshwater ecosystems. Such data are critical for evaluating reproductive success and habitat suitability and have been identified as a top priority in anadromous fish research and management. We developed a state‐of‐the‐art underwater video and computational system to collect videos to estimate abundances and migration timing for juvenile river herring (Alosa pseudoharengus; Alosa aestivalis). We collected continuous video in the Monument River (Bourne, Massachusetts, USA) from June to November 2017. We trained three types of neural network models to detect and count fish in video frames and evaluated model performance by comparing human counts to model outputs. Our top model assessed presence and absence (F1 = 87%) and counted fish (counting error 9.4%) with an accuracy comparable to human counters (F1 = 88%). Our system's capability to collect accurate counts of emigrating juveniles will provide critical information that could be related to the numbers of spawning adults, system‐specific productivity, and spawning and nursery habitat suitability. Both the video collection system and computational model may be transferrable to other sites and for other species where tracking juvenile emigration may inform management efforts. [ABSTRACT FROM AUTHOR]

Introducing anadromous fish upstream of migration barriers has frequently been proposed as a conservation strategy, but existing conditions and future changes to the ecosystems above barriers such as invasive species, climate change, and varying water operations influence the capacity to support such introductions. In the Upper Skagit River, Washington, USA, introduction of anadromous salmonids above three high-head dams was proposed; however, the proliferation of invasive redside shiner Richardsonius balteatus fundamentally altered reservoir food web interactions, presenting potential challenges for the growth and production of introduced anadromous salmonids. By combining empirical measurements of zooplankton availability and temporal patterns in thermal structure of the reservoir with bioenergetics model simulations to quantify the rearing capacity of Ross Lake, we estimated the lake could support millions of sockeye salmon fry entering in spring after accounting for temporal consumption demand by the existing planktivore community dominated by redside shiner. The initial fry estimates varied according to the expected fry-to-smolt survival rate, and whether salmonids would be thermally restricted from prey in the epilimnion. This translated to estimates of 189,000 to 285,000 smolts leaving the following spring, and 7700 to 11,700 returning adults, using mean fry-to-smolt and smolt-to-adult survival rates from a nearby sockeye salmon population. We also estimated that predation potential could pose substantial mortality for lake-rearing sockeye or Chinook salmon, although it is expected to play a lesser role in limiting survival of species that only migrate through the reservoir. These results provide a case study and framework for examining bottom-up and top-down food web processes that influence growth and survival of introduced anadromous salmonids in reservoir habitats, thus guiding the direction of future feasibility studies in Ross Lake and other regulated rivers where introduction programs are considered. [ABSTRACT FROM AUTHOR]

Diadromous fish have exhibited a dramatic decline since the end of the 20th century. The allis shad (Alosa alosa) population in the Gironde‐Garonne‐Dordogne (GGD) system, once considered as a reference in Europe, remains low despite a fishing ban in 2008. One hypothesis to explain this decline is that the downstream migration and growth dynamics of young stages have changed due to environmental modifications in the rivers and estuary. We retrospectively analysed juvenile growth and migration patterns using otoliths from adults caught in the GGD system 30 years apart during their spawning migration, in 1987 and 2016. We coupled otolith daily growth increments and laser ablation inductively‐coupled plasma mass spectrometry measurements of Sr:Ca, Ba:Ca, and Mn:Ca ratios along the longest growth axis from hatching to an age of 100 days (i.e., during the juvenile stage). A back‐calculation allowed us to estimate the size of juveniles at the entrance into the brackish estuary. Based on the geochemistry data, we distinguished four different zones that juveniles encountered during their downstream migration: freshwater, fluvial estuary, brackish estuary, and lower estuary. We identified three migration patterns during the first 100 days of their life: (a) Individuals that reached the lower estuary zone, (b) individuals that reached the brackish estuary zone, and (c) individuals that reached the fluvial estuary zone. On average, juveniles from the 1987 subsample stayed slightly longer in freshwater than juveniles from the 2016 subsample. In addition, juveniles from the 2016 subsample entered the brackish estuary at a smaller size. This result suggests that juveniles from the 2016 subsample might have encountered more difficult conditions during their downstream migration, which we attribute to a longer exposure to the turbid maximum zone. This assumption is supported by the microchemical analyses of the otoliths, which suggests based on wider Mn:Ca peaks that juveniles in 2010s experienced a longer period of physiological stress during their downstream migration than juveniles in 1980s. Finally, juveniles from the 2016 subsample took longer than 100 days to exit the lower estuary than we would have expected from previous studies. Adding a new marker (i.e., Ba:Ca) helped us refine the interpretation of the downstream migration for each individual. [ABSTRACT FROM AUTHOR]

Removing small legacy dams can help improve habitat connectivity along rivers, re-establish the flow of sediments and nutrients, support anadromous fish populations, and reduce the risk of flooding to upstream and downstream communities. However, decisions around dams are often controversial and can lead to lengthy and contested debates. Given the growing number of dam removal cases that have failed or been delayed due to community resistance, new methods for involving communities in the decision-making process around dams are necessary. Yet there is limited research and few examples to guide efforts to create quality public engagement that might contribute to a better decision-making process around dams. This article presents a structured and collaborative approach to involving the public in the decision-making process around small legacy dams. We take a transdisciplinary approach, bringing together public engagement tools from the design and planning disciplines with Structured Decision Making. We tested these methods in a public workshop in Keene, New Hampshire, using a pre- and post-survey. This article contributes to existing discourses concerning the role of the public in ecological restoration and environmental management decisions and aims to provide a framework that can help support decision making at other complex dam sites. [ABSTRACT FROM AUTHOR]

Modern fisheries management strives to balance opposing goals of protection for weak stocks and opportunity for harvesting healthy stocks. Test fisheries can aid management of anadromous fishes if they can forecast the strength and timing of an annual run with adequate time to allow fisheries planning. Integration of genetic stock identification (GSI) can further maximize utility of test fisheries by resolving run forecasts into weak‐ and healthy‐stock subcomponents. Using 5 years (2017–2022) of test fishery data, our study evaluated accuracy, resolution, and lead time of predictions for stock‐specific run timing and abundance of Columbia River spring Chinook salmon (Oncorhynchus tshawytscha). We determined if this test fishery (1) could use visual stock identification (VSI) to forecast at the coarse stock resolution (i.e., classification of "lower" vs. "upriver" stocks) upon which current management is based and (2) could be enhanced with GSI to forecast at higher stock resolution. VSI accurately identified coarse stocks (83.3% GSI concordance), and estimated a proxy for abundance (catch per unit effort, CPUE) of the upriver stock in the test fishery that was correlated (R2 = 0.90) with spring Chinook salmon abundance at Bonneville dam (Rkm 235). Salmon travel rates (~8.6 Rkm/day) provided predictions with 2‐week lead time prior to dam passage. Importantly, GSI resolved this predictive ability as finely as the hatchery broodstock level. Lower river stock CPUE in the test fishery was correlated with abundance at Willamette Falls (Rkm 196, R2 = 0.62), but could not be as finely resolved as achieved for upriver stocks. We described steps to combine VSI and GSI to provide timely in‐season information and with prediction accuracy of ~12.4 mean absolute percentage error and high stock resolution to help plan Columbia River mainstem fisheries. [ABSTRACT FROM AUTHOR]

The longjaw tapertail anchovy Coilia nasus, which migrates from ocean to freshwater for spawning in spring, is an important anadromous fish with ecological and cultural significance. To determine parasite infection in anadromous C. nasus, a total of 103 fish from the Yangtze River were collected and examined in 2021 and 2022. The overall infection prevalence of nematodes in C. nasus was 100%, with a mean intensity of 13.81 ± 16.45. The mean intensity of nematode infections in 2022 was significantly higher than that observed in 2021 across all sampling sites (p < 0.05). Nematodes were widely detected in the mesentery, pyloric cecum, stomach, and liver, among which the mesentery accounted for the highest proportion, reaching up to 53.52%. A total of eight ascaridoid nematodes belonging to the family Anisakidae and Raphidascarididae were identified by using morphological characters and molecular biological techniques, including two species of Anisakis, five species of Hysterothylacium, and one species of Raphidascaris. A. pegreffii was found as the predominant species, accounting for 48.65% of all identified parasitic nematodes in liver, while Raphidascaris sp. was the most common nematode in the mesentery, pyloric cecum, and stomach, reaching up to 39.81%, 36.21%, and 74.36%, respectively. The present study systematically investigated the parasitic status and community structure of the nematode in C. nasus during its migration in the Yangtze River. This research provides a foundation for studying the impact of nematode parasitism on the reproductive migration and population recruitment of C. nasus, and offers valuable insights for biomarker screening and nematode identification in C. nasus. [ABSTRACT FROM AUTHOR]

Background: The ice-free season (typically late-June to early-October) is crucial for anadromous species of fish in the Arctic, including Arctic Char (Salvelinus alpinus), which must acquire adequate resources for growth, reproduction, and survival during a brief period of feeding in the marine environment. Arctic Char is an important food fish for Inuit communities across the Arctic. Understanding drivers and patterns of migration in the marine environment is thus essential for conservation and management of the species. Methods: We used passive acoustic telemetry to characterize migration patterns of 51 individual anadromous Arctic Char during the ice-free season in the marine environment of Coronation Gulf (Nunavut, Canada; 2019–2022). Based on recent genetic evidence, some tagged individuals were likely Dolly Varden (Salvelinus malma malma), a closely related species to Arctic Char. Using local Getis G* and network analysis, we described movement patterns and identified high-use locations in the marine environment. We also related freshwater overwintering location to migration timing and movement pattern. Results: Comparing groups of fish that overwintered in distinct locations, we found: (i) limited evidence that marine movements were associated with overwintering location; (ii) minor differences in use of marine space; and, (iii) timing of freshwater return differed significantly between overwintering groups, and was related to length and difficulty of the migratory pathway in freshwater. Results from both network analysis and local Getis G* revealed that, regardless of overwintering location, coastal locations were highly used by fish. Conclusions: Overwintering locations, and the migratory routes to access overwintering locations, affect the timing of freshwater return. Preference of fish for coastal marine locations is likely due to abundance of forage and patterns in break-up of sea ice. Similarities in marine space use and movement patterns present challenges for managing this and other mixed stock fisheries of anadromous Salvelinus spp. Absences or periods of time when fish were not detected prevented comprehensive assessment of movement patterns. Local Getis G*, a local indicator of spatial association, is a helpful tool in identifying locations associated with absences in acoustic telemetry arrays, and is a complementary method to network analysis. [ABSTRACT FROM AUTHOR]

In undammed watersheds in Mediterranean climates, the timing and abruptness of the transition from the dry season to the wet season have major implications for aquatic ecosystems. Of particular concern in many coastal areas is whether this transition can provide sufficient flows at the right time to allow passage for spawning anadromous fish, which is determined by dry season baseflow rates and the timing of the onset of the rainy season. In (semi-) ephemeral watershed systems, these functional flows also dictate the timing of full reconnection of the stream system. In this study, we propose methods to predict, approximately 5 months in advance, two key hydrologic metrics in the undammed rural Scott River watershed in northern California. The two metrics are intended to characterize (1) the severity of a dry year and (2) the relative timing of the transition from the dry to the wet season. The ability to predict these metrics in advance could support seasonal adaptive management. The first metric is the minimum 30 d dry season baseflow volume, Vmin , which occurs at the end of the dry season (September–October) in this Mediterranean climate. The second metric is the cumulative precipitation, starting 1 September, necessary to bring the watershed to a "full" or "spilling" condition (i.e., initiate the onset of wet season storm- or baseflows) after the end of the dry season, referred to here as Pspill. As potential predictors of these two metrics, we assess maximum snowpack, cumulative precipitation, the timing of the snowpack and precipitation, spring groundwater levels, spring river flows, reference evapotranspiration, and a subset of these metrics from the previous water year. Though many of these predictors are correlated with the two metrics of interest, we find that the best prediction for both metrics is a linear combination of the maximum snowpack water content and total October–April precipitation. These two linear models could reproduce historical values of Vmin and Pspill with an average model error (RMSE) of 1.4 Mm 3 per 30 d (19.4 cfs) and 25.4 mm (1 in.), corresponding to 49 % and 37 % of mean observed values, respectively. Although these predictive indices could be used by governance entities to support local water management, careful consideration of baseline conditions used as a basis for prediction is necessary. [ABSTRACT FROM AUTHOR]

Objective: The Pacific Lamprey Entosphenus tridentatus is an anadromous fish that provides many ecosystem services and is important to Native American tribes, which harvest lamprey for cultural uses. The largest harvest of adult Pacific Lamprey in North America occurs at Willamette Falls (WF) in Oregon, USA. Lamprey have experienced conservation problems for decades, and harvest numbers at WF have plummeted approximately 33‐fold from a maximum of over 500,000 during 1946 to an average of about 16,000 per year during 1969–1999. Recent harvest has dropped to less than 6,000 lamprey/year during 2000–2021. However, existing conservation plans do not provide a historical perspective on the lamprey harvest at WF and the cumulative threats in the Willamette River basin that likely caused lamprey harvest reductions. Land development, river impoundment, and water pollution coincided with significant decreases in lamprey harvest at WF. These (and other) threats have resulted in reduced habitat for larval lamprey in the basin and, thus, a reduction in the pheromones they release, which attract adults. Conservation plans do not identify specific, basinwide goals to increase the number of lamprey in general and to provide more lamprey for harvest in particular. Methods: Therefore, in this review paper, we provide a conservation perspective on three topics to increase and monitor lamprey abundance at WF. Result: We (1) provide an up‐to‐date history of the lamprey harvest and its regulation at WF; (2) recommend addressing the lack of basinwide, lamprey‐specific passage and habitat restoration to improve the quantity and quality of lamprey spawning and rearing habitats; and (3) identify research needs for monitoring the lamprey population(s) at WF. Conclusion: We conclude that lamprey‐specific passage improvements and habitat restoration throughout the Willamette River basin above WF are necessary to bolster abundance. Impact statementWe provide an up‐to‐date history of Pacific Lamprey harvest at Willamette Falls, Oregon. The species is ecologically and culturally important. Conservation‐focused perspectives could be used in basinwide plans to increase and monitor lamprey abundance. [ABSTRACT FROM AUTHOR]

Hilsa shad (Tenualosa ilisha, Hamilton, 1822), the highly coveted table fish within the Indian subcontinent, is Bangladesh's most significant single-species fishery. To assess the risk that toxic metals pose to human health, certain health risk indices—estimated daily intake (EDI), target hazard quotient (THQ), total target hazard quotient (TTHQ), and target cancer risk (TR)—were calculated. The hierarchy of toxic metals (µg/g-ww) in Hilsa shad of the bay showed as Zn (13.64 ± 2.18) > Fe (9.25 ± 1.47) > Mn (2.98 ± 0.75) > Cu (0.57 ± 0.18) > Cr (0.23 ± 0.06) > Pb (0.22 ± 0.04) > As (0.08 ± 0.02) > Ni (0.06 ± 0.02) > Co (0.04 ± 0.01) > Cd (0.01 ± 0.003) in the wet season and Zn (11.45 ± 1.97) > Fe (10.51 ± 1.38) > Mn (3.80 ± 0.75) > Cu (0.73 ± 0.17) > Pb (0.30 ± 0.03) > Cr (0.20 ± 0.05) > As (0.09 ± 0.01) > Ni (0.08 ± 0.02) > Co (0.07 ± 0.02) > Cd (0.02 ± 0.004) in the dry season. The EDI of all the examined trace metals indicated no risk to human health from consuming Hilsa fish. The estimation of THQ and TTHQ suggested that the ingestion of both individual and combined trace metals through Hilsa shad consumption was safe from the perspective of human health. Also, there was no evidence of carcinogenic risk for consumers based on the evaluation of the TR value of metals (As, Pb, Cd, and Ni) due to Hilsa shad consumption. [ABSTRACT FROM AUTHOR]

The river shad, hilsa (Tenualosa ilisha), is an anadromous fish that migrates from marine to freshwater for spawning. This transition/migration poses severe osmotic stress that hilsa needs to successfully minimize. The present study was conducted to evaluate the cellular (ultrastructure of gill and kidney), physiological (hemocyte counts, blood, and water osmolality), and biochemical (free amino acids, free fatty acids, blood glucose, and cortisol levels) parameters of hilsa collected from four different environmental salinity levels (0‰, 10‰, 20‰, and 30‰ salinity levels; using 10 fish samples from each salinity). Results indicate that increased amount of fatty/mucus cells in the gill and lower number of glomerular capsules in kidney were observed at low (0‰ and 10‰) salinities compared to high (20‰ and 30‰) salinities. Water and blood osmolality (total ionic content) showed declining trends with lowering salinities. Total number of blood cells was also found to vary significantly P < 0.05 among salinities. Salinity-specific blood cortisol and glucose levels of hilsa were observed as 0‰ > 10‰ > 30‰ > 20‰. Total essential and free amino acids of hilsa blood showed significantly declining trends P < 0.05 with salinity reductions. No significant differences were observed between 10‰, 20‰, and 30‰ salinities for different types of fatty acids, while hilsa collected from 0‰ showed significantly higher P < 0.05 levels of fatty acids compared to the remaining three salinities. Findings indicate that hilsa rapidly alters cellular, physiological, and biochemical traits for successful transition between different salinity habitats. [ABSTRACT FROM AUTHOR]

Species detection using eDNA is revolutionizing global capacity to monitor biodiversity. However, the lack of regional, vouchered, genomic sequence information—especially sequence information that includes intraspecific variation—creates a bottleneck for management agencies wanting to harness the complete power of eDNA to monitor taxa and implement eDNA analyses. eDNA studies depend upon regional databases of mitogenomic sequence information to evaluate the effectiveness of such data to detect and identify taxa. We created the Oregon Biodiversity Genome Project to create a database of complete, nearly error‐free mitogenomic sequences for all of Oregon's fishes. We have successfully assembled the complete mitogenomes of 313 specimens of freshwater, anadromous and estuarine fishes representing 24 families, 55 genera and 129 species and lineages. Comparative analyses of these sequences illustrate that many regions of the mitogenome are taxonomically informative, that the short (~150 bp) mitochondrial 'barcode' regions typically used for eDNA assays do not consistently diagnose for species and that complete single or multiple genes of the mitogenome are preferable for identifying Oregon's fishes. This project provides a blueprint for other researchers to follow as they build regional databases, illustrates the taxonomic value and limits of complete mitogenomic sequences and offers clues as to how current eDNA assays and environmental genomics methods of the future can best leverage this information. [ABSTRACT FROM AUTHOR]

Many Norwegian rivers and lakes are regulated for hydropower, which affects freshwater ecosystems and anadromous fish species, such as sea trout (Salmo trutta). Lakes are an important feature of many anadromous river systems. However, there is limited knowledge on the importance of lakes as habitat for sea trout and how hydropower affects the behaviour of sea trout in lakes. To investigate this, we conducted an acoustic telemetry study. A total of 31 adult sea trout (532 ± 93 mm total length) were captured by angling in river Aurlandselva, Norway, and tagged between July 20 and August 12, 2021. The tags were instrumented with accelerometer, temperature, and depth sensors, which provided information on the sea trout's presence and behaviour in lake Vassbygdevatnet. Our results indicate that there was a large prevalence of sea trout in the lake during the spawning migration, and that the sea trout were less active in the lake compared to the riverine habitats. An increase in activity of sea trout in the lake during autumn might indicate that sea trout spawn in the lake. However, the discharge from the high-head storage plant into the lake did not affect the depth use or activity of sea trout in the lake. Furthermore, the large prevalence of spawners in the lake during autumn will likely cause an underestimation of the size of the sea trout population in rivers with lakes during annual stock assessment. In conclusion, our results could not find evidence of a large impact of the discharge on the behaviour of sea trout in the lake. [ABSTRACT FROM AUTHOR]

The rapid warming of the Northern hemisphere has especially challenged the evolvability of anadromous fish species, such as Atlantic salmon (Salmo salar), which must cope with drastically different environments depending on their life-history stage. We studied the long-term trends in, and the effects of environmental factors and life-history traits on, Atlantic salmon growth rates in both freshwater and in the ocean using c. 35,000 scale samples collected across 48 years from spawners returning to three tributaries of the subarctic River Teno in the northernmost parts of Finland and Norway (70° N). The freshwater growth has decreased in all three populations and spending more than three juvenile years in freshwater before the sea migration comes at the expense of growth. On the other hand, returning mature salmon (one-sea-winter, 1SW) showed increased growth at the sea with increasing marine temperatures, which results in larger sizes at return in 1SW spawners. We did not observe such trends in growth rates in larger, two-sea-winter salmon. Here, we report the contrasting responses in Atlantic salmon growth rates to a warming climate depending on the life-history stage. [ABSTRACT FROM AUTHOR]

Aims The bacterial communities associated with the gastrointestinal (GI) tract are primarily involved in digestion, physiology, and the immune response against pathogenic bacteria for the overall development and health of the host. Hilsa shad (Tenualosa ilisha), a tropical anadromous fish, found predominantly in Bangladesh and India, has so far been poorly investigated for its gut bacterial communities. In this study, both culture-based and metagenomic approaches were used to detect intestinal isolates of hilsa, captured from both freshwater and seawater to investigate the community structure of intestinal microbiota. Methods and results Culture-dependent approach allowed to isolate a total of 23 distinct bacterial species comprising 16 Gram-negative, and 7 Gram-positive isolates, where Proteobacteria and Firmicutes were identified as the two most dominant phyla. While metagenomic approach explored a wide range of important GI bacteria, primarily dominated by Proteobacteria, Firmicutes, and Bacteroidetes, with Proteobacteria and Firmicutes, being the most abundant in freshwater and seawater samples, respectively. Conclusions A combination of these approaches provided the differential GI-associated bacterial diversity in freshwater and seawater hilsa with the prediction of overall functional potential. Impact Statement The study explored the diversity of gut microbiota in hilsa, one of the most preferred nutritious dietary fish, captured from freshwater and seawater habitats, which may encourage to comprehend the composition of the gut microbiome in relation to the migratory behavior and polyunsaturated fatty acid profile of anadromous fish in general. [ABSTRACT FROM AUTHOR]

The broad scale distribution of population‐specific genetic diversity (GDP) across taxa remains understudied relative to species diversity gradients, despite its relevance for systematic conservation planning. We used nuclear DNA data collected from 3678 vertebrate populations across the Americas to assess the role of environmental and spatial variables in structuring the distribution of GDP, a key component of adaptive potential in the face of environmental change. We specifically assessed non‐linear trends for a metric of GDP, expected heterozygosity (HE), and found more evidence for spatial hotspots and cold spots in HE rather than a strict pattern with latitude. We also detected inconsistent relationships between HE and environmental variables, where only 11 of 30 environmental comparisons among taxa groups were statistically significant at the.05 level, and the shape of significant trends differed substantially across vertebrate groups. Only one of six taxonomic groups, freshwater fishes, consistently showed significant relationships between HE and most (four of five) environmental variables. The remaining groups had statistically significant relationships for either two (amphibians, reptiles), one (birds, mammals), or no variables (anadromous fishes). Our study highlights gaps in the theoretical foundation upon which macrogenetic predictions have been made thus far in the literature, as well as the nuances for assessing broad patterns in GDP among vertebrate groups. Overall, our results suggest a disconnect between patterns of species and genetic diversity, and underscores that large‐scale factors affecting genetic diversity may not be the same factors as those shaping taxonomic diversity. Thus, careful spatial and taxonomic‐specific considerations are needed for applying macrogenetics to conservation planning. [ABSTRACT FROM AUTHOR]

The Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) is a anadromous fish found along the east coast of North America and is listed under the Endangered Species Act of the United States. Decades of overfishing and habitat degradation have caused range-wide population declines, and the population status of this species in many rivers is unclear. Quantifying annual recruitment is one way to assess the status of sturgeon populations, but such assessments are few. The objective of this study was to quantify recruitment of juvenile Atlantic sturgeon in the Savannah, Ogeechee, and Satilla Rivers in Georgia. Because we used identical methods in 3 rivers simultaneously, we were able to directly compare recruitment among these populations with that of other populations observed in contemporaneous studies in other rivers. We conducted mark-recapture sampling during 2014-2017 and used Huggins closed population models to estimate annual recruitment. Because there are no historical data for comparison, we evaluated the 3 populations in terms of recruit abundance, variation, and consistency. The Savannah River annually had relatively large numbers of recruits (639-937 individuals). Fewer recruits were produced in the Ogeechee (27-57 individuals) and Satilla (51-134 individuals) Rivers; recruitment occurred in only some years in those rivers. Populations that produce few recruits or do not have regular recruitment may be experiencing bottlenecks preventing recovery. [ABSTRACT FROM AUTHOR]

This work aims to present an historical review of the ecosystem services provided by anadromous fish (i.e., species that migrate from the sea to the river to spawn) throughout Human time, as well as of the main related threats, focusing on the Iberian Peninsula region. Anadromous fish provide important provision, cultural, regulatory and supporting ecosystem services across their distribution range and have been extensively exploited by humans since prehistoric times. In the Iberian Peninsula, sea lamprey, allis and twaite shads, sea trout, Atlantic salmon and European sturgeon were once abundantly present in several river basins covering what is now Portuguese and Spanish territory. These species have suffered a severe decline across their distribution range, mainly due to habitat loss and overexploitation. Considered regal delicacies, these fishes were once a statement on the tables of the highest social classes, a much appreciated bounty for the poorer population and are still an important part of the local gastronomy and economy. Such high economic and cultural interest encouraged intensive fishing. Currently, management efforts are being implemented, pairing habitat rehabilitation (e.g., construction of fish passes in obstacles to migration such as weirs and dams) with sustainable fisheries. Considering the present climate change scenario, these species are bound to endure increased pressures, demanding novel management approaches to ensure population numbers that are able to secure their sustainable exploitation. [ABSTRACT FROM AUTHOR]

Predicting and mitigating the impact of anthropogenic barriers on migratory fish requires an understanding of the individual and environmental factors that influence barrier passage. Here, the upstream spawning migrations of iteroparous twaite shad Alosa fallax were investigated over three successive spawning migrations in a highly fragmented river basin using passive acoustic telemetry (n = 184). More fish approached and passed barriers in the lower river reaches than further upstream, with the median cumulative weir passage time of 4.6 (1.8–9.2) days representing 18% of their time in the river. Returning fish in their second year had significantly higher weir passage rates than in their tagging year, with passage rates also positively influenced by previous passage success. Higher water temperatures and river levels also had positive impacts on passage rates. Lower weir passage rates by newly tagged individuals suggest that reliance on within-year passage estimates in telemetry-based barrier impact assessments could result in conservative results, while the higher passage rates of previously successful versus unsuccessful individuals suggest a conserved motivation and/or inherent ability to pass barriers. [ABSTRACT FROM AUTHOR]

Stream restorations are increasingly critical for managing and recovering freshwater biodiversity in human‐dominated landscapes. However, few studies have quantified how rehabilitative actions promulgate through aquatic communities over decades. Here, a long‐term dataset is analyzed for fish assemblage change, incorporating data pre‐ and post‐restoration periods, and testing the extent to which native assemblage stability has increased over time. In the late 1950s, a large capacity dam was installed on Putah Creek (Solano County, CA, USA), which altered the natural flow regime, channel structure, geomorphic processes, and overall ecological function. Notably, downstream flows were reduced (especially during summer months) resulting in an aquatic assemblage dominated by warm‐water nonnative species, while endemic native species subsisted at low levels as subordinates. A court‐mediated Accord was ratified in 2000, providing a more natural flow regime, specifically for native and anadromous fishes in the stream. The richness of nonnative species decreased at every site following the Accord, while the richness of native species increased or stayed constant. At the three most upstream sites, native species richness increased over time and ultimately exceeded nonnative richness. Native assemblage recovery was strongest upriver, closer to flow releases and habitat restoration activities, and decreased longitudinally downstream. Rank–abundance curves through time revealed that, while species evenness was low throughout the study, dominance shifted from nonnative to native species in the upstream sites coincident with rehabilitation efforts. Mean rank shifts decreased following flow rehabilitation; thus the assemblage became increasingly stable over time following flow rehabilitation. Putah Creek's rehabilitation may represent a model for others interested in improving endemic freshwater communities in degraded ecosystems. [ABSTRACT FROM AUTHOR]

The Chinese tapertail anchovy, Coilia nasus, is a socioeconomically important anadromous fish that migrates from near ocean waters to freshwater to spawn every spring. The analysis of genomic architecture and information of C. nasus were hindered by the previously released versions of reference genomes with gaps. Here, we report the assembly of a chromosome-level gap-free genome of C. nasus by incorporating high-coverage and accurate long-read sequence data with multiple assembly strategies. All 24 chromosomes were assembled without gaps, representing the highest completeness and assembly quality. We assembled the genome with a size of 851.67 Mb and used BUSCO to estimate the completeness of the assembly as 92.5%. Using a combination of de novo prediction, protein homology and RNA-seq annotation, 21,900 genes were functionally annotated, representing 99.68% of the total predicted protein-coding genes. The availability of gap-free reference genomes for C. nasus will provide the opportunity for understanding genome structure and function, and will also lay a solid foundation for further management and conservation of this important species. Measurement(s) Coilia nasus • Gap-free genome assembly • sequence annotation Technology Type(s) DNBSEQ • Pacbio HiFi Sequencing • Nanopore Sequencing • Hi-C Sample Characteristic - Organism Coilia nasus Sample Characteristic - Environment freshwater Sample Characteristic - Location Taizhou City, Jiangsu Province, China [ABSTRACT FROM AUTHOR]

As we confront novel environmental challenges, a full understanding of the physical and biological processes that govern species responses to climate change will help maintain biodiversity and support conservation measures that are more robust to irreducible uncertainty. However, climate impacts are so complex, and the literature on salmon and trout is so vast that researchers and decision makers scramble to make sense of it all. Therefore, we conducted a systematic literature review of climate impacts on salmon and anadromous trout as a resource for stakeholders, managers, and researchers. We reviewed studies published from 2010 to 2021 that address climate impacts on these fish and organized them in a database of 1169 physical and 1853 biological papers. Papers are labeled with keywords across eight categories related to subject matter and study methods. We compared the literature by biological process and life stage and used these comparisons to assess strengths and weaknesses. We then summarized expected phenotypic and genetic responses and management actions by life stage. Overall, we found the largest research gaps related to species interactions, behavioral responses, and effects that carry over across life stages. With this collection of the literature, we can better apply scarce conservation resources, fill knowledge gaps, and make informed decisions that do not ignore uncertainty. [ABSTRACT FROM AUTHOR]

Sea lamprey Petromyzon marinus is an anadromous and semelparous fish without homing behaviors. Despite being a freshwater, free‐living organism for a large part of their life cycle, its adulthood is spent as a parasite of marine vertebrates. In their native European range, while it is well‐established that sea lampreys comprise a single nearly‐panmictic population, few studies have further explored the evolutionary history of natural populations. Here, we performed the first genome‐wide characterization of sea lamprey's genetic diversity in their European natural range. The objectives were to investigate the connectivity among river basins and explore evolutionary processes mediating dispersal during the marine phase, with the sequencing of 186 individuals from 8 locations spanning the North Eastern Atlantic coast and the North Sea with double‐digest RAD‐sequencing, obtaining a total of 30,910 bi‐allelic SNPs. Population genetic analyses reinforced the existence of a single metapopulation encompassing freshwater spawning sites within the North Eastern Atlantic and the North Sea, though the prevalence of private alleles at northern latitudes suggested some limits to the species' dispersal. Seascape genomics suggested a scenario where oxygen concentration and river runoffs impose spatially varying selection across their distribution range. Exploring associations with the abundance of potential hosts further suggested that hake and cod could also impose selective pressures, although the nature of such putative biotic interactions was unresolved. Overall, the identification of adaptive seascapes in a panmictic anadromous species could contribute to conservation practices by providing information for restoration activities to mitigate local extinctions on freshwater sites. [ABSTRACT FROM AUTHOR]

Objective: We used passive acoustic monitoring (PAM) and automatic detection of spawning splashes to examine the timing and environmental drivers of spawning in river herring (Alewife Alosa pseudoharengus and Blueback Herring A. aestivalis). Methods: Acoustic recordings of spawning splashes were collected from March to May 2021 in the Choptank River, Maryland, using an AudioMoth recorder. Recordings were analyzed using a random forest model on the Rainforest Connection ARBIMON platform to determine hourly presence–absence of splashes. Result: At a seasonal scale, our results suggested two peaks in spawning activity: early March and mid‐April, corresponding with the known phenologies of Alewife and Blueback Herring. Hourly patterns in spawning activity suggested distinct diel cycles, with spawning most concentrated at dawn. In contrast, sonar fish counts collected for 1 week during the season indicated that migration occurred throughout daylight hours. We also found a potential relationship between spawning activity and the presence of great blue herons Ardea herodias. Conclusion: Overall, our results demonstrate that PAM can be an efficient and affordable method for studying the spawning ecology of anadromous fish. Impact statementMonitoring fish migrations in rivers can be costly and time consuming. We present a method that automatically detects the splashing sounds some fish make when they spawn. Using this efficient method, we examine the timing and environmental drivers (e.g., temperature, tides, predators) of spawning in river herring. [ABSTRACT FROM AUTHOR]

Historical loss of river and stream habitats due to impassable dams has contributed to the severe decline of many fish species. Anadromous fishes that migrate from the sea to freshwater streams to spawn have been especially impacted as dams restrict these fish from accessing ancestral spawning grounds. In 2018, Bloede Dam was removed from the Patapsco River near Baltimore, Maryland, restoring approximately 100 km of potential habitat for migratory fish. We assessed the response of anadromous river herring, alewife (Alosa pseudoharengus) and blueback herring (Alosa aestivalis), to this dam removal by monitoring environmental DNA (eDNA) and eggs from 2015 to 2021 at locations upstream and downstream of the dam site during their spawning migrations. We additionally assessed the presence of fish by collecting electrofishing samples and tracked the movements of individual adult fish within the river using passive integrated transponder (PIT) tags. No adult river herring, eDNA, or eggs were detected upstream of Bloede Dam in the four years prior to its removal despite the presence of a fish ladder. Our results suggest initial habitat use recovery by spawning river herring in the first year post-removal, although a relatively small proportion of the population in the river used the newly accessible habitat. In the three years post-removal, the likelihood of detecting river herring eDNA upstream of the former dam site increased to 5% for alewife and 13% for blueback herring. Two adult fish were also collected in electrofishing samples upstream of the dam site in 2021. We found no evidence of changes in egg abundance and no tagged fish were detected upstream of the dam site post-removal. While long term monitoring is needed to assess population changes, this study highlights the value of integrating methods for comprehensive understanding of habitat use following dam removal. [ABSTRACT FROM AUTHOR]

Fecundity is an important demographic parameter that contributes to the productivity of anadromous fish stock dynamics. Yet, studies on fecundity patterns in Pacific salmon (Onchorhynchus spp.) often only include a few years of data, limiting our ability to understand spatio‐temporal trends. Here, we used data on 43 hatchery Chinook salmon (O. tshawytscha, Salmonidae) populations in Washington State to evaluate whether average fecundity changed over the past three decades. We then used data from a subset of stocks (18) to evaluate the relationship between fecundity and body length. Our results revealed significant changes in fecundity across the 25‐year study period with most stocks showing declines in fecundity over the past decade. Results further showed that Chinook salmon have decreased in length over this same period and that annual variation in mean length explains a majority (62%) of annual variation in mean fecundity. Specifically, we estimated that a 1‐mm reduction in length results in 7.8 fewer eggs (95% CI = 6.6–8.9). Given that the majority of Pacific Northwest Chinook salmon in the environment and harvested in fisheries originate from hatchery releases and that nearby hatchery and wild populations generally have similar ocean distributions, these results likely reflect patterns for many populations not included. Combined, our results highlight the need to consider changes in body size and egg production when assessing the dynamics of anadromous fish populations and designing management or conservation plans, particularly for depressed populations. [ABSTRACT FROM AUTHOR]

Chinese sturgeon Acipenser sinensis is an anadromous migratory fish species, which inhabits the Chinese coastal continental shelf areas and breeds in the Yangtze River Basin. The Yangtze River Estuary is an important migration passage of Chinese sturgeon. The Chinese sturgeon population is endangered, which has been listed as a top-level protected animal in List of key protected wild animals in China and a critically endangered species in IUCN Red List of Threatened Species. Since 2013, the abnormal interruption of natural spawning behavior of wild Chinese sturgeon has brought great challenges to the protection and restoration of Chinese sturgeon population. Based on the survey data of juvenile Chinese sturgeon resources in the Yangtze River estuary from 2004 to 2020, this study analyzed the population characteristics and migration habits of juvenile Chinese sturgeon in the Yangtze River estuary following the abnormal natural spawning behavior. The results show that:(1) Since natural spawning behavioral interruption of Chinese sturgeon, juvenile Chinese sturgeon was not detected in the Yangtze River estuary in 2014, 2016, 2018, 2019 and 2020. The recruitment of juvenile Chinese sturgeon population in the Yangtze River estuary showed an interannual continuous fracture. (2) The migration habits of juvenile Chinese sturgeon changed significantly, it showed that the first occurrence time was advanced to mid-April, and the final occurrence time was delayed to late October, and the occurrence time span was extended by 118 days. (3) The biological indexes of juvenile Chinese sturgeon were lower than the same period in history; Their minimum body length and minimum body weight were 7. 0 cm and 4. 9 g respectively. To sum up, the juvenile Chinese sturgeon recruitment decreased year by year. The migration habit shows that the earlier the first occurrence time, the later the final occurrence time in the Yangtze River estuary. The results of this study will provide an important scientific reference for the protection of rare and endangered species of Chinese sturgeon. [ABSTRACT FROM AUTHOR]

Anadromous fishes, such as Pacific salmon, spend portions of their life cycle in freshwater and marine systems, thus rendering them susceptible to a variety of natural and anthropogenic stressors. These stressors operate at different spatiotemporal scales, whereby freshwater conditions are more likely to impact single populations or subpopulations, while marine conditions are more likely to act on entire evolutionarily significant units (ESUs). Coherence in population parameters like survival and productivity can therefore serve as an indicator of relative influence. The goal of this study was to elucidate scale‐dependent shifts in Oregon Coast coho salmon productivity. We used a multivariate state‐space approach to analyze almost 60 years of stock‐recruitment data for the Oregon Coast ESU. Analyses were conducted separately for time periods prior to and after 1990 to account for improvements in abundance estimation methods and significant changes in conservation and management strategies. Prior to 1990, productivity declined for most Oregon Coast populations, especially through the 1980s. From 1990–onward, coherence increased, and trends tracked closely with the North Pacific Gyre Oscillation (NPGO). The latter period is associated with reductions in harvest rates and hatchery production such that the relative influence of the marine environment may have grown more apparent following the removal of these stressors. Furthermore, the link between productivity and NPGO is consistent with trends observed for several other Pacific salmon ESUs. If Oregon Coast coho salmon populations become more synchronous, managers can expect to face new challenges driven by reductions in the population portfolio effect and increasingly variable marine conditions due to climate change. [ABSTRACT FROM AUTHOR]

Declining body size is believed to be a universal response to climate warming and has been documented in numerous studies of marine and anadromous fishes. The Salmonidae are a family of coldwater fishes considered to be among the most sensitive species to climate warming; however, whether the shrinking body size response holds true for freshwater salmonids has yet to be examined at a broad spatial scale. We compiled observations of individual fish lengths from long‐term surveys across the Northern Hemisphere for 12 species of freshwater salmonids and used linear mixed models to test for spatial and temporal trends in body size (fish length) spanning recent decades. Contrary to expectations, we found a significant increase in length overall but with high variability in trends among populations and species. More than two‐thirds of the populations we examined increased in length over time. Secondary regressions revealed larger‐bodied populations are experiencing greater increases in length than smaller‐bodied populations. Mean water temperature was weakly predictive of changes in body length but overall minimal influences of environmental variables suggest that it is difficult to predict an organism's response to changing temperatures by solely looking at climatic factors. Our results suggest that declining body size is not universal, and the response of fishes to climate change may be largely influenced by local factors. It is important to know that we cannot assume the effects of climate change are predictable and negative at a large spatial scale. [ABSTRACT FROM AUTHOR]

Understanding the reproductive strategy of an organism is important in conservation ecology as it directly affects the population performance under changing environmental conditions. Chinese sturgeon (Acipenser sinensis) are the largest anadromous fish in the Yangtze River, China. Currently, the species has only one spawning ground and has failed to spawn in recent years, leading it to the brink of extinction. To develop effective conservation measures, a further understanding of its reproductive strategy is needed. In our study, we conducted kinship analyses by using mitochondrial and microsatellite DNA data from 216 wild juveniles collected over nine years (2006-2013, 2015) to understand the mating system, breeding interval, effective number of breeding adults, and reproductive success. The results from these analyses suggested polygynandry, with some parents contributing up to eight half-sibling juvenile genotypes. Although the spawning ground was restricted to a limited area, genetic diversity was maintained at a relatively high level (observed heterozygosity from 0.698 to 0.787 and expected heterozygosity from 0.763 to 0.787) and inbreeding coefficients in each year-class ranged from -1% to 9% (low to modest detrimental effects on offspring). A parental inference analysis revealed that Chinese sturgeon have a breeding interval of 2-6 years, indicating that it has the potential to feed, accumulate nutrition in the ocean, and then migrate back to the Yangtze River for iteroparous reproduction. The annual effective number of breeders in the Yangtze River ranged from 14 to 161 during the study period, and it decreased by 62.1% from the 2011-2014 year-classes. This sharp population decline likely contributes to the reproduction failure. However, the ratios of effective to census population size (Ne/Nc) were all larger than 0.20 after the 2010 year-class, indicating relatively even reproductive success. Based on these results, a suggested approach to protect this species is to restock parent fish to increase the reproductive stock size and optimize the discharge of the Three Gorges Dam to reduce the unsuitable hydrological conditions and rehabilitate spawning ground habitats. [ABSTRACT FROM AUTHOR]

The timing of entry into freshwater by mature anadromous fishes is an important component of their migration phenology. In iteroparous anadromous species, identifying the migration cues that influence the timing of entry into freshwater, and annual variability in these timings, is important in assessing the extent of individual repeatability. Here, passive acoustic telemetry tracked 71 twaite shad Alosa fallax ('shad') returning to the River Severn's upper estuary over successive spawning seasons, providing individual information on river entry timing and the associated migration cues. The timing of shad returning to the river (defined as their first detection on an acoustic receiver in the upper estuary) was consistent between years and strongly predicted by photoperiod (as increasing day length), whereas water temperatures and flow rates were weak predictors and highly variable between years. Timings of river entry of individual shad were also consistent across consecutive spawning migrations, indicating high individual repeatability. The application of acoustic telemetry in this study thus revealed high repeatability in the timing of annual spawning migrations, which were cued mainly by increasing day length and that, being seasonally predictable, should ensure entry into freshwater with sufficient time to arrive in spawning areas when in-river environmental conditions are favourable. [ABSTRACT FROM AUTHOR]

Rivers and their watersheds have been subject to various changes, including biodiversity, caused by anthropogenic activities. The Danube River and Black Sea basin overlap the geographic range of six species of anadromous fish belonging to the family Acipenseridae. Nowadays, they are endangered or at risk of extinction in the wild due to fragmentation and destruction of natural habitats, blocking their migration to spawning grounds, water pollution, and poaching. This paper presents a review of the historical and current distribution and evolution of the sturgeon populations of sturgeon species in the Danube River basin. Conservation status and measures for species protection, conservation, and restoration at the national, regional, and global levels are presented. [ABSTRACT FROM AUTHOR]