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Not much is known about the changes in bone density due to fish reproductive migration. We used micro-CT and inferential statistics to determine whether the relative bone density in the skulls of adult Three-lips fish, that seasonally upstream migrated to a Lake Biwa tributary, changed across their known reproductive season. The relative bone density significantly decreased as standard length and condition factor (K) increased in both sexes. This negative relationship is likely due to age and hormonal effects in the fish. Results from the bone density analysis also revealed that male Three-lips fish had potentially lower relative bone density (although not significantly different) than females during peak reproductive migration, i.e., July to August. On average, male Three-lips fish are larger in length and weight than females, and in many species, females prefer larger males to smaller males, viewing their size as an indicator of genetic fitness and their ability to provide protection. Resources in the skulls of Three-lips males may be distributed in such a way that increases reproductive success, i.e., size at the expense of quality. In addition, individuals with slightly less dense bones, particularly males, appeared later than those with denser bones during the peak of the reproductive season. The high energy demands involved with aggression in males, often requires resource mobilization from various tissue compartments and could explain the slightly lower density in the latter half of the peak migration. Furthermore, Three-lips individuals that migrated earlier and later during the reproductive season may have more energy reserves than those that had been in the river for some time, hence the variable bone density between individuals. This study serves as a foundation for future studies on the effects of migration, changes in physiology and age on bone density analysis of Three-lips fish and other species in various ecosystems. [ABSTRACT FROM AUTHOR]

Migratory animals play a crucial role in connecting distinct habitats by transferring matter and energy across ecosystem boundaries. In the North Atlantic, anadromous species exemplify this through their movement between freshwater and marine environments. Alosids, including species such as alewife (Alosa pseudoharengus), blueback herring (Alosa aestivalis), and American shad (Alosa sapidissima), exhibit this migratory behavior to maximize growth and fecundity and are, therefore, vital components of Atlantic coastal ecosystems. Despite their ecological importance, these species have experienced considerable population declines. Due to a research focus on dams and the freshwater phase of their ecology, the marine ecology of Alosids remains much less understood, potentially hindering effective management. This paper synthesizes current knowledge on the marine ecology of anadromous alewife, blueback herring, and American shad in the northwest Atlantic, focusing on life‐history aspects, migratory patterns, and foraging behavior at sea. The paper also outlines current fisheries management and the anthropogenic threats these species face during their marine phase. We identified knowledge gaps regarding marine distribution, migration routes, impacts of climate change on movement and behavior, population dynamics, and the identification of gaspereau. By identifying gaps in the literature, we highlight research needs, emphasizing the role of telemetry studies in tracking marine movements and the impact of climate change on habitat use. Addressing these gaps through targeted research on marine ecology and movement patterns is essential for developing informed management strategies aimed at increasing Alosid populations. [ABSTRACT FROM AUTHOR]

When changing habitat during migration or ontogenesis, fish encounter changes of the visual environment, among which the most important is the spectral composition of light and turbidity. This leads to changes in the behavior, morphology, and physiology of the visual system, in particular, spectral sensitivity, which is determined by the properties of visual pigments. The masked greenling Hexagrammos octogrammus has an additional factor that shapes effective spectral sensitivity—the presence of densely bright orange cornea which reversibly changes its density depending on the state of light/dark adaptation. However, it is unknown to what extent the properties of visual pigments are matched to spectral properties of the developing cornea at the final stage of metamorphosis during migration of juveniles from pelagic to coastal environments. The findings of this microspectrophotometric study show that the range of the spectral sensitivity of rods and cones in juveniles is much wider than in adults, with a shift in the sensitivity of a significant part of the cells to shorter wavelengths. The reason for this is the large variation in the ratio of chromophores A1:A2 in the pigment mixture in each cell and possible expression of new opsins. This also indicates the asynchrony of pigment transformations in different types of photoreceptors and the incompleteness of these transformations on the eve of the transition to bottom life in shallow water. [ABSTRACT FROM AUTHOR]

Climate change is impacting marine ecosystems throughout the circumpolar Arctic, altering seasonal habitats and the food bases for fishes, seabirds, and marine mammals. Arctic and Subarctic regions provide resources for resident species and for species that migrate to the north from more southerly regions. Changes in northerly latitudes thus impact endemic as well as non-endemic animals. Herein, we review what is known about climate-driven changes in the migration patterns of Arctic and Subarctic marine vertebrates, including: 1) Arctic residents with seasonal movements – those fishes, seabirds, and marine mammals that complete their entire life cycle within the Arctic but exhibit seasonal movements; 2) Breeding migrants – many seabirds enter the Arctic to breed and subsequently migrate south in the fall; and 3) Summer visitors for feeding – certain species of boreal fishes, seabirds and marine mammals arrive during the northern summer to feed on abundant prey though they breed elsewhere. Migratory movements are often driven by the timing and extent of sea ice, which defines suitable habitat for some animals and limits access to open water and prey for others. Longer open-water seasons, warmer ocean temperatures, and stronger winds have resulted in earlier production blooms in spring and often, extended open-ocean plankton blooms into late summer, resulting in altered prey types and distributions. A common thread among taxa is that shifts in distribution and timing of migrating animals indicate they are traveling farther north, or shifting longitudinally, and migrations are occurring over longer seasonal time frames. Species performing multiple lifetime migrations or long-distance migrants may need to adjust migration timing or routing iteratively to match changes in marine productivity. Altered animal distributions or phenology, and reduced sea ice, affects access to animals that are critical nutritional, economical, and cultural components of Indigenous people's lives in the Arctic. Ongoing changes challenge the resilience and adaptability of Arctic people and ecosystems, and will require adaptive research and management approaches. [ABSTRACT FROM AUTHOR]

Tributaries provide temporal and spatial habitat heterogeneity in river networks that can be critical for parts of the life history of a species. Tributary fidelity can benefit individual fish undergoing spawning migrations by reducing time and energy spent exploring new areas and leveraging previous experience, but anthropogenic activities that fragment or degrade these systems can eliminate those benefits. We used multistate models based on passive integrated transponder (PIT) detection data from 2013 to 2023 to estimate the proportion of flannelmouth suckers (Catostomus latipinnis) migrating to a tributary, McElmo Creek, from the mainstem San Juan River for spawning. Survival varied among years and among states. The top model for migration probability included sex, with males slightly more likely to migrate (0.93 vs 0.90), and the next model identified interannual variation in migration probability ranging from 0.875 to 0.999 across years, indicating high site fidelity. Individuals showed consistency in relative arrival timing across years, with the highest correlation generally during years with greater spring discharge and extended tributary residence time. Successful tributary spawning may be important for the maintenance of the mainstem San Juan River flannelmouth sucker population, but site fidelity may be maladaptive where tributaries are vulnerable to human alterations. [ABSTRACT FROM AUTHOR]

Anadromous migration of salmonid fish has been extensively studied, primarily focusing on the dichotomous framework of anadromy and residency. However, there remains a limited understanding of intermediate migratory behaviors within the anadromous framework. Our study aimed to classify the lifetime migration patterns of the anadromous white-spotted charr Salvelinus leucomaenis within and among populations using otolith annuli and Sr:Ca ratios. Initially, the migratory histories of anadromous charr were divided into two stages: "virgin sea-run stage" and "veteran sea-run stage". The former was further categorized into three types: ocean entry at age ≥1+ years old, early descending, and brackish use. The latter was grouped into four types: annual migrants, frequent migrants, retired migrants, and ocean residents. We found that the proportion of migration patterns varied among rivers, with multiple patterns coexisting within the same river. Migration patterns typically involving 1–5 years spent in freshwater rivers followed by annual oceanic migrations were the most abundant, although diverse patterns were also observed. In the virgin sea-run stage, some individuals experienced the ocean at age 0+ years or brackish environments before their first sea entry. In the veteran sea-run stage, we found individuals who had resided in either the ocean or rivers for over a year. Retired migrants, characterized by stopping oceanic migration at a certain age and subsequently spending time in rivers, were exclusive to southern rivers. Conversely, ocean residents who spend one or more years in the ocean were more frequent in northern regions. Consequently, the lifetime migration patterns of anadromous white-spotted charr may exhibit stronger ocean dependency at higher latitudes. The implications of this study highlight the complexity and flexibility of migratory behaviors within and among white-spotted charr populations. [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 Pak Mun Dam remains one of north-eastern Thailand's most disputed infrastructure projects. Local livelihoods, particularly those of women, have been negatively impacted, leading many to participate in the resistance movement against the dam. While the fight continues, protests have waned in frequency and vigour. Many in the resistance movement have grown older and passed on, while others have accepted their fate brought on, in part, by the Electricity Generating Authority of Thailand's use of resources to foment division among villagers. In the place of mass protests, local actors have turned to organising memorial events, in which resistance is expressed through local practices. This article explores the roles of women, particularly Mae Sompong, in shaping new geographies of resistance through the embodied performances of two memorial events—the Liang Luang Liang Wang and the Boon Mot ceremonies. While the annual Liang Luang Liang Wang ceremony commemorates the migration of fish upriver from the Mekong River into the Mun River and the Boon Mot ceremony commemorates Wanida (Mot) Tantiwittayapitak and other Pak Mun Dam activists who have died, both serve as reminders of past resistance and are catalysts for future contestations. Using a feminist political ecology approach, we contend that ethnic Lao villagers, especially women, constitute local geographies of anti-dam resistance through commemorative practices, ones that have been rendered unimportant and invisible by uneven power relations. [ABSTRACT FROM AUTHOR]

In order to describe large-scale spatial structure of sockeye salmon on the Asian part of the range, the variability of 45 SNP loci was analyzed in 22 samples from the Northwest coast of the Pacific Ocean. Three large regional population complexes were identified: Southwest Kamchatka, Kamchatka River basin, and the Northeast (comprising stocks from Koryak Highlands). Populations within the identified complexes are connected by gene migration and have a common origin, close geographic proximity, comparable climatic, landscape, and environmental conditions in the freshwater and early marine periods of sockeye salmon life. Populations confined to watersheds of the North coast of the Sea of Okhotsk (Palana and Okhota rivers), along with island populations, displayed distinctions from the isolated population complexes. It is hypothesized that the marked divergence observed in island populations is primarily caused by genetic drift occurring during long periods of isolation. The pronounced divergence of Palana River population may be the result of both genetic drift and natural selection, driven by the challenging smoltification and specific conditions of freshwater period in this watershed. At the same time in the Okhota River population, demographic factors such as genetic drift and bottlenecks played a key role. [ABSTRACT FROM AUTHOR]

Fish passes are a key element enabling the migration of aquatic organisms in the context of restrictions resulting from the presence of weirs. Multi-criteria decision analysis, AHP, and Rembrandt methods were used to assess the effectiveness of fish passes on mountain rivers. Three common types of fish passes were considered: slotted fish pass, block ramps, and a circulation channel with boulders. The results of the study indicated that block ramps proved to be the most favourable solution, achieving the highest preference values in both methods (Rembrandt: 0.77, AHP: 0.63). The key factors influencing the effectiveness of the fish passes are the availability of space and the water requirements, which reached values of 0.38 and 0.27 in the Rembrandt method and 0.33 and 0.28 in the AHP method, respectively. The differences between the results of both methods were minimal and did not have a significant impact on the final choice. The discussion emphasised the advantage of nature-like fish passes, such as block ramps, which better preserve the ecological continuity of rivers and can be more easily adapted to local hydrological conditions. The study also indicated the need for continuous monitoring of the fish passes and their optimisation to reduce problems related to sedimentation and flow blocking. The obtained results can provide a valuable basis for decision making in the planning and construction of fish passes, especially in demanding mountainous conditions, contributing to improving the effectiveness of fish migration and minimising negative impacts on the natural environment. [ABSTRACT FROM AUTHOR]

Migration is a high‐risk behavior. For the Atlantic salmon, Salmo salar, migrating from its river nursery area to marine feeding grounds, the magnitude of risk varies with habitat type. Passage through lakes, in particular, is associated with low rates of migration success. Downstream migrating salmon smolts are rheotactic when migrating in rivers, but lakes typically provide poorer directional currents for migrating salmon. In this study we tested if, in the absence of clear navigational cues in lakes, Atlantic salmon smolts switch to a random search strategy to find the outflowing river. We constructed random search simulations to test if lake basin shape has an effect on migration success. We also compared simulated migration characteristics with migrations of salmon smolts through five real lakes for which there are telemetry data for migrating salmon. Correlated random walk simulations showed that a random search strategy could be successful for all lake shapes tested but was more successful in curved (round and elliptical) than rectangular basin shapes. Rectangular basins with the migration start and stop points at the ends of the lake had a higher success than those where these points were perpendicular to the axis of the lake. In general, a random walk model predicted the migration success rate of fish tracked through real lakes. However, for two lakes the simulated migration success exceeded that of actual success, suggesting that fish passing through these lakes were not adopting a random search strategy. We speculate that this is the result of either conflicting navigational cues which inhibit smolts from finding the lake outlet or that they temporarily suspend migration (e.g., to feed). Modelling predicted that for small lakes, directional swimming in short steps (ca. 100 m) followed by turns with very low variation from the direction of travel resulted in the highest migration success. For larger lakes, longer step lengths but also with low turn variation (simulated turning angle drawn from distributions of standard deviation 2° and 5° around a mean of 0°) resulted in the highest migration success. We conclude that navigation in downstream migrating salmon smolts switches from rheotaxis in rivers to a random search tactic in lakes except where residual flow cues in some lakes prevent this, at times resulting in suboptimal navigation outcomes. [ABSTRACT FROM AUTHOR]

Walleye (Sander vitreus) are a sexually dimorphic species in which females are larger than males in adulthood. Walleye can also exhibit sex‐ and population‐based differences in migration behavior. In Lake Erie, we used acoustic telemetry to test the prediction that female walleye exhibit larger broad‐scale movements than males during the summer and autumn. This prediction was based on the hypothesis that greater foraging in females would be needed to satisfy their higher energy requirements. We quantified movements of males and females from distinct spawning populations from Lake Erie's west and east basins using a lake‐wide grid of acoustic receivers in 2017 and 2018. We found no differences between male and female home range sizes, core range sizes, or distances travelled in either population. Fish length‐at‐tagging was unrelated to the size of a fish's home range or to its distance travelled, contrary to previous predictions about body size as a driver of migration distance in the Lake Erie population. We found that west basin walleye occupied large and indiscrete portions of the lake, but the core range of females extended into the central basin, whereas males were concentrated in the west basin. Walleye originating from the east basin confined their movements primarily to the east basin and showed stronger home range overlap among members of their population than did walleye from the west basin population. Within either population, walleye had more home range overlap with members of the same sex, which likely reflects differences in the migratory tendencies of males and females. [ABSTRACT FROM AUTHOR]

Iberian barbel (Luciobarbus bocagei Steindachner, 1864) and Iberian nase (Pseudochondrostoma polylepis Steindachner, 1864) are two Mediterranean potamodromous fish species known to perform annual upstream migrations to reach spring spawning grounds. In the Mondego River basin, at the Coimbra dam, migratory movement patterns and individual size structure were assessed through a video recording monitoring system installed on an upstream section of a vertical-slot fish pass. Visual census for these target species during two consecutive annual cycles (2013–2014) revealed alternative migratory patterns, with the first peak of upstream movements in autumn, for both barbel (October–November) and nase (November–December). Circadian movements of both species showed a diurnal preference, contrary to what is usually described for these species. Size structure analysis for individuals of both species showed significant intra-annual differences in the size of migrating fish. Boosted regression trees models applied to the 2013–2014 visual count data identified flow and temperature as the most influential environmental predictors, triggering both species' movements in each direction in the study years. These results provide novel information on the timing of the migratory movements of these potamodromous fish, which can be used to adapt current management and conservation measures to the specificities of their migratory behaviour. [ABSTRACT FROM AUTHOR]

Trap and transport, the capture and subsequent translocation of fish during the freshwater phase of their migration, is becoming more common as a management intervention. Although the technique can be successful, it is costly and can have unintended effects on the fish being transported. This study investigates whether trap and transport can be used to increase the migration success of Atlantic salmon, Salmo salar, smolts in naturally flowing rivers. Seaward‐migrating S. salar (n = 294) from two UK rivers were tracked using acoustic telemetric techniques. Outmigration success and timing were compared between non‐transported (released at the original in‐river capture site) and transported (released ca. 23 km downstream of the capture site) individuals. Downstream translocation increased the proportion of fish that successfully migrated to marine waters, and there was no indication that transport reduced post‐release survival. The post‐release migration speed of transported fish was slower than expected but this was likely a function of their advanced migration timing rather than an inhibition of their capacity to migrate. These results suggest that trap and transport can increase the outmigration success of S. salar smolts, but the earlier river exit dates of transported fish could negatively affect their survival at sea. [ABSTRACT FROM AUTHOR]

ABSTRACT Riverine habitat fragmentation by barriers, including impoundments, is common and their effects on obligate aquatic organisms are manifold. Organisms, such as Atlantic salmon (Salmo salar), that make extensive river migrations are particularly vulnerable to the effects of impoundments. In this study, we use acoustic telemetry to examine the migratory behaviour of Atlantic salmon, as they migrate to sea for the first time as juvenile ‘smolts’, in a river with a series of dams that form a complex hydropower scheme. We demonstrate that overall migration success in the River Dee catchment and particularly through standing waters was remarkably high. We speculate that high migration success in standing waters could be due to relative current speeds providing good quality directional cues to migrants. Migration success past the two dams in this study, was relatively high, although the number of unsuccessful passage attempts before a successful passage was also very high. The vast majority of smolts passed the dams when the turbines were operational. At one dam, smolts did not use an available fish pass but migrated through the generating turbines. These findings provide several routes through which generation could be managed to enhance the success of downstream smolt migration in rivers where there are similar patterns of hydrogeneration in place. [ABSTRACT FROM AUTHOR]

Microplastics, as an emerging pollutant, are widely distributed in freshwater environments such as rivers and lakes, posing immeasurable potential risks to aquatic ecosystems and human health. The migration behaviour of microplastics can exacerbate the degree or scope of risk. A complete understanding of the migration behaviour of microplastics in freshwater environments, such as rivers and lakes, can help assess the state of occurrence and environmental risk of microplastics and provide a theoretical basis for microplastic pollution control. Firstly, this review presents the hazards of microplastics in freshwater environments and the current research focus. Then, this review systematically describes the migration behaviours of microplastics, such as aggregation, horizontal transport, sedimentation, infiltration, stranding, resuspension, bed load, and the affecting factors. These migration behaviours are influenced by the nature of the microplastics themselves (shape, size, density, surface modifications, ageing), environmental conditions (ionic strength, cation type, pH, co-existing pollutants, rainfall, flow regime), biology (vegetation, microbes, fish), etc. They can occur cyclically or can end spontaneously. Finally, an outlook for future research is given. [ABSTRACT FROM AUTHOR]

The study of fish movement patterns in freshwater ecosystems remains a major focus of contemporary research. Small freshwater fish are known to migrate between tributaries and mainstems at different stages of their life history, helping to avoid local extinctions and maintain genetic diversity. However, fitting small fish species with telemetry equipment is difficult, making it challenging to track their detailed migration history via conventional methods. We evaluated the suitability of otolith isotope analysis for verifying the migration histories of small fish species inhabiting small tributaries. In this study, we compared the 87Sr/86Sr values in stream water and the otoliths of two fish species (Nipponocypris temminckii and Rhinogobius flumineus) inhabiting small streams. The 87Sr/86Sr ratios in the stream waters within the study area displayed considerable variation among tributaries. Our findings indicate that only N. temminckii has high dispersal ability, showing movement between tributaries or between tributaries and the mainstem in some locations. In contrast, such movement was not generally observed for R. flumineus. These results show the potential of 87Sr/86Sr as a tool for estimating the migration patterns of small fishes that inhabit small tributaries. [ABSTRACT FROM AUTHOR]

Tracking of animal migrations using telemetry technologies needs to take into consideration the burden that the tag exerts on the animal. Here, we examined the potential impacts of acoustic tags of two sizes (nominally a 'V6' [smaller] and 'V7' [larger]) on the downstream riverine migration success and behaviour of wild Atlantic salmon (Salmo salar L.) smolts. One hundred fish were tagged with either a V6 or V7 tag. Tag burden (tag: fish weight) ranged from 1.88% to 7.39% and differed significantly between fish tagged with the V6 (mean [SD] = 3.63% [0.51%]) and the V7 tags (mean [SD] = 5.84% [0.95%]). There was no significant difference in the in‐river migration failure between the two groups when tested with a time‐to‐event analysis. There were also no differences in other elements of the migratory behaviour (rate of movement, time of detection and residency time) between the two tagging groups. These data support the use of acoustic tracking for monitoring smolt migration and highlight that tagging of smaller smolts at up to 7.39% tag burden to gain a more representative understanding of migration success and behaviours across a smolt population. [ABSTRACT FROM AUTHOR]

In the Laurentian Great Lakes, the issue of barrier removal is complicated by the presence of non‐native species below barriers. A fish tracking study was conducted to guide efforts for barrier remediation decisions for the restoration of fish populations with a focus on Walleye (Scander vitreus) and Lake Sturgeon (Acipenser fulvescens) in the Black Sturgeon River, a river system fragmented by a dam which blocks access of fishes to the majority of a large, otherwise barrier‐free watershed. Data from 3 years of spawning migrations (2018–2020) indicated that the Walleye population in Black Bay likely consists of both river (65%) and lake spawners (27%), with the remaining individuals spawning in the bay or river in different years. Walleye and Lake Sturgeon showed consistent differences in the extent to which individuals migrated upstream in the river during the spawning season, despite expectations that both species would spawn at the base of the dam when prevented from further migration. The dam was presumably a barrier to migration for Lake Sturgeon, as nearly all Lake Sturgeon that entered the river migrated to the base of the dam. In contrast, few Walleye entering the river during the spawning season migrated to the dam annually. These findings suggest that Walleye and Lake Sturgeon may not benefit equally, at least in the short term, from barrier remediation or dam removal. [ABSTRACT FROM AUTHOR]

Migratory fish species have been particularly impacted by changes to the hydrologic and climatic cues to which migration and spawning behaviours have been adapted across generations. While conservation and recovery programs increasingly implement flow management actions to promote successful migration and spawning, uncertainty regarding how spawning migration phenology responds to changing environmental conditions can limit the ability to effectively target such recovery actions. Here, we use a Bayesian hierarchical modelling framework to analyse spawning migration phenology of individually tagged June suckers (Chasmistes liorus) – a federally threatened, long‐lived, iteroparous, adfluvial species endemic to Utah Lake (Utah, USA) and its tributaries. We then examine how annual hydrologic and thermal conditions relate to different components of annual migration phenology, including peak migration date, in‐stream residence time, and among‐individual variation in migration timing. Peak migration date occurred earlier in years with warmer spring air temperatures (a proxy for water temperatures), though this effect interacted with peak runoff timing. Both residence time and among‐individual variation in migration timing were greater in years with larger spring discharge and later peak flows. Residence time was also longer in warmer years. These results highlight how natural and anthropogenic changes to river flow and thermal regimes are likely to impact June sucker migration timing and duration, and our approach can be applied to other migratory species to identify the external drivers of the different components of migration phenology. [ABSTRACT FROM AUTHOR]

Dams and other in‐stream obstacles disrupt longitudinal connectivity and hinder fish from moving between habitats. Fishways and other fish passage solutions are used to pass fish over these artificial migration barriers. Fish passage functionality, however, varies greatly with fish passage design and environmental conditions and depends on fish species and characteristics. In particular, swimming performance and fish behaviour are considered key characteristics to predict fish passage performance. It is also well known, but not well quantified, that the presence of conspecifics affects fish passage behaviour. In this study, we quantified individual passage rates of PIT‐tagged gudgeons (Gobio gobio) over a scaled deep side notch weir in an hydraulic flume. We then quantified individual swimming capability (time to fatigue) and activity level (distance moved in an open field test) for the same individual fish and tested for potential effects on fish passage rate. To check for potential group effects, we then repeated the passage experiment for fish individually or in groups of five. More active fish displayed higher passage rates compared to less active fish, and fish passed the obstacle at higher rates in groups of five compared to alone. No effect of fish swimming capability on passage rates was detected. This result highlights the need to take both individual variation as well as the presence and behaviour of conspecifics into account in fish passage studies and evaluations. Doing so has the potential to improve the understanding of fish behaviour, and in the end, the design of fish passage solutions. Future studies should explore these results on free ranging fish and in relation to in‐situ fish passage solutions. [ABSTRACT FROM AUTHOR]

The Black Sea is a semi‐enclosed inland sea with an unevenly distributed extensive coastal shelf area and anoxic deep waters. It is inhabited by common and bottlenose dolphins, as well as harbour porpoises, all represented by local subspecies. Between September 2020 and October 2022, 19 F‐PODs deployed by research teams from Bulgaria, Georgia, Romania, Türkiye and Ukraine collected data on acoustic activity of Black Sea harbour porpoises. Strong seasonal and diel patterns were found, which varied in three regions. In the south‐eastern part of the Black Sea, harbour porpoise acoustic activity was higher from January to May, with a peak in April. This pattern agrees with the seasonal anchovy migration from the winter spawning grounds in warmer waters in the south‐eastern region to feeding grounds on the productive shallow north‐west shelf. The diel pattern showed strong nocturnal acoustic activity, which is consistent with anchovy vertical migration. Porpoises on the western side of the Black Sea exhibited a bimodal seasonal pattern in acoustic activity, with a larger peak in April and a smaller one in October. Diel activity was primarily nocturnal. On the north‐west shelf, harbour porpoise acoustic activity was mostly recorded during the warm period from April to October. The diel pattern showed activity mainly during daylight with two peaks: a smaller one approximately at dawn and a larger one at dusk. This pattern is similar to the vertical migrations of sprat. Overall, the results of the study were consistent with the prey being an important driver of seasonal and diel dynamics of harbour porpoise acoustic activity. [ABSTRACT FROM AUTHOR]

Coilia nasus are an important fish resource in the Yangtze River, and the Yangtze River Estuary is a crucial migration pathway for them. In this study, we used otolith microchemistry to analyze the strontium/calcium (Sr/Ca) ratios and Sr contents in the sagitta otolith of C. nasus from the south branch (SB) and north branch (NB) of the Yangtze River Estuary and obtained the diversity of migration patterns and spawning ground distribution for C. nasus. The results indicate that C. nasus from both branches include two types of habitat history: freshwater (F)–brackish water (B) (Type I) and F-B seawater (S) (Type II), with Type I being dominant at 62.50% in both branches. The C. nasus from the SB comprise six migration patterns, while that from the NB has seven migration patterns. The C. nasus from both branches hatch in F habitats. At the time of capture, the C. nasus from the SB predominantly remain in F, accounting for 62.5%, while C. nasus from the NB primarily stay in B, accounting for 87.5%. Throughout the migration process, C. nasus from both branches switch between different habitats, with C. nasus from the NB exhibiting more frequent transitions between F and B, showing a greater reliance on the estuarine brackish habitat. The radius of the first blue region near the core (Lf) and freshwater coefficient (Fc) of the otolith for C. nasus from both branches are divided into three groups: long-distance freshwater dependence (LD), medium-distance freshwater dependence (MD), and short-distance freshwater dependence (SD), with the LD only appearing in the SB, while the NB is primarily represented by MD. There is a correlation between the differences in Lf among different groups of C. nasus and the differences in the distance from the spawning grounds of C. nasus in different sections of the Yangtze River to the estuary (DYRE), reflecting the distribution pattern of C. nasus spawning grounds in different sections of the Yangtze River. This study provides theoretical guidance for the protection of migration pathways and maintenance of spawning grounds for C. nasus, which have significant practical value in the precise management of C. nasus resources in the Yangtze River Estuary. [ABSTRACT FROM AUTHOR]

Genetic data have greatly increased means to understand fish marine migration behaviours at large spatial scale within a quantitative framework. The anadromous sea trout is a prized target of recreational fishery and an important ecosystem component in freshwater and marine coastal habitats in large parts of temperate northern Europe. Nonetheless, little is known about population distributions while feeding at sea. To reconcile notions about feeding migrations being predominantly locally restricted or not, we used SNP data for 3465 trout representing >100 rivers to identify population origins of 903 coastally feeding fish captured throughout the North Sea-Baltic Sea transition area. Across areas, coastal collections generally showed marked stock-mixing and overall 70:30% native:non-native fish with seasonal variation corresponding with spawning run timing. Data revealed strong spatial feeding segregation between trout from the Scandinavian Peninsula and the European continent. This is surprising given the short distances between areas, but is in alignment with strong genetic differentiation between populations in these areas. Estimation of stock complexity of coastal feeding aggregations showed no spatial trend through the North Sea-Baltic Sea transition area otherwise characterized by strong environmental clines. Analyses of scale samples collected in the 1950s indicate that stock-mixing was consistent over almost 70 years. [ABSTRACT FROM AUTHOR]

When the conditions encountered by a migratory species are highly variable, the timing of migration and migrant condition are critical to survival. Spring/summer‐run Chinook salmon in the Snake River of the Northwestern United States are listed as threatened under the US Endangered Species Act because of decades of poor adult returns; juvenile smolt‐to‐adult survival is often 0.5% or less. Juveniles pass through eight dams en route to the ocean, and despite changes to the hydropower system, there has been little recent improvement in survival. We analysed the effect of migration date, fish length and their interaction on the survival to adulthood of 409,747 wild juvenile Chinook salmon as they migrated through a large, dynamic riverine ecosystem and through the ocean, measured over 20 years.Parametric 2D smoothers are often used to quantify unexplained heterogeneity across space; however, these same statistical models can be applied to 2D datasets to uncover patterns of covariance of two biological processes important to management actions. We employ Gaussian Markov random fields (GMRF) and a 2D smoother to estimates of fish survival to identify the nature of variation in survival with respect to length and date.We found significant effects of fish length, migration date and the date‐by‐length interaction on resulting riverine and marine survival. Unlike the gradual shifts in survival in relation to length and timing that marginal estimates demonstrate, we show that within year, changes in survival with length and timing are often more abrupt than previously thought.Synthesis and applications: Inter‐annually, we identified distinct combinations of fish length and migration timing leading to elevated smolt‐to‐adult survival. Often these 'hotspots' are compressed into fish sizes greater than 100 mm and passage timings of only a week or two, demonstrating that even modest shifts in body size and migration timing can have a significant effect on survival. Further research can concentrate on these distinct annual periods to identify sources of mortality that have previously been elusive. [ABSTRACT FROM AUTHOR]

A 60‐year precipitation and streamflow record from the Caspar Creek Experimental Watersheds in northern California was used to explore the propagation of meteorological drought to hydrological drought. Standardized precipitation and runoff indices were calculated for the two forested catchments using integration periods of 12, 24, and 36 months. The resulting time series were used to define three severe drought events (1976–1977, 2013–2014, and 2020–2022). The earliest drought followed the 1971–1973 harvest of the 417 ha South Fork (SF) watershed, a second followed the 1989–1992 harvest of the 479 ha North Fork watershed, and a third followed the 2017–2019 reentry harvest of the SF. From these time series, we calculated drought metrics and anomalies to model differences in catchment responses in the context of climate and management. The meteorological drought in the 1977 event was more severe and extreme than the streamflow response. Both of the 21st Century droughts were hydrologically more severe than the 1977 drought. Timber harvest initially shortened and reduced streamflow drought (1977 and 2021) but prolonged and intensified the 2014 streamflow drought. Declining fall precipitation has reduced streamflows, thereby impeding salmonid migration and exacerbating impacts on native fish. Our results provide new insights into the role of climate variation, particularly long‐term and seasonal drought dynamics, in managed forests along the North American Pacific coast. [ABSTRACT FROM AUTHOR]

Atlantic salmon (Salmo salar), brown trout (Salmo trutta), and steelhead trout (Oncorhynchus mykiss) kelts have important conservation value for population resilience. Nonetheless, relative to other salmonid life-stages, knowledge on their behavior and survival is poor. This is especially true for kelt downstream migration in rivers fragmented by hydropower plants (HPP), even though the physiological and survival consequences are severe when functional connectivity is not provided. Here, the existing information about kelt downstream migration past HPP was summarized, beginning with an overview of iteroparous salmonid lifecycles. Then, the importance of kelts for conservation and the threats they encounter while migrating was discussed. Finally, the current corpus of peer-reviewed literature and reports focused on kelt migration in these systems was presented. Kelt studies have focused on five main themes: (1) postspawning survival, (2) migration delays before dam passage, (3) passage selection and guidance efficiency, (4) passage mortality, and (5) the swimming behaviors of kelts during migration. Overall, there was a paucity of information about kelts and several information gaps and needs for future research. This work should inform scientists and river managers on the conservation of salmonids in regulated rivers, including the development of passage solutions for safe downstream migration. [ABSTRACT FROM AUTHOR]

Swimming performance is important for fish migration, habitat selection, and predator-prey interaction, as well as for fish passage design. Procedural choices made when experimentally estimating it may influence the results. Systematic experiments were conducted to study the effect of different in-flume habituation times, habituation behaviour, and the use of external encouragement on burst swimming performance of Rutilus aula, a small-sized cyprinid, in a fixed velocity testing protocol. Increasing habituation times from 30 s to 5 or 20 min substantially increased the success proportion of swimming trials and estimated fish swimming performance, with no difference between the latter two habituation times. Fish resting on the downstream grid before the start of testing velocity outperformed those who swam during habituation and transition periods. Fish swimming volitionally in response to flow at testing velocity showed a significantly improved performance compared to fish motivated by external poking. The results of this study highlight that in-flume habituation time is important, and fish behaviour before actual testing may influence the outcomes of swimming performance results. [ABSTRACT FROM AUTHOR]

Streamgage designs often include a full-width artificial hydraulic control (e.g., concrete weir) to aid in the computation of streamflow. While important to water resource managers, these weirs also tend to act as full or partial barriers to fish migration, effectively hindering the health and survival of these populations. In this study, we conducted experiments to quantify the effect of head drop and submergence of a common streamgage weir on the passage performance of an important migratory fish species, the American Shad. Three treatment conditions were selected based on the tailwater surface elevation (ElTW): unsubmerged (ElTW = 1.05 m; head drop = 0.46 m), equal to the weir crest (ElTW = 1.20 m; head drop = 0.31 m), and submerged (ElTW = 1.36 m; head drop = 0.15 m). Fish movements were recorded via passive integrated transponder telemetry techniques. Results revealed that the backwatered Columbus-type weir was not a complete barrier at any of the three treatments, but passage was shown to be significantly impaired when the weir was unsubmerged. Passage efficiency for the unsubmerged, equal, and submerged treatments was 20.2 ± 6.2, 49.2 ± 7.2, and 64.2 ± 7.4%. Backwatering a weir, rather than removal or other major alterations that would affect weir calibration, may be an acceptable retrofit to increase fish passage. [ABSTRACT FROM AUTHOR]

Background: Colombian migration globally has had many consequences. This study focuses solely on the smoking behavior of Colombian migrants. This study addresses how the act of international migration impacts the likelihood of smoking among Colombian migrants. Smoking has been found to be the leading cause of preventable death, and so it is a health risk for Colombian migrants. This study pertains to the communities sampled in Colombia, where the migrants originated. Methods: This study uses the Latin American Migration Project to address how migration impacts the smoking behavior of Colombian migrants. Using logit regression, the statistical analysis predicts the likelihood of smoking among Colombian migrants. This study compares the smoking behavior of current and former Colombian migrants, which is compared to that of Colombian non-migrants. Because current and former migrants were surveyed, the study is able to control for health selectivity. Results: After accounting for health selectivity, the study finds that migration, as measured by time abroad, is positively and significantly related with the smoking of migrants relative to non-migrants. Additionally, this study shows that former migrants are more likely to smoke than current migrants, providing support for the salmon effect. Conclusion: This study adds to the literature on smoking by studying an understudied group, Colombian migrants, and provides a better measure of the impact of migration on smoking behavior by accounting for health selectivity and the salmon bias. The findings are consistent with assimilation theory, as more time abroad is associated with negative health behaviors that are common in the culture of the receiving country, but the fact that such behaviors are harmful to the migrant is consistent with the immigrant health paradox. [ABSTRACT FROM AUTHOR]

A survey was conducted to identify fish fauna related to the dominant habitats, viz., (i) Honduwa Lake (stagnate water), (ii) estuarine area with mangroves, (iii) freshwater marshy area with floating aquatic vegetation, (iv) canals, and (v) river, in the Dedduwa estuary of southern Sri Lanka. Thirty-nine species of fish including two endemics (to the island) were identified, including members of major migratory families such as Anguillidae and Megalopidae. Based on the abundance of species, the Bray-Curtis similarity index indicated a clear separation of the canal and freshwater marsh, with other studied areas. Similarly, the taxonomic diversity of the canal and freshwater marsh was high, indicating high variation and diversity of the species and genera. Protection of mangroves and related habitats is important to maintain the stability and long-term existence of fish fauna in the estuary. Effective monitoring is proposed for detecting and eliminating illegal encroachments, mangrove clearance, and illegal fishing activities. Moreover, improving the knowledge and awareness among members of the local community, politicians, and environment officers about the importance of the region's biodiversity implementing strong policies, and creating a strong responsible stakeholder bond are required to ensure the long-term sustainability of the estuary. [ABSTRACT FROM AUTHOR]

Gillnets are widely used in research and commercial fishery activities. As passive gear, gillnets can be selective and dependent on the diel migration of fish. In areas with limited littoral extent, inshore–offshore migration may cause bias in the gillnet catch. Our hypothesis was that some factors, such as gillnet saturation, fish depletion, or chemical cues, could be the cause of the bias. We used a total of 66 CEN gillnets deployed at Římov Reservoir parallel to the shore at different positions of littoral-pelagic gradient. Individual fish direction was recorded from inshore, offshore, or unknown direction (i.e., entangled fish). A total of 5791 fishes from nine different species were caught. For most fish, it was possible to determine their directivity, and most fish were captured in littoral or first pelagic gillnets. Shallower and deeper benthic gillnets differed in their bleak (Alburnus alburnus) catch. No significant differences were found between fish directions. At the species level, only asp (Leuciscus aspius) and ruffe (Gymnocephalus cernua) showed differences between the captured directions in one case. The results support the assumption that gillnet capture is a random process that to a great extent is connected to random local movements. This is good news for fish monitoring projects. Sampling catch is likely to reflect true changes in the fish community, and not the effects of the deployment of the sampling gear. The experiment also showed that fish directivity statistics can be used for investigation of fish behavior and gear performance. [ABSTRACT FROM AUTHOR]

AbstractThe Mekong River basin is a biodiversity hotspot and supports the largest inland capture fishery globally. The fish and fisheries, especially migratory species that underpin the capture fisheries, are, however, under threat from multiple pressures, not least hydropower development, expansion of irrigated agriculture and aggregate mining. In this paper the Global Swimways concept was used to understand migratory patterns in different Mekong fish species and provide insights for management and conservation of migratory species in the basin. Information was collated from existing databases, FishBase, and literature searches to determine the significance of migration routes of the river system. A total of 1393 fish species was recorded. About 21% of these are truly migratory species, mostly potamodromous and amphidromous species, and contribute ≈70% of catch. Distribution of fish in the Mekong River exhibits a well-defined zonation pattern, with species diversity highest in the lower floodplains and delta reaches. Three main migration zones occur in the Lower Mekong Basin (LMB) but with considerable migration of some species between zones. Some species adopt multiple migration strategies as opposed to simple longitudinal or lateral migrations, with sub-populations occurring in different reaches of the LMB exhibiting different migratory strategies. Approximately 11% of native fish species are threatened; among migratory species 35% are threatened. There are multiple challenges to maintaining swimways in the Mekong, including improved understanding of migratory pathways, managing intensification of environmental pressures and managing heavy fishing pressure. Measures to conserve and protect the migratory fish species in the Mekong are suggested. [ABSTRACT FROM AUTHOR]

Dams commonly restrict fish movements in large rivers but can also help curtail the spread of invasive species, such as invasive bigheaded carps (Hypophthalmichthys spp). To determine how dams in the upper Mississippi River (UMR) affect large-scale invasive and native fish migrations, we tracked American paddlefish (Polyodon spathula) and bigheaded carp across > 600 river km (rkm) and 16 navigation locks and dams (LD) of the UMR during 2 years with contrasting water levels. In 2022, a low-water year, both native paddlefish and invasive bigheaded carp had low passage rates (4% and 0.6% respectively) through LD15, a movement bottleneck being studied for invasive carp control. In contrast, flooding in 2023 led to open-river conditions across multiple dams simultaneously, allowing 53% of paddlefish and 46% of bigheaded carp detected in Pool 16 to move upstream through LD15. Bigheaded carp passed upstream through LD15 rapidly (μ = 32 rkm per day) a maximum of 381 rkm, whereas paddlefish moved an average of 9 upstream rkm per day (maximum of 337 rkm). Our results can inform managers examining trade-offs between actions that enhance native fish passage or deter movements of invasive species. This understanding is critical because current climate change models project increases in flooding events like that observed during 2023. [ABSTRACT FROM AUTHOR]

Hydropower expansion is increasingly responsible for connectivity and biodiversity loss in freshwater ecosystems. The Amazon basin, which supports the highest level of freshwater biodiversity globally, faces such unplanned expansion. Here, we demonstrate and quantify the impacts of two major hydroelectric dams on the Madeira River on the trans‐Amazonian movements of one apex predatory catfish (Brachyplatystoma rousseauxii) performing the longest migration known in freshwaters. Using otolith microchemistry, we show that above the dams, the former basin‐wide homing migration between the breeding grounds in the upper Madeira and the nursery in the Amazon estuary has now been replaced by residency. We found evidence suggesting downstream migration of juvenile fish past dams into the Amazon estuary and that some of them home back to the Madeira. However, we did not find evidence that the fish homing back from the estuary can access their breeding grounds in the upper Madeira; they remain blocked below the dams. Our results provide undisputable evidence that the conservation of the species primarily requires the reestablishment of connectivity with the construction of efficient fishways. [ABSTRACT FROM AUTHOR]

Passive integrated transponder (PIT) technology is a leading tool for tracking fish in freshwater systems. PIT is highly applicable for assessing fish passage at anthropogenic infrastructure (e.g., dams and floodgates); however, there are often complications in operating PIT antennas near these structures due to the ambient electromagnetic interference of metal and power‐supply equipment. We designed a PIT antenna that is resistant to the effects of ambient electromagnetic interference (AEMI). This design uses lobes with balanced polarity within the antenna to neutralize AEMI within the vicinity of the antenna. This novel PIT antenna provides a more effective and cost‐efficient option for researchers tracking fish in environments with high AEMI. [ABSTRACT FROM AUTHOR]

China has experienced substantial coastal reclamation and damming of rivers. These changes have the potential to impact migrations of diadromous fishes between the sea and fresh waters, but the composition of these fishes and the impacts of barriers to their movement in China have received little attention. We inventoried the species composition and distribution of diadromous fishes, and the impacts of barriers on them, in the Hong Kong Special Administrative Region (HKSAR), southern China. Fish assemblages were surveyed using hand‐nets, supplemented by cast‐netting and single‐pass snorkel surveys, in 24 small coastal streams across three regions. Surveys were undertaken on multiple occasions during the wet and dry seasons to account for the monsoonal tropical climate. Twenty‐eight diadromous fishes were collected, mostly gobies, amounting to over half (53%) of the total richness of primary freshwater fishes; four additional species are known from literature records. Diadromous richness was 48% greater during the wet season, when all species were encountered. Richness varied substantially among streams, from a maximum of 17 (2 streams that were diversity hot spots) to none (3 streams). The most widespread diadromous fish was Glossogobius giuris (71% frequency of occurrence), followed by Mugil cephalus (58% occurrence) and Eleotris oxycephala (50% occurrence). The remaining 25 diadromous fishes occurred in fewer than half of the streams; 12 species were confined to a single stream and may be locally threatened. There were conspicuous spatial differences in diadromous assemblages across HKSAR, despite its limited extent (1114 km2), the proximity of the surveyed streams, and the broad geographic distribution of most species. Regional species assemblages were influenced by localized habitat characteristics, with a noticeable distinction between areas with and without large, fast‐flowing, and highly oxygenated streams. The presence of in‐stream barriers (weirs: 0.3–8.7 m high) did not affect spatial patterns in species assemblages, although, on average, diadromous richness was lower in weir‐obstructed streams (4.0 vs. 6.9 species in unobstructed streams). In total, 18 species were confined to unobstructed streams or sections below weirs, whereas the remaining 10 species were recorded both above and below weirs. Only the mottled eel (Anguilla marmorata) and a goby (Stiphodon multisquamus) were able to ascend weirs over 2 m. Although at least 400 m of the lower course of each stream was sampled, diadromous fishes were confined to the first 300 m in 12 of the 13 weir‐obstructed streams. Remarkably, the tally of 32 diadromous species in HKSAR exceeds the 19 known from mainland China, highlighting the need for further research on composition and conservation status of diadromous fishes. [ABSTRACT FROM AUTHOR]

Deep‐pelagic fishes are among the most abundant vertebrates on Earth. They play a critical role in sequestering carbon, providing prey for harvestable fishing stocks and linking oceanic layers and trophic levels. However, knowledge of these fishes is scarce and fragmented, hampering the ability of both the scientific community and stakeholders to address them effectively. While modelling approaches incorporating these organisms have advanced, they often oversimplify their functional and ecological diversity, potentially leading to misconceptions. To address these gaps, this synthesis examines the biodiversity and ecology of global deep‐pelagic fishes. We review pelagic ecosystem classifications and propose a new semantic framework for deep‐pelagic fishes. We evaluate different sampling methods, detailing their strengths, limitations and complementarities. We provide an assessment of the world's deep‐pelagic fishes comprising 1554 species, highlighting major groups and discussing regional variability. By describing their morphological, behavioural and ecological diversity, we show that these organisms are far from homogeneous. Building on this, we call for a more realistic approach to the ecology of deep‐pelagic fishes transitioning between very different ecological niches during diel vertical migrations. To facilitate this, we introduce the concept of 'diel‐modulated realised niche' and propose a conceptual model synthesising the multiple drivers responsible for such transitions. [ABSTRACT FROM AUTHOR]

In marine fishes of commercial interest, defining habitat use and migration strategies through ontogeny can help better understand the structure and dynamics of harvested populations and guide their management. The present study relied on otolith chemistry to identify three contingents within the Atlantic halibut (Hippoglossus hippoglossus) stock in the Gulf of St. Lawrence (GSL). We differentiated two chemical signatures from otolith edges, one for shallow (<100 m) and another one for deep (>100 m) waters. By identifying transitions between the deep and shallow habitats, we found that most halibut display migrations from the deep waters to shallow waters during the first 3 years of life. After reaching maturity, most halibut distributing in northern regions of the GSL became full-time residents in deep areas of the GSL. In contrast, halibut found in summer on the shallow plateau of the southern GSL displayed migrating behaviour between shallow (summer) and deep (winter) waters throughout their lives, either on an annual or irregular basis. Overall, our results demonstrate that otolith chemical signatures serve as natural markers of geographically distinct marine environments, facilitating the identification and reconstruction of environmental histories of long-lived marine fishes. [ABSTRACT FROM AUTHOR]

Freshwater fisheries in developing regions provide livelihoods and nutrition for millions of people worldwide. These fisheries are frequently data poor, which limits fisheries management. The seasonal Cambodian dai platform fishery on the Tonle Sap River is one of the best monitored inland fisheries in Southeast Asia, yet catch sampling is limited and there is no fishery-independent monitoring. A monitoring system is needed to characterize fish migration and mortality, be cost effective, and be deployable in areas with minimal infrastructure. We integrated a Simrad wide-band transceiver mini echosounder (200 kHz), solar power, and an Internet of Things communications module as an autonomous, automated monitoring package for the deployment on upstream and downstream commercial fishing platforms. The solar panel and controller supply direct current power to the echosounder, communications module, and battery for power during dark hours. Echosounders are programmed to sample at 1 Hz for 15 min every hour. The communications module is a built cellular endpoint that accesses a local wireless network to transmit raw data files to a data server. Data are downloaded from the server for processing and analysis. This expandable system provides a flexible management tool that can be deployed at any location with wireless communication capability. [ABSTRACT FROM AUTHOR]

In a warming world, the input of glacier meltwater to inland water ecosystems is predicted to change, potentially affecting their productivity. Meta-ecosystem theory, which posits that the nutrient availability in the recipient ecosystem can determine the extent of cross-ecosystem boundary utilization, can be useful for studying landscape-scale influences of glacier meltwater on inland waters. Here, we investigate how the input of glacier meltwater in a river system in Southern Greenland influences the utilization of marine subsidies in freshwater fish. Our study system comprised four sites, with controls for glacial meltwater and marine subsidies, harboring a partially migrating population of arctic char, meaning that some individuals migrate to the ocean and others remain in freshwaters, and two fully resident populations as a freshwater reference. We assessed the incorporation of marine carbon in freshwater resident char using both bulk and amino acid stable isotope analysis of muscle tissue. In the population with partial migration, marine subsidies were a significant resource for resident char individuals, and estimates of trophic position suggest that egg cannibalism is an important mechanism underlying the assimilation of these marine subsidies. In proglacial streams, namely those with high glacial meltwater, the total dependence on marine subsidies increased and reached 83% because char become cannibals at smaller sizes. In the configuration of our focal meta-ecosystem, our results suggest that the importance of marine subsidies to freshwater fish strengthens within increasing meltwater flux from upstream glaciers. [ABSTRACT FROM AUTHOR]

This study determined the particle release area based on the abundance data of largehead hairtail eggs obtained from the ichthyoplankton surveys in the coastal waters of the southern Yellow Sea (119°E-124°E, 33°N-37°N) conducted from 2018 to 2022. Subsequently, the Finite-volume Coastal Ocean Model (FVCOM) coupled with the Lagrangian particle tracking algorithm was employed to simulate the transport process of largehead hairtail larvae. The primary objectives were to investigate the spawning periods, spawning grounds, and nursery habitats of largehead hairtail in the southern Yellow Sea, and to assess the influence of oceanic conditions on the egg distribution and larval dispersion. The results indicated that largehead hairtail spawned in spring and summer in the southern Yellow Sea with the eggs appeared in May, July and August, and August was the peak spawning month. Largehead hairtail eggs were mainly distributed in 19-30 m waters in May, and 11-70 m waters in August. The sea surface temperature and sea bottom temperature for the presence of largehead hairtail eggs was 13.9-29.7 and 8.4-28.3 ℃ respectively. The Random Forest model analysis revealed that water temperature and depth were the main environmental factors affecting the abundance of largehead hairtail eggs. Particle tracking simulation experiments showed that the particles released in May mainly spread towards the northwest coastal area of the particle release area after 1-15 days (when the swim bladder has not formed and the larvae have no vertical migration behavior), and after 16-40 days (when the swim bladder has formed and the larvae have vertical migration behavior), the particles left the coastal area and spread eastward to the waters near a 50 m water depth. In July and August, before the occurrence of vertical migration behavior, larvae stayed in the near-bottom layer and most of them were transported from the release area to the nearshore waters; After that, larvae moved slightly from the near-shore aggregation area to the off-shore direction. Some of the individuals released in the offshore deep-water areas spread to the north and northeast in a short-distance, and the dispersal pattern did not change significantly in their whole larval stages. Aggregation areas of largehead hairtail larvae derived from field surveys in August were consistent with the results of particle transport simulation experiments. We thought that the season and spatial location of spawning, the diel vertical migration characteristics of larval fish, and the regional upwelling and frontal structure were the dominant factors that shape the transport pattern of largehead hairtail larvae. The results of the study provide a scientific basis for understanding the habitat selection mechanism of egg and larvae stages and early replenishment dynamics of this fish in the southern Yellow Sea, and help for the management and protection of largehead hairtail resources. [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]

The Amazon basin, spanning approximately 540,000 km2, exhibits distinct fluviometric surfaces that differentiate between the dry and rainy seasons. This seasonality, along with hydrological connectivity and the creation of new habitats during the rainy season, significantly promotes the migration, reproduction, and feeding of potamodromous fishes. To estimate the realized niches of species, species distribution models (SDMs) employ the extrapolation of environmental predictors and species occurrence data. Our objective was to compare the spatial distribution of migratory fish species in the Amazon basin using SDMs based on variables characterizing the dry season, rainy season, and a combination of both. All evaluated treatments demonstrated high performance and exhibited different distribution ranges in the applied SDMs, particularly when combining environmental variables with occurrence data during the rainy season. These findings support the hypothesis that spatial distribution is influenced by seasonality. The increased fluviometric surface and enhanced connectivity of the rainy season favor both longitudinal and lateral migrations of Amazonian migratory catfishes. Moreover, the spatial distribution reveals four critical spatial overlap (CSO) regions with higher population densities regardless of the season. These CSOs primarily coincide with the Amazon alluvial plain, which exhibits the highest rates of endemism, species richness, and abundance of organisms. Considering the discontinuous and heterogeneous nature of fluviometry when performing niche modeling processes is pivotal, although SDMs applied in the Amazon generally ignore such regional seasonality. [ABSTRACT FROM AUTHOR]

The European eel is critically endangered due to heavy impact of anthropogenic factors, such as habitat fragmentation, overexploitation and climate change. During downstream migration, silver eels may encounter hydropower plants, which often result in delay or mortality from impingement on trash-racks or turbine passage. These problems can be mitigated with downstream passage solutions, such as angled racks that guide downstream-migrating eels to safe passage routes. The importance of bar spacing and phenotypic diversity for passage performance is, however, largely unknown. In this study, we investigated how morphological parameters (body mass, eye and fin indices) and behavioral score (open field test) influenced passage rate at an experimental intake equipped with a bypass and angled racks with either 15 or 30 mm bar spacing. Both racks were efficient in guiding eels into a bypass. There was a strong positive effect of body mass and a weak positive effect of open field test score on passage rate. Other factors such as eye and fin indices played a minor role. These results demonstrate the performance of angled racks with bypasses and form a useful starting point for further research regarding the relationships between individual variation in behavior, morphology and passage solutions for silver eels. [ABSTRACT FROM AUTHOR]

A challenge in the design of fishways especially for large rivers is the trade‐off between attraction and passability of the entrance. High flow velocities in the entrance slot generate a strong attraction flow and tend to have better attraction efficiency for upstream migration. However, these velocities may also prevent small‐bodied species or juvenile fish from entering the fishway. With our experiment, we reproduced fish swimming behaviour and passage of a fishway entrance for small‐bodied fish under realistic hydraulic conditions. At an entrance slot with 0.4 m width four velocities 0.8, 1.2, 1.5 and 1.8 ms−1 were investigated. In total, 326 juvenile roach (Rutilus rutilus (Linnaeus, 1758)), gudgeons (Gobio gobio (Linnaeus, 1758)) and spirlins (Alburnoides bipunctatus (Bloch, 1782)) were tested. The passage rates of the three tested species were altogether higher than would have been predicted from reported swimming capacities. They increased from gudgeons to roach and (significantly) to spirlins and we could show how passage rates of the three species decreased with increasing flow velocities, suggesting species‐specific critical slot velocities. Still, these velocities are lower than those currently proposed to generate sufficient attraction flow, which may have implications for fishway design. [ABSTRACT FROM AUTHOR]

The migration of fishes through the Thoranit Naruemit water gate on the Nam Kam River, a tributary of the Mekong River in Thailand, was studied to understand the hydrological factors and improve the operational regime of the fish pass at the structure. Migration of fish from the Mekong River started in May, at the onset of the rainy season. During 60 days of operation, the fish pass supported the upstream migration of 83 species and ~440,000 fish, equivalent to about 11 tonnes of fish. The fish pass supported migration of a range of fish sizes between 45 and 700 mm, and most fish were larger than the size of first maturity for the species in question. The factors influencing migration into the Nam Kam included the lunar cycle, flow in the fish pass, water level in the river and flow variability. Migration tended to increase in the 3–4‐day period before or after a full moon. Discharge and water level changes associated with rising water levels in the Mekong River synchronize with the spawning season of most Mekong fish and trigger upstream migration. Many fish migrated through the pass at discharges through sluice gates between 8 and 323 m3/s and with a flow through the fish pass between 0.95 and 3.08 m/s. Restrictions to fish migration were found and recommendations for water gate and fish pass operation are provided. [ABSTRACT FROM AUTHOR]

This study assessed the functional effectiveness in terms of passage proportion of three fishways, one nature‐like bypass, one partly nature‐like bypass and partly technical pool‐type and one completely technical, on the Pinka Stream and Rába River in Western Hungary. Radio frequency identification with passive integrated transponder tagging was used to collect data on the upstream passage. A total of 2976 tagged individuals were included in the study; 2863 individuals were either barbel (Barbus barbus), nase (Chondrostoma nasus) or chub (Squalius cephalus), which were chosen as model species, and 113 individuals belonged to four other species. There were 540 individuals of the model species and 18 individuals of the other species detected while successfully ascending one of the fishways. The time‐to‐event (survival) analysis of the data of the model species revealed significant species‐specific differences in passage probability between the fishways. The passage probabilities of the barbel were the highest, and those of the nase were the lowest at all three fishways. The findings demonstrate that single‐species evaluations can lead to incorrect conclusions on fishway effectiveness. Technical fishways can be as effective as nature‐like ones, and hybrid solutions (technical fishways combined with nature‐like bypasses) can function as well. The limitations of the study and recommendations for further evaluations are also discussed. [ABSTRACT FROM AUTHOR]

A fishway is an engineered structure designed to help fish navigate past artificial barriers in rivers such as dams, weirs and regulators. An effective fishway design requires an understanding of fish biodiversity and migration requirements of the local community.To support the conceptual design of a fishway at Caringin Weir in the Cibareno River in Indonesia, we conducted a fish biodiversity assessment using experimental fishing and an eDNA metabarcoding survey.In early 2022, experimental fishing was conducted at six sampling sites upstream and downstream of the planned weir, and the sites were surveyed seven times during the wet and dry seasons. Environmental DNA sampling was conducted once during the wet season at each of the six sites.A total of 72 aquatic species were detected, comprising 40 fish species and seven crustacean species captured during experimental fishing and 36 fish species identified through eDNA metabarcoding. Twelve fish species (18.46%) were detected using both methods. The total number of all species caught and species richness in experimental fishing varied significantly between upstream and downstream areas (p < 0.05), but there were no significant differences between seasons (p > 0.05). There was also a significant difference in the species richness upstream and downstream using eDNA metabarcoding (p < 0.05). There were also numerous diadromous species, further highlighting the need for an effective fishway design to enable these species to complete their migrations.Based on the species present, migratory behaviour and economic importance, we prioritised six fish and crustacean species to be considered during fishway design: Anguilla marmorata, Tor tambra, Barbodes binotatus, Sicyopterus sp., Rhyacichthys aspro and Macrobrachium sp. [ABSTRACT FROM AUTHOR]

Rationale: Carbon (δ13C) and nitrogen (δ15N) stable isotope analysis is a powerful tool to investigate diverse questions in fish ecology, such as their trophic position or migration strategies. These questions appear particularly important to protect endangered European eel. However, elevated lipid content in eel muscle can bias δ13C values, as lipids are 13C‐depleted compared to proteins and carbohydrates. Methods: We measured δ13C and δ15N values of bulk and lipid‐free samples of eel muscle. Lipid‐free samples were obtained after the extraction of lipids with cyclohexane. Lipid‐corrected δ13C values, using five different mathematical equations based on bulk δ13C values, were compared to lipid‐free δ13C values. We also evaluated the effect of lipid extraction on δ15N values. The analyses were based on linear regression performed on 333 individuals captured in nine lagoons and four rivers. Results: Independently to the capture site or habitat (river or lagoon), the predicted lipid‐corrected δ13C values were highly consistent with the measured lipid‐free δ13C values (R2 > 0.90). The application of specific equations for each habitat or capture site only slightly increases these R2 (1.5% or less). The lipid extraction treatment significantly decreased by 0.2‰ the δ15N values compared to bulk samples. Conclusions: Given the excellent prediction of mathematical equations and the small decrease of δ15N values after lipids extraction, we propose to use mathematical correction to estimate δ13C values of eel muscle. As the habitats or sites did not strongly influence the results, the coefficients from our study can be applied to other studies on European eel. [ABSTRACT FROM AUTHOR]