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ABSTRACT Aim Location Methods Results Main Conclusions The Mediterranean Sea is one of the most anthropized seas in the world but also a marine biodiversity hotspot with many fish species under threat. The main goal of the study is to test whether on the heavily fished and anthropized Mediterranean coast, the less impacted Corsica and Balearic Islands, can be considered as refugia for threatened and elasmobranch fishes independently of protection by marine reserves.The French Mediterranean coast and three north‐western Mediterranean islands: Corsica and also Mallorca and Minorca from the Balearic archipelago.We performed 187 fish surveys using environmental DNA metabarcoding on three islands and 109 along the continental coast. Of the 78 surveys on islands 22 correspond to no‐take marine reserves and of the 109 continental surveys 26 were carried out within reserves. After eDNA filtration, extraction, amplification, and sequencing we estimated the number of fish species but also the number commercial, threatened and elasmobranch fish species on each sample. We then performed an ANOVA by permutation to test the effect of insularity and protection on these four biodiversity metrics. We also modelled these four biodiversity metrics as a function of protection and human pressure but also environmental, habitat and sampling conditions. We also built species accumulation curves to obtain asymptotes representing the potential regional pools for each species category on both island and continental coasts.We obtained a total of 175,982,610 reads over the 187 eDNA samples that were assigned to 153 fish species including 17 elasmobranch species among which 7 were only detected on islands. We observed a higher total fish richness on continental than island surveys regardless of protection but a higher threatened and elasmobranch fish richness on the island than on continental surveys. We obtained a significant, negative and predominant human gravity impact on the diversity of elasmobranch species. The modelled asymptote reached 148 teleostean fish species on islands and 196 on the continental coastline with a very similar rate of diversity increase with sampling effort but the shape of the species accumulation curves differed markedly for elasmobranchs with a stronger increase in diversity with sampling effort on islands.Our findings highlight that Mediterranean islands can be refugia for sharks and rays but also threatened fishes in this overexploited region. Our results also suggest that reducing or banning trawling activities may play a key role for conserving vulnerable fishes, beyond the benefits of no‐take marine reserves, which appear limited on these large home‐range species. [ABSTRACT FROM AUTHOR]

Aim: Mobulid rays are a group of threatened batoid fishes susceptible to population decline from targeted fisheries and accidental capture. Spatial distributions of mobulid rays remain poorly known. Prior studies found commonalities between favourable environments and prey among various mobulid species, yet most were conducted in tropical waters. To explore the habitat use and distribution of mobulid rays in a temperate environment we model the habitat suitability of two mobulid rays (Mobula mobular and Mobula birostris) in Aotearoa New Zealand using fisheries and citizen science occurrence data spanning almost two decades. Location: Northeastern coast of Aotearoa, New Zealand. Methods: Boosted Regression Tree models were used to predict the annual habitat suitability and favourable environmental conditions of the two species based on available sightings records in conjunction with high resolution (1 km2) environmental data. Results: The sympatric study species had contrasting habitat requirements. We found a separation in their spatial distribution defined by the 200 m isobath – the onshore extent for M. birostris and the offshore extent for M. mobular. While there were only subtle variations in relative habitat suitability for M. mobular over the study period, M. birostris exhibited greater interannual variability. Despite differing interannual patterns, spatial separation, as a function of environmental properties, persisted regardless of the year. Main Conclusions: Our results suggest that associations between mobulid species may differ from tropical regions due to regional adaptations to prey availability or local environmental conditions unique to colder and more productive temperate waters. Our findings highlight the importance of multi‐species surveys and the inclusion of temporal variability in support of separate species‐specific management plans to account for differing stressors impacting each species. [ABSTRACT FROM AUTHOR]

It remains unknown how species' populations across their geographic range are constrained by multiple coincident natural and anthropogenic environmental gradients. Conservation actions are likely undermined without this knowledge because the relative importance of the multiple anthropogenic threats is not set within the context of the natural determinants of species' distributions. We introduce the concept of a species 'shadow distribution' to address this knowledge gap, using explainable artificial intelligence to deconstruct the environmental building blocks of current species distributions. We assess shadow distributions for multiple threatened freshwater fishes in Switzerland which indicated how and where species respond negatively to threats — with negative threat impacts covering 88% of locations inside species' environmental niches leading to a 25% reduction in environmental suitability. Our findings highlight that conservation of species' geographic distributions is likely insufficient when biodiversity mapping is based on species distribution models, or threat mapping, without also quantifying species' expected or shadow distributions. Overall, we show how priority actions for nature's recovery can be identified and contextualised within the multiple natural constraints on biodiversity to better meet national and international biodiversity targets. Explainable Artificial Intelligence can improve conservation decisions by revealing hidden insights to where human impacts on biodiversity are greatest. In this investigation of freshwater fish in Switzerland, around 90% of potentially habitable areas were negatively impacted human influences - these areas form the species' "shadow distribution". [ABSTRACT FROM AUTHOR]

Simple Summary: The wild population of Brachymystax tsinlingensis is a Grade II protected species in China. This population faces slow growth, harsh habitats, serious individual miniaturization, and a low survival rate of fry during its early developmental stage. To improve the aquaculture efficiency of this species, and thus increase the survival rate, protect species resources, and enrich species numbers, the present study was conducted to investigate the changes in intestinal tissue structure, digestive enzymes, malondialdehyde content, and antioxidant enzyme activities and their patterns during the early developmental stage of Brachymystax tsinlingensis. This information can be applied to increase the survival rate of fry, protect the species resources, and enrich the species' quantity. This work explores the digestive system characteristics of Brachymystax tsinlingensis during early developmental stages and aims to solve the problem of high lethality of fry during the transgression period, which is crucial for the artificial propagation and population conservation of endangered fishes. This study was carried out on intestinal tissue, digestive enzymes, and antioxidant enzyme activities in the early development stage of Brachymystax tsinlingensis. Ten random samples during endogenous nutrition (7, 10, and 11 days after hatching), mixed nutrition (13 and 19 DAH), and exogenous nutrition (31, 33, 39, 45, and 73 DAH) were collected by histological and biochemical analysis methods. The results showed that the intestine of Brachymystax tsinlingensis already has four layers initially at 7 DAH, and the intestinal gland tissue is evident at 73 DAH. The contents of total protein (TP) and the activities of lipase (LPS) and trypsin (TPS) were maximal at 39 DAH, and the activities were 3.20 ± 0.26 mg/mL, 2.52 ± 0.69 U/g, and 2717.45 ± 295.26 U/mg, respectively. Catalase (CAT) and glutathione peroxidase (GSH-PX) activities both showed the lowest values at 39 DAH, which were 0.57 ± 0.11 U/mg and 3.35 ± 0.94 U/mg, respectively. The activity of amylase (AMS) and the content of malonaldehyde (MDA) increased, and the highest values were reached at 45 DAH (1.32 ± 0.41 U/mg) and 73 DAH (1.29 ± 0.43 nmoL/mg), respectively. Superoxide dismutase (SOD) and GSH-PX activities both showed a peak value at 7 DAH (126.58 ± 20.13 U/mg and 6.47 ± 1.86 U/mg). Overall, the changes in intestinal tissue, digestive enzymes, and antioxidant enzyme activities at 39 DAH of Brachymystax tsinlingensis are inseparable from different vegetative stages during the developmental period, and these results can provide a reference for the proliferation and cultivation of Brachymystax tsinlingensis resources. [ABSTRACT FROM AUTHOR]

Genetic diversity is rapidly lost from small, isolated populations by genetic drift. Measuring the level of genetic drift using effective population size (Ne) is highly useful for management. Single‐cohort genetic Ne estimators approximate the number of breeders in one season (Nb): a value < 100 signals likely inbreeding depression. Per‐generation Ne < 1000 estimated from multiple cohort signals reduced adaptive potential. Natural populations rarely meet assumptions of Ne‐estimation, so interpreting estimates is challenging. Macquarie perch is an endangered Australian freshwater fish threatened by severely reduced range, habitat loss, and fragmentation. To counteract low Ne, augmented gene flow is being implemented in several populations. In the Murrumbidgee River, unknown effects of water management on among‐site connectivity impede the design of effective interventions. Using DArT SNPs for 328 Murrumbidgee individuals sampled across several sites and years with different flow conditions, we assessed population structure, site isolation, heterozygosity, inbreeding, and Ne. We tested for inbreeding depression, assessed genetic diversity and dispersal, and evaluated whether individuals translocated from Cataract Reservoir to the Murrumbidgee River bred, and interbred with local fish. We found strong genetic structure, indicating complete or partial isolation of river fragments. This structure violates assumptions of Ne estimation, resulting in strongly downwardly biased Nb estimates unless assessed per‐site, highlighting the necessity to account for population structure while estimating Ne. Inbreeding depression was not detected, but with low Nb at each site, inbreeding and inbreeding depression are likely. These results flagged the necessity to address within‐river population connectivity through flow management and genetic mixing through translocations among sites and from other populations. Three detected genetically diverse offspring of a translocated Cataract fish and a local parent indicated that genetic mixing is in progress. Including admixed individuals in estimates yielded lower Ne but higher heterozygosity, suggesting heterozygosity is a preferable indicator of genetic augmentation. [ABSTRACT FROM AUTHOR]

A growing body of evidence indicates that the global diversity of freshwater fishes has not been fully documented. Studies of freshwater fishes that were previously thought to be morphologically variable have revealed the existence of deeply divergent lineages, with many distinct species. In southern Africa a number of Enteromius species exhibit either exceedingly wide or divided distribution patterns that should be rare for freshwater fishes with limited dispersal opportunities between river systems. One such species is the sidespot barb, Enteromius neefi. As currently defined, E. neefi has a disjunct distribution that is divided between rivers in the northeast escarpment in South Africa and Eswatini, and tributaries of the Upper Zambezi in Zambia and southern Congo in the Democratic Republic of Congo, with a large geographic gap between these two populations. With the use of molecular and morphological methods, the level of divergence between the two populations was examined, and a new species was described from the Steelpoort River in the Limpopo River system of South Africa. Findings from this study provide further evidence for a number of taxonomic problems within the goldie barbs of southern Africa, and some taxonomic rearrangements are proposed for this group. [ABSTRACT FROM AUTHOR]

Elucidating the habitat-related processes driving variation in juvenile recruitment and population fluctuation is crucial for the conservation of freshwater fishes, particularly for species at risk, where recruitment variation increases at small population size. We used a combination of instream enclosure experiments and habitat use observations to investigate between-stream variation in recruitment of juvenile Nooksack Dace (Rhinichthys cataractae sp. cataractae), a federally endangered stream-dwelling minnow endemic to the lower Fraser Valley of British Columbia, Canada. There was no evidence for a recruitment bottleneck associated with high water temperature or low dissolved oxygen in spawning riffles at the egg incubation stage, or during a two week juvenile growth experiment in enclosures, although larval dace growth was positively related to temperature. However, immediately post-hatch dace showed a very strong preference for shallow (<5 cm deep), fine substrate marginal shoal habitat with no overhanging or instream vegetation cover, which was associated with well-developed pool-riffle channel structure. Shallow marginal rearing habitat was absent in the stream experiencing recruitment failure, where encroachment by invasive reed canarygrass (Phalaris arundinacea) created homogenous steep-bank habitat with heavy instream vegetation cover, implicating loss of critical marginal rearing habitat as the recruitment bottleneck. [ABSTRACT FROM AUTHOR]

Objective: Delta Smelt Hypomesus transpacificus are thought to be close to extinction in the wild, which has spurred multiagency conservation efforts to supplement their population. In conjunction with these recent efforts, monitoring is needed to assess the effectiveness of supplementation. Such monitoring often uses large‐scale tagging of released fish to distinguish between hatchery‐origin and wild‐origin fish that are captured during later surveys. Methods: In this study, we evaluated the use of visible implant elastomer tags in Delta Smelt to determine whether fish survival, fish growth, or tag retention were dependent on (1) tag color (blue, green, orange, red, yellow, and no‐tag control) or (2) tagged body location (posterior‐dorsal, anterior‐dorsal, mid‐lateral line, and no‐tag control). Result: Over 165 days of the first experiment, there was no difference in growth rate (mean = 0.073 mm/d, range = 0.072–0.075 mm/d) or survival (mean = 68%, range = 63–77%) among tag colors. Across all color groups, mean tag retention was 96% (range = 87–100%). Over 120 days of the second experiment, tag location had no influence on fish growth rate (mean = 0.064 mm/d, range = 0.062–0.067 mm/d) or survival (mean = 50%, range = 43–58%). For the surviving fish in experiment 2, the tags that were placed at the mid‐lateral line were retained less (84% on day 120) than those at posterior‐dorsal and anterior‐dorsal locations (99% and 98%, respectively). Conclusion: Our results suggest that visible implant elastomer tagging is a suitable method for marking adult Delta Smelt (>50 mm fork length) for up to 165 days and can be useful for conservation monitoring efforts. Impact StatementSupplementation of Delta Smelt is needed to avert its extinction in the wild. We experimentally evaluated the suitability of visible implant elastomer tags to distinguish hatchery‐ and wild‐origin Delta Smelt captured in monitoring surveys during the spawning season. [ABSTRACT FROM AUTHOR]

Population recruitment of endangered rheophilic fish species strongly depends on the survival of critical egg and larvae life stages. However, the transportation and fate of eggs after spawning is not well understood. This study analyzed the egg transport and settling patterns of the endangered European nase (Chondrostoma nasus L.), testing the transferability of physical modelling approaches for simple solid sphere surrogates to the eggs of lithophilic fish species. This model approach was validated in both laboratory and field experiments, where wetted millet corns were used as surrogates excluding deformability and stickiness of natural nase eggs. The transport and settling patterns of the surrogate accurately modelled the transportation of C. nasus eggs. Results further indicate that transport processes of eggs are small-scaled in the range of less than 3 m. These findings provide helpful information for restoration planning of C. nasus spawning grounds. They also highlight the importance of the interstitial zone regarding the egg settling process, which conforms to the ecological relevance of this habitat for egg and larval development. While the surrogate was suitable for this model approach, further investigations are necessary to confirm transferability to other species. [ABSTRACT FROM AUTHOR]

Endemic fish species have long supported the livelihoods of local communities in the Rufiji River Basin (RRB). However, destructive fishing practices have led to a concerning decline in endemic fish stocks. To assess these changes, this study employed key informant interviews, focus group discussions (FGDs), and fishery surveys to assess the historical and contemporary distribution of endemic fishes within the RRB. DNA barcoding was also used to verify species identities. Out of 37 reported fish species, 33 species (54.55% endemic and 45.45% exotic to RRB) were confirmed through DNA barcoding and morphological characteristics. About 5 species including, Heterobranchus longifilis, Citharinus congicus, Labeo congoro, Mormyrus longirostris, and Labeobarbus leleupanus were rarely found in the field, despite being classified as Least Concern by IUCN. Additionally, five species that were reported to be present in the RRB by experienced fishers were not captured during sampling. This highlights the need for validation of the existence of such species through eDNA metabarcoding. Moreover, due to the rarity of some species in the area, their IUCN assessment should be revisited. [ABSTRACT FROM AUTHOR]

Freshwater ecosystems are crucial for global biodiversity through supporting plant and animal species and providing essential resources. These ecosystems are under significant threat, particularly in island environments such as Madagascar. Our study focuses on the Amboaboa River basin, home to the rare and endemic fish species Rheocles derhami, last recorded in 2013. To assess the status of this and other threatened fish species including Ptychochromis insolitus and Paretroplus gymnopreopercularis, and to understand freshwater fish population dynamics in this biodiversity hotspot, we conducted a comprehensive survey using both environmental DNA (eDNA) and traditional fishing methods. While traditional methods effectively captured a diverse range of species, including several invasive aliens and the critically endangered endemic species that were the focus of this study, the eDNA approach detected only a fraction of these introduced species and struggled to identify some critically endangered endemics at the species level. This highlights the value of combining methods to enhance species detection. We also investigated the trade-offs associated with multi-primer assessments in eDNA analysis, focusing on three different primer combinations targeting the 12S mitochondrial gene: MiFish, Tele02, and Riaz. Additionally, we provided 12S reference barcodes for 10 species across 9 genera of fishes from the region to increase the coverage of the public reference databases. Overall, our study elucidates the current state of freshwater biodiversity in the Amboaboa River basin and underscores the value of employing multiple methods for effective conservation strategies. [ABSTRACT FROM AUTHOR]

Understanding the mechanism by which non‐native fish species integrate into native communities is crucial for evaluating the possibility of their establishment success. The genus Pangasianodon, comprising Pangasianodon gigas and Pangasianodon hypophthalmus, has been introduced into reservoirs, which are non‐native habitats, for fishery stock enhancement. P. gigas and P. hypophthalmus often successfully establish and co‐occur in several Thai reservoirs, but there is little information on differences in food resource use between the two species. To investigate the trophic niche width of P. gigas and P. hypophthalmus in a Thai reservoir, we conducted stable carbon and nitrogen ratio (δ13C and δ15N) analyses. We examined the degree of individual specialization in both species using the δ13C and δ15N values of muscle and liver tissues, which provides long‐ and short‐term diet information. The isotopic niches did not overlap between P. gigas and P. hypophthalmus. The δ15N value of P. gigas was significantly higher than that of P. hypophthalmus, whereas the δ13C value did not significantly differ between the two species. The isotopic niche sizes were larger in P. hypophthalmus than in P. gigas. Individual specialization was observed in P. hypophthalmus but not in P. gigas, indicating that intraspecific variation in food resource use was larger in P. hypophthalmus compared to P. gigas. These findings suggest that trophic niche partitioning was one of the factors facilitating the establishment success of P. gigas and P. hypophthalmus in a reservoir, but the establishment process may differ between the two 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]

Goodeinae is a subfamily of critically endangered fish native to central Mexico. Populations of Skiffia lermae, a species belonging to this subfamily, have significantly decreased in the past two decades. A previous study showed that S. lermae is sensitive to acute nitrate-nitrogen (NO3-N) exposure, leading to noticeable changes in both behavioral and histopathological bioindicators. The aim herein was to determine the vulnerability of S. lermae to NO3-N exposure at realistic concentrations registered in freshwater ecosystems in central Mexico where the species was historically reported. Offspring of S. lermae were chronically exposed during 60 days to concentrations of 5, 10 and 20 mg NO3-N/L, with 2 mg NO3-N/L used as the reference value (control). Survival rate, feeding behavior, aquatic surface respiration, body growth, scaled mass index, immature red blood cells, as well as histopathological changes in branchial, hepatic and gonadal tissues were evaluated. Additionally, this study analyzed water quality in freshwater ecosystems where S. lermae presently persists. The results showed decreased survival as NO3-N concentration increased, as well as increased feeding latency, aquatic surface respiration and histological damage in the gills and liver. These organs showed differential sex-dependent responses to NO3-N exposure; females were more sensitive than males. In the ovaries, a decreased density of stage III oocytes was associated with increased NO3-N concentrations. No changes were observed in body growth and number of immature red blood cells. Concentrations recorded in the three freshwater ecosystems that S. lermae inhabit were below 2 mg NO3-N/L. Together, the results could explain why the species has disappeared from more contaminated freshwater ecosystems where NO3-N levels exceed 5 mg/L. Moreover, the study warns about the risks of increasing NO3-N concentrations in the current sites where the species lives. [ABSTRACT FROM AUTHOR]

Stocking density has a significant influence on animal homeostasis, and so is a crucial factor in the management of organisms in captivity and an important role in ex situ conservation of endangered species. Hypancistrus zebra is an endemic fish species from the Xingu River, Brazil—considered Critically Endangered since the early 2000s due to fishing pressure and, more recently, due to habitat changes resulting from the construction of the Belo Monte Hydroelectric Dam. The implementation of appropriate management in captivity is of utmost importance for its conservation, particularly in enhancing ex situ breeding techniques. This study aimed to evaluate the growth of juvenile H. zebra in aquaria with three different stocking density levels. The juveniles were distributed between three treatments, each with three replicates. We found no correlation between biomass and aquarium volume. The highest correlation coefficients were found between growth rates and densities related to the number of individuals per shelter area. Mortality rate was directly related to the initial number of fish in the aquariums, with larger populations showing higher mortality rates. The treatment with lower stocking density displayed the highest growth rate, while the treatment with higher density had the lowest growth rate, with a 23.38% difference between them. [ABSTRACT FROM AUTHOR]

The article in the Journal of Fish Diseases discusses a case of spontaneous fibromyxoma in senescent turquoise killifish Nothobranchius furzeri, a model organism in aging research. The tumour was predominantly formed by connective tissue and displayed typical traits of fibromyxoma. The research findings suggest that the tumour was not pathogen-induced, and its rapid growth was likely associated with senescence. This report provides valuable insights into spontaneous neoplasia in Nothobranchius furzeri, a species previously not considered tumour-prone. [Extracted from the article]

The Chinese bahaba (Bahaba taipingensis), belonging to the Sciaenidae family, is one of the largest croakers with a limited geographical distribution. It is a critically endangered fish species according to the IUCN and a protected animal in China. In this study, the morphological characteristics of Chinese bahaba were observed and analyzed across different developmental stages, namely, the embryonic, larval, juvenile, and young fish stages. The results demonstrated that the mature eggs had a terminal yolk and a single oil globule. The eggs remained floating, and the mean diameters of the fertilized egg and oil globules were 1.14 ± 0.09 mm and 0.35 ± 0.07 mm, respectively. The findings revealed that the embryonic development of Chinese bahaba occurs broadly in seven stages, including the blastogenesis, cleavage, blastocyst, gastrula, neuro embryonic, organ differentiation, and membrane emergence stages, which lasted approximately 27 h and 10 min until hatching under 22.5 ± 0.5 °C. After 70 d, the larvae developed into young fish with a mean total length and body length of 97.75 ± 12.61 and 75.27 ± 13.27 mm, respectively. The digestive organs and the swim bladder began to differentiate, and the swim bladder, bladder duct, intestine, stomach, and mouth gradually formed at 2 d after hatching. Juvenile development occurred via six stages and there were certain differences in the morphological characteristics of Chinese bahaba across the different stages of growth and development. This study provides a theoretical reference for studying the growth, development, and artificial breeding of Chinese bahaba. [ABSTRACT FROM AUTHOR]

The wild population of Eutropiichthys vacha, Ompok pabda, Chitala chitala and Botio lohachata has been declined and is presently listed under threatened category owing to reduced abundance. A total of 308 samples of 4 threatened species representing three order and four families were procured during April 2019 to March 2020 from 6 different sites (Ghazipur, Jaunpur, Sultanpur, Barabanki, Lucknow and Sitapur) of River Gomti. A minimum of 10 female individuals were selected to study the biometry, health and absolute fecundity. In the present study, the value of regression coefficient (b) obtained via length-weight relationship showed high heterogeneity where a fish showed isometric (b = 3), negative (b < 3) to positive (b > 3) allometric growth patterns. Fulton's condition factor (K) was relatively found to be less than "1" which reflects the depletion in energy reserves except in B. lohachata collected from Jaunpur. The absolute fecundity varied from 1604 to 9049 and significantly different at the level of p < 0.001. In E. vacha, the "b" value ranges between 1.006–3.035, in O. pabda, the "b" value ranges between 2.628–3.469 and in C. chitala the "b" value was 2.426 and 3.255 and in B. lohachata the value of "b" was 3.180 which are considered to be the new record for fishdata base. The present study contributes the comparative description of growth pattern, well being and reproductive parameters of selected threatened fishes of River Gomti, India. The information gathered will be considered as baseline knowledge for future research to contemplate management strategies with reference to selected threatened fish species. [ABSTRACT FROM AUTHOR]

Gaining insight into the diversity, structure, and metabolic functions of microbial communities is essential for understanding their roles in host health and ecosystem dynamics. However, research on the seahorse-associated microbiome remains limited, despite these threatened fish facing increasing human pressures worldwide. Here, we explored the microbial diversity and metabolic functions of the skin and gut of the tiger tail seahorse (Hippocampus comes) and its surrounding environment using shotgun metagenomics and bioinformatics. Members of the Pseudomonadota phylum were dominant in the skin microbiome, whereas Bacteroidota was dominant in the gut. Bacillota, Actinomycetota, and Planctomycetota were also detected in the seahorse-associated microbiome. Statistical analysis revealed significant differences (P < 0.01) in species diversity between skin and gut microbiomes, with members belonging to the Moraxellaceae family being dominant on the skin and the Bacteroidaceae family in the gut. Moreover, the surrounding environment (water or sediment) did not have a direct effect on the seahorse microbiome composition. The skin microbiome exhibited a higher abundance of functional genes related to energy, lipid, and amino acid metabolism as well as terpenoids and polyketides metabolism, xenobiotics biodegradation, and metabolism compared with the gut. Despite differences among classes, the total abundance of bacteriocins was similar in both gut and skin microbiomes, which is significant in shaping microbial communities due to their antimicrobial properties. A better knowledge of seahorse microbiomes benefits conservation and sustainable aquaculture efforts, offering insights into habitat protection, disease management, and optimizing aquaculture environments, thereby promoting seahorse health and welfare while minimizing environmental impact and enhancing aquaculture sustainability. NEW & NOTEWORTHY: To the best of our knowledge, this study represents the first comprehensive examination of the taxonomic and functional patterns of the skin and gut microbiome in the tiger tail seahorse. These findings have the potential to significantly enhance our understanding of the seahorse-associated microbiome, thereby contributing to the prediction and control of bacterial infections in seahorses, which are a leading cause of high mass mortality rates in seahorse aquaculture and other fish species. [ABSTRACT FROM AUTHOR]

Species occupying dryland river ecosystems often experience "boom‐and‐bust" demographic cycles that coincide with shifts in habitat availability. Knowing whether declines are within natural thresholds versus those caused by acute human disturbance is critical for managing protected species. We investigated temporal shifts in abundance and habitat use of an endangered population of the threespine stickleback Gasterosteus aculeatus in southern California, where a Mediterranean climate leads to ephemeral habitat in one of the regions' least hydrologically modified rivers, the Santa Clara River. We conducted population surveys over a period of below‐average rainfall in the upper watershed in Soledad Canyon, with predefined reaches surveyed multiple times per year to capture different hydrologic conditions. Abundances were stable across years but varied significantly depending on location, with some reaches remaining dry and others drying seasonally to varying degrees. Occupancy models showed that the presence of stable perennial reaches, drying regime, and other site‐specific factors were important predictors of habitat use, and that certain reaches may be key to ensuring source‐sink dynamics as flow dissipates over the dry season. Low occupancy in two sections was driven by different predominant mechanisms, one by diel cycles of evapotranspiration and the other by cattails (Typha spp.), with both having greater effects during the hotter, drier parts of the year. As dryland river ecosystems are vulnerable to the effects of anthropogenic‐induced climate change, this study demonstrates how temporal monitoring can delimit dry‐state benchmarks for improving management interventions (i.e., translocation and habitat restoration) for protected species under conditions that are predicted to worsen in the coming years. [ABSTRACT FROM AUTHOR]

Animal personalities are differences in behaviour among individuals of the same species that are consistent over time and contexts. The integration of animal personality into conservation actions is hampered by limited understanding of personality traits in non‐model organisms. We estimated repeatabilities and correlations between behaviours in Nannoperca vittata (western pygmy perch), a threatened freshwater fish species endemic to south‐western Australia, and, for comparative purposes, in Poecilia reticulata (guppy), a species frequently used in personality research. Each fish was measured four times for each of seven behavioural variables, presumed to reflect five underlying personality traits. Track length (TL) was used as a measure of activity; time spent in a risky zone (RZ) and time to emergence (ET) for boldness; latency to approach a novel object (LA) and time spent close to the object (TS) for exploration; time spent close to a mirror (CV) for sociability and number of attacks at the mirror (AT) for aggression. Four behavioural variables (TL, RZ, ET and CV) were significantly repeatable in N. vittata and also in P. reticulata. There was a lack of concordance in phenotypic and among‐individual correlations between variables in both species, emphasising the importance of partitioning trait covariances to infer behavioural syndromes. Significant among‐individual correlations were found between activity, boldness and sociability variables in N. vittata, consistent with a proactive‐reactive personality axis, but not in P. reticulata. Personality variation should be considered in conservation actions for N. vittata, particularly with respect to unintended consequences of domestication selection in captive breeding and release. [ABSTRACT FROM AUTHOR]

The Common Black Carp Strain (Cyprinus carpio var. baisenensis), known for its black skin, is commonly cultured in the integrated rice-agriculture (IRA) system in Guangxi province, China. This study aimed to compare the genetic diversity of three common carp strains/populations (Common Black Carp Strain, Huanghe, and Songpu) using resequencing data. The genome-based method reveals a significant difference (p < 0.05) in identified loci and SNP frequency (p < 1 × 10−6) between the Songpu (Sp) or mirror carp and Huanghe (Hh) new strain. Additionally, the Common Black Carp Strain (Bk) exhibits a higher number of Tajima's D values, possibly due to its population size and mutations within its entire genome. The average value of population nucleotide diversity (π) for the Bk is 1.706 × 10−4 while the mean number for the Hh and Sp strains is 1.691 × 10−4 Heterozygosity analysis results indicate that the Bk has the highest F coefficient compared to the Sp and Hh hybrids. This suggests that the isolated population of the Bk may have experienced a decrease in population size as a result of environmental disturbances in the IRA system. PCA results further reveal that all individuals of the Bk, except for one, are clustered together, while individuals of the Hh form a separate group. On the other hand, Sp displays a distinct distribution pattern. The comparative study of the genetic diversity of the Bk provides baseline data on its genome makeup. Assessing genetic diversity and genetic structure is critical for fisheries management and the conservation of critically endangered fish species. [ABSTRACT FROM AUTHOR]

The lake minnow Eupallasella percnurus is a small leuciscid fish. In Poland, this species has been in a continuous decline since the mid-20th century and is presently considered as a extremely endangered. According to Polish law, E. percnurus is a strictly protected species that requires active conservation measures. In Poland, one the most common and effective measure of active protection E. percnurus is initiation of new populations. For this purpose, in 2004–2012, juvenile individuals originating from aquaculture conditions were translocated to group of isolated water bodies not inhabited by this species. The juveniles were offspring of parental fish belonging to the same local population, which is extinct at present. Five of those attempts were successful. The aim of the present study was to assess the genetic variation in a group new populations and compare genetic variation indicators with 13 old populations that had existed for decades. The polymorphism of 13 microsatellite markers was investigated, significance of differences in the genetic variation indicators between the groups were tested using a one-way analysis of variance (ANOVA). The mean values of all summary statistics under study, i.e. observed heterozygosity, expected heterozygosity and the total number of alleles, were higher in the group of new populations compared to almost all old ones. A similar dependence was found for Garza—Williamson M values, where the mean for the group of new populations was higher than in almost all old populations. Our results indicate that all recently established E. percnurus populations have not yet experienced any extensive founder effects or bottlenecks. They have preserved a large part of the genetic variability typical of their maternal population, which might also have been relatively high. This feature of new populations, may give them a relatively high ability to adapt to changing environments in the future. [ABSTRACT FROM AUTHOR]

Coastal areas host a major part of marine biodiversity but are seriously threatened by ever‐increasing human pressures. Transforming natural coastlines into urban seascapes through habitat artificialization may result in loss of biodiversity and key ecosystem functions. Yet, the extent to which seaports differ from nearby natural habitats and marine reserves across the whole Tree of Life is still unknown. This study aimed to assess the level of α and β‐diversity between seaports and reserves, and whether these biodiversity patterns are conserved across taxa and evolutionary lineages. For that, we used environmental DNA (eDNA) metabarcoding to survey six seaports on the French Mediterranean coast and four strictly no‐take marine reserves nearby. By targeting four different groups—prokaryotes, eukaryotes, metazoans and fish—with appropriate markers, we provide a holistic view of biodiversity on contrasted habitats. In the absence of comprehensive reference databases, we used bioinformatic pipelines to gather similar sequences into molecular operational taxonomic units (MOTUs). In contrast to our expectations, we obtained no difference in MOTU richness (α‐diversity) between habitats except for prokaryotes and threatened fishes with higher diversity in reserves than in seaports. However, we observed a marked dissimilarity (β‐diversity) between seaports and reserves for all taxa. Surprisingly, this biodiversity signature of seaports was preserved across the Tree of Life, up to the order. This result reveals that seaports and nearby marine reserves share few taxa and evolutionary lineages along urbanized coasts and suggests major differences in terms of ecosystem functioning between both habitats. [ABSTRACT FROM AUTHOR]

We comprehensively assessed marine recreational fishing of threatened guitarfishes (Pseudobatos percellens, Pseudobatos horkelii, and Zapteryx brevirostris) in Southern and Southeastern regions of Brazil. A dual‐method approach integrated social media reports and in‐person interviews to explore practices, challenges, and conservation implications of recreational fishing for these species. Seasonal capture of guitarfishes required the promotion of catch‐and‐release, especially during gestational periods, and the development of best angling practices manuals and accessible identification guides. With a focus on land‐based activities and engagement with fishers, our findings underscored the importance of collaboration among researchers, recreational fishers, and conservationists to map and monitor recreational capture and contribute to guitarfish research and conservation. [ABSTRACT FROM AUTHOR]

Objective: The objective of this study was to estimate the population size and annual recruitment of Shortnose Sturgeon Accipenser brevirostrum in the Altamaha River estuary in Georgia, United States, during an 11‐year period. The Shortnose Sturgeon is an endangered fish species that occupies rivers on the east coast of North America. Previous studies have suggested that the Altamaha River supports the largest population of Shortnose Sturgeon in the southern United States; however, the status of the population has not been assessed in 11 years. Methods: We used entanglement gears to capture Shortnose Sturgeon in the Altamaha River estuary, marked them, and used an established statistical method (closed‐population capture–mark–recapture models) to annually estimate total population size and age‐1 recruitment from 2012 to 2022. Result: We were able to estimate the size of age‐1 cohorts in 7 of the 11 years of data collection. Point estimates of annual age‐1 recruitment varied between 113 and 1021 individuals, and total population size varied between 452 (95% confidence interval [CI]: 116–2277) and 5054 individuals (95% CI: 2155–13,267). Conclusion: Recruitment of age‐1 juveniles was variable, suggesting that reproduction success is inconsistent between years in the Altamaha River. The results of this study, in combination with previous work, do not show any clear trends in Altamaha River Shortnose Sturgeon population abundance or recruitment. The population seems to be stable, but in the absence of historical population numbers, it is unclear whether the population should be considered recovered or is stagnated in its recovery. Impact statementEleven years of data indicate that the Altamaha River population of Shortnose Sturgeon appears stable but reproduction is not successful every year. In the absence of historical data, it is unclear whether the population is recovered or stagnated in its recovery. [ABSTRACT FROM AUTHOR]

A population of weatherloach has been detected in the Schadgraben stream at Geispolsheim, near Strasbourg (Rhine catchment, eastern France) in autumn 2021 and identified as the European weatherloach Misgurnus fossilis. Another sixty-three weatherloaches were caught in this location the 19 May 2022. The examination of morphological and molecular data on preserved specimens allows to identify this population as the dojo weatherloch Misgurnus anguillicaudatus, an invasive species. The length-weight relationship of this population highlights its good health and establishment. A review of the knowledges about this invasive species is also done. Monitoring must be put in place in order to follow its propagation, threatening native fishes like the endangered M. fossilis. [ABSTRACT FROM AUTHOR]

Globally, suitable freshwater habitats are undergoing alterations and fish population declines, primarily attributed to the swift changes in climate and land use. Developing an effective conservation policy for freshwater fish necessitates careful consideration of the impacts of climatic and spatial factors. This study focuses on the analysis of 64 threatened freshwater fish in Bangladesh to anticipate their current and future climatically suitable habitats, utilizing the bioclim() species distribution model. Additionally, this study examines existing inland hydrographic networks and their corresponding harvest rates. The findings indicate that approximately 75% of the area of occupancy for the studied species is currently climatically suitable, but this is expected to decrease to 13% under future climate scenarios. Notably, 27 threatened species are at risk of lacking climatically suitable habitats in the future within their current area of occupancy. The three components of hydrographic networks—floodplains, rivers, and natural lakes—play varying roles in providing a climatically suitable habitat for the studied species. For instance, only 34% of threatened fish species are projected to find a suitable habitat over flood areas, 23% over rivers, and 16% over lakes. Existing protected areas presently offer limited protection (21% suitable area), expected to decline to 6% in the future, with no dedicated protected areas for freshwater fish. Floodplains are highlighted for providing habitat connectivity and facilitating brood fish dispersal. However, the unregulated and unmonitored annual harvest of freshwater fish from floodwater and rivers poses a potential silent cause for rapid population decline. Prioritizing the management of hydrographic components to maintain habitat connectivity, legal protection for threatened fish species, and establishing permanent protected areas for fish are crucial aspects in developing a conservation policy to mitigate the impact of future climate scenarios on threatened freshwater species. [ABSTRACT FROM AUTHOR]

Sturgeons are among the most endangered fishes in the world. Identifying habitat use characteristics to inform restoration projects is crucial for recovery. However, small sample sizes, inadequate replication of studies, and limited spatial extents complicate our ability to effectively apply the findings of single studies to endangered species conservation across the larger riverscape. We synthesized information from amphidromous and anadromous sturgeons in North America to identify species-specific knowledge gaps and conduct a quantitative comparison of species–habitat relationships. We provided a qualitative summary of substrate use and synthesized estimates of depth and velocity during spawning and non-spawning activity. Generalized patterns among species were identified, such as spawning in fast water on hard substrate and then using slow water with soft substrate areas when not spawning. We noted species-specific variability during spawning that may be attributed to historical maximum length, egg characteristics, and watershed features. This study provides some of the first estimates of habitat use that can be adapted for many populations. Results can contribute to empirically grounded decision-support tools used to prioritize information needs for recovery. [ABSTRACT FROM AUTHOR]

Identifying the most important factors affecting population growth in animal life cycles is an important tool of species conservation. Delta Smelt (Hypomesus transpacificus), an annual fish endemic to the San Francisco Estuary in California (USA), has been provided legal protection since 1993 but 30 years later exists in a conservation‐reliant state on the brink of extinction. Despite considerable controversies about what factors are most responsible for the species' decline, no population growth rate sensitivity comparisons between the most important factors regulating growth have been done. Nor has anyone attempted to quantitatively identify habitat conditions needed to support positive population growth. We developed a set of stage‐structured population models to link habitat indices regulating recruitment of new generations of fish as they metamorphosed into juveniles and the subsequent survival of those fish over several seasons until they reached adulthood. These models are used to quantify drivers of growth rate variation over 30 years. Several complimentary sensitivity analyses indicated freshwater outflow to the estuary during summer had the largest potential to change population growth. Multiple habitat metrics (e.g., food availability, temperature) influencing recruitment and life stage specific survival rates across different seasons interacted in nonlinear ways to determine habitat conditions and water management targets associated with positive population growth. We discuss the implications for freshwater management, Delta Smelt conservation, and the challenges climate change poses for co‐implementation of these two societal priorities. [ABSTRACT FROM AUTHOR]

Like most rivers worldwide, the transboundary North American Kootenay/i River has experienced multiple impacts including watershed developments, river channelization, and floodplain clearing, draining, and diking. Construction of Libby Dam was authorized by the 1964 Columbia River Treaty (CRT) between the United States and Canada, and in 1975 began regulating downstream flows for flood risk management and hydropower generation. Following cumulative impacts, the endemic Kootenai River White Sturgeon population collapsed and was designated as endangered in 1994 (U.S. Endangered Species Act). Subsequent Biological Opinions from the U.S. Fish and Wildlife Service prescribed Libby Dam operations to provide springtime flow pulses for sturgeon spawning. These provided the unanticipated benefit of substantial seedling recruitment of native and introduced riparian cottonwoods and willows. The regulated flow regime was further adaptively managed to provide a more normative (natural) regime, to balance ecological functions with flood risk management and hydropower generation. The broadened ecological considerations would be consistent with the proposed priorities for the modernization of the international CRT. The observed responses revealed that (1) diverse aquatic and riparian organisms are dependent on common river flow characteristics; (2) a normalized flow regime provided substantial ecological benefits; and (3) due to multiple influences, hybrid ecosystems develop along regulated rivers, with a blending of natural and altered processes and communities. For other regulated rivers, we recommend that (1) high springtime flows be allowed, as feasible; (2) followed by the gradual post‐peak recession; and (3) the maintenance of sufficient flows through the warm and dry interval of mid to late summer. [ABSTRACT FROM AUTHOR]

Population estimates are required for effective conservation of many rare marine species, but can be difficult to obtain. The critically endangered red handfish (Thymichthys politus) is a coastal anglerfish known only from two fragmented populations in southeast Tasmania, Australia. It is at a high risk of extinction due to low numbers, loss of habitat, and the impacts of climate change. To aid conservation efforts, we provide the first empirical population size estimates of red handfish and investigate other important aspects of the species' life history, such as growth, habitat association, and movement. We surveyed both red handfish local populations via underwater visual census on scuba over 3 years and used photographic mark‐recapture techniques to estimate biological parameters. In 2020, the local adult population size was estimated to be 94 (95% confidence interval [CI] 40–231) adults at one site, and 7 (95% CI 5–10) at the other site, suggesting an estimated global population of 101 adults. Movement of individuals was extremely limited at 48.5 m (± 77.7 S.D.) per year. We also found evidence of declining fish density, a declining proportion of juveniles, and increasing average fish size during the study. These results provide a serious warning that red handfish are likely sliding toward extinction, and highlight the urgent need to expand efforts for ex situ captive breeding to bolster numbers in the wild and maintain captive insurance populations, and to protect vital habitat to safeguard the species' ongoing survival in the wild. [ABSTRACT FROM AUTHOR]

Endangered wild fish populations are commonly supported by hatchery propagation. However, hatchery‐reared fish experience very different selective pressures compared to their wild counterparts, potentially causing genotype‐by‐environment interactions (G × E) in essential fitness traits. We experimentally studied early selection in a critically endangered landlocked Atlantic salmon population, first from fertilization to the swim‐up stage in a common hatchery setting, and thereafter until the age of 5 months in two contrasting rearing environments. Swim‐up progeny were moved either to standard indoor hatchery tanks involving conventional husbandry or to seminatural outdoor channels providing only natural food. After the first summer, sampled survivors were assigned to their families by genotyping. Early survival until the swim‐up stage was mostly determined by maternal effects, but also involved significant variation due to sires and full‐sib families (potential genetic effects). High on‐growing survival in hatchery tanks (88.7%) maintained a more even distribution among families (relative share 1.5%–4.2%) than the seminatural environment (0.0%–5.4%). This heterogeneity was mostly maternal, whereas no independent paternal effect occurred. Heritability estimates were high for body size traits in both environments (0.62–0.69). Genetic correlations between the environments were significantly positive for body size traits (0.67–0.69), and high body condition in hatchery was also genetically linked to rapid growth in the seminatural environment (0.54). Additive and phenotypic growth variation increased in the seminatural environment, but scaling effects probably played a less significant role for G × E, compared to re‐ranking of genotypes. Our results suggest that not only maternal effects, but also genetic effects, direct selection according to the environmental conditions experienced. Consistently high genetic variation in growth implies that, despite its low overall genetic diversity and long history in captive rearing (>50 years), this landlocked Atlantic salmon population still possesses adaptive potential for response to change from hatchery rearing back to more natural conditions. [ABSTRACT FROM AUTHOR]

Augmenting depleted genetic diversity can improve the fitness and evolutionary potential of wildlife populations, but developing effective management approaches requires genetically monitored test cases. One such case is the small, isolated and inbred Cotter River population of an endangered Australian freshwater fish, the Macquarie perch Macquaria australasica, which over 3 years (2017–2019) received 71 translocated migrants from a closely related, genetically more diverse population. We used genetic monitoring to test whether immigrants bred, interbred with local fish and augmented population genetic diversity. We also investigated whether levels of river flow affected recruitment, inbreeding depression and juvenile dispersal. Fish length was used to estimate the age, birth year cohort and growth of 524 individuals born between 2016 and 2020 under variable flow conditions. DArT genome‐wide genotypes were used to assess individual ancestry, heterozygosity, short‐term effective population size and identify parent‐offspring and full‐sibling families. Of 442 individuals born after translocations commenced, only two (0.45%) were of mixed ancestry; these were half‐sibs with one translocated parent in common. Numbers of breeders and genetic diversity for five birth year cohorts of the Cotter River fish were low, especially in low‐flow years. Additionally, individuals born in the year of lowest flow evidently suffered from inbreeding depression for juvenile growth. The year of highest flow was associated with the largest number of breeders, lowest inbreeding in the offspring and greatest juvenile dispersal distances. Genetic diversity decreased in the upstream direction, flagging restricted access of breeders to the most upstream breeding sites, exacerbated by low river flow. Our results suggest that the effectiveness of translocations could be increased by focussing on upstream sites and moving more individuals per year; using riverine sources should be considered. Our results indicate that river flow sufficient to facilitate fish movement through the system would increase the number of breeders, promote individuals' growth, reduce inbreeding depression and promote genetic rescue. [ABSTRACT FROM AUTHOR]

Headwaters are important refuges for threatened fishes and play an important role in their conservation. The effects of large flooding events on headwater fish assemblages are under studied in southern Africa. In January 2012, heavy rainfall resulted in a large flood in the upper Klaserie River, Olifants River, and Limpopo River System, South Africa. This flood had an estimated return level of 225 years and caused significant ecological and economic damage. This study aimed to explore the impact of a large flood on the fish assemblage and substrate in the headwaters of a subtropical stream. The fish communities and selected substrate variables were collected at 10 sites. Sites were sampled at six monthly intervals thrice before and thrice after the flood. Fish were collected by electrofishing and substrate quantified visually. Changes in habitat resulted from the flood included the scouring of gravel sand and mud, greater exposure of bedrock and boulder substrates, increased stream width, and decreased stream depth in all zones. The fish community showed an increase in abundance for all but three species, colonisation of upstream sites, and colonisation of the study area by six species. The majority of fish species have opportunistic life‐history traits, which could explain the rapid colonisation and increase in abundance. The flood occurred in the middle of the breeding season for most the fish species, and this resulted in an increase of abundance for these species and provided opportunities for upstream colonisation. The impact of late season and aseasonal large floods requires further study. [ABSTRACT FROM AUTHOR]

Currently, the effects of dietary levels of n-3 highly unsaturated fatty acids (HUFAs) on the growth performance, antioxidant capacity, immunity, and serum oxylipin profiles of female F2-generation Yangtze sturgeon remain unknown. A total of 75 Yangtze sturgeons, an endangered freshwater fish species, with an average body weight of 3.60 ± 0.83 kg, were randomly allocated to 15 concrete pools, with each dietary group represented by 5 fish per pool. The fish were fed five different experimental diets containing various levels of n-3 HUFAs (0.5%, 1.0%, 1.5%, 2.0%, and 2.4%). After a feeding period of 5 months, no significant differences in the growth performances of the fish were observed among the five dietary groups (p > 0.05). However, we did note that the serum levels of low-density lipoprotein cholesterol (LDL-C), triglycerides (TGs), and total cholesterol (TCHO) exhibited a marked increase in the fish that consumed higher dietary n-3 HUFA levels (p < 0.05). Conversely, alkaline phosphatase (ALP) activities showed a notable decrease as dietary n-3 HUFA levels increased (p < 0.05). Serum antioxidant indices, such as the activity levels of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), were significantly higher in the 2.4% HUFA group compared to the 0.5% HUFA group. Additionally, muscle antioxidant indices, including total antioxidant capacity (T-AOC), catalase (CAT), and SOD activity, exhibited notable increases as dietary n-3 HUFA levels increased (p < 0.05). Furthermore, there was a decrease in malondialdehyde (MDA) levels as dietary n-3 HUFA levels increased (p < 0.05). In relation to immune indices, only serum immunoglobulin M (IgM) and muscle complement 3 (C3) were found to be influenced by dietary n-3 HUFA levels (p < 0.05). A total of 80 oxylipins were quantified, and our subsequent K-means cluster analysis resulted in the classification of 62 oxylipins into 10 subclasses. Among the different n-3 HUFA diets, a total of 14 differential oxylipins were identified in the sera. These findings demonstrate that dietary supplementation with n-3 HUFAs exceeding a 1.0% level can enhance antioxidant capacity and regulate serum lipid metabolism, potentially through modulation of oxylipins derived from ARA, DHA, and EPA. These insights provide novel perspectives on the mechanisms underlying these observations. [ABSTRACT FROM AUTHOR]

Dianchi Lake is a eutrophic lake in Yunnan, China with many endangered or extinct native fish species. We sought to explore the fish community using traditional netting and trapping methods and, for the critically endangered golden-line barbel Sinocyclocheilus grahami, environmental DNA (eDNA) to detect its presence and distribution in the lake. Using traditional netting and trapping methods, we found that the fish community has been almost completely converted to non-native species (~ 97% of abundance). Loss of native fishes was serious, with ~ 77% of historical species not detected. We designed sensitive species-specific primers for the mitochondrial COI gene specific to S. grahami and sampled eDNA throughout the lake. While only two individuals of S. grahami were caught using traditional methods, eDNA analysis revealed its presence across the central and southern regions of the lake. While it is unclear if the fish detected represent a recovering population or recently stocked individuals, occurrence records for the fish were inversely related to regions of the lake that suffered serious cyanobacteria blooms. This study highlights the sensitivity and utility of eDNA for non-destructively detecting presence of an endangered fish species. [ABSTRACT FROM AUTHOR]

Black ruff (Centrolophus niger) is a rare and poorly studied species found in both the Atlantic and Pacific Oceans and also in the Mediterranean Sea. It is sporadically caught south of Iceland during the annual International Ecosystem Summer Survey of the Nordic Seas. In total, 43 specimens were caught from 2009 to 2021, of which 41 specimens were caught during 2017–2021. All specimens, except one, were caught using a pelagic trawl (cod‐end mesh‐size: 50 mm) close to the surface (trawl depth: 0–35 m) with in situ temperature ranging from 9 to 13°C. The area south of Iceland is characterized by having warmer temperatures than other areas around the island, which might be indicative of a northern limit for the distribution of black ruff. The fish were primarily in the range of 29–46 cm with a few larger individuals up to 71 cm. Fourteen fish, caught in 2017 and 2021, were dissected to gather biological information on this species. These fish were all juveniles with no obvious sign of gonad development. Correlations between total length, fork length, and standard length are presented. Otoliths were thin and delicate with a length of ~13–16 mm, and otolith size (length, width, and area) was correlated with fish size. Much of the stomach content was at an advanced stage of digestion, but some contents could be identified and consisted of invertebrates, primarily of the orders Amphipoda and Calanoida with some unidentified fish also present. [ABSTRACT FROM AUTHOR]

Bitterling fishes evolve an idiosyncratic symbiosis with freshwater mussels, in which they are obligated to spawn in the gills of mussels for reproduction. In recent years, freshwater mussel populations have been drastically diminishing, due to accelerating anthropogenic impacts, which can be large threats to the risk of bitterling's extinction cascade (i.e. 'coextinction'). The host mussel size may be an important factor driving the adaptation and evolution of bitterling's reproductive phenotypes. Here we examined the host size preference and morphological adaptation of female bitterling to the host size from 17 localities at the Han River in Korea. Using our developed molecular‐based species identification for bitterling's eggs/larvae inside the mussels, we further determined the spawning patterns of seven bitterling species. Mean length of spawned mussels (N = 453) was significantly larger than that of unspawned mussels (N = 1814), suggesting that bitterling prefers to use larger hosts as a spawning ground. Spawning probability was clearly greater as mussel size increases. Results of our reciprocal transplant experiments do provide some evidence supporting the 'bitterling's larger host preference' hypothesis. Interspecific competition appeared to be intense as two fish species often spawned eggs in the same mussel individuals simultaneously. Longer ovipositor and more elongated egg may evolve in females of Tanakia signifer in response to larger host environments. The observed bitterling's spawning preference for large‐sized mussels may evolve perhaps because of the fitness advantage in relation to the offspring survival. Our findings further inform on the development of effective conservation and management strategy for the endangered bitterling fishes. [ABSTRACT FROM AUTHOR]

Animal body-size variation influences multiple processes in marine ecosystems, but habitat heterogeneity has prevented a comprehensive assessment of size across pelagic (midwater) and benthic (seabed) systems along anthropic gradients. In this work, we derive fish size indicators from 17,411 stereo baited-video deployments to test for differences between pelagic and benthic responses to remoteness from human pressures and effectiveness of marine protected areas (MPAs). From records of 823,849 individual fish, we report divergent responses between systems, with pelagic size structure more profoundly eroded near human markets than benthic size structure, signifying greater vulnerability of pelagic systems to human pressure. Effective protection of benthic size structure can be achieved through MPAs placed near markets, thereby contributing to benthic habitat restoration and the recovery of associated fishes. By contrast, recovery of the world's largest and most endangered fishes in pelagic systems requires the creation of highly protected areas in remote locations, including on the High Seas, where protection efforts lag. [ABSTRACT FROM AUTHOR]

Climate changes have altered biodiversity and ultimately induced community changes that have threatened the survival of certain aquatic organisms such as fish species. Obtaining biological and genetic information on endangered fish species is critical for ecological population management. Thamnaconus multilineatus, registered as an endangered species by the IUCN in 2019, is a Data Deficient (DD) species with a remarkably small number of habitats worldwide and no known information other than its habitat and external form. In this study, we characterized the external and osteological morphology of a T. multilineatus specimen collected from eastern Jeju Island, South Korea, in 2020. We also investigated the phylogenetic relationships among related fish species through complete mitochondrial DNA (mtDNA) analysis of the T. multilineatus specimen. The external and skeletal characteristics of T. multilineatus were similar to those of previous reports describing other fish of the genus Thamnaconus, making it difficult to classify T. multilineatus as a similar species based only on morphological characteristics. As a result of analyzing the complete mtDNA of T. multilineatus, the length of the mtDNA was determined to be 16,435 bp, and the mitochondrial genome was found to have 37 CDCs, including 13 PCGs, 22 tRNAs, and 2 rRNAs. In the phylogenetic analysis within the suborder Balistoidei, T. multilineatus mtDNA formed a cluster with fish of the genus Thamnaconus. This study is the first to report on the skeletal structure and complete mtDNA of T. multilineatus. Since the current research on T. multilineatus has only been reported on morphology, the results of this study will be utilized as important information for the management and restoration of T. multilineatus as an endangered species and significant fishery resource. [ABSTRACT FROM AUTHOR]

Sawfishes are considered one of the most threatened marine fish taxa globally, with major contractions in their geographic range documented over the last 50 years across all oceans. Two sawfish species used to be found in Mexico, but a historical lack of research and first-hand baseline information recently raised concerns about the fate of both species there. Recent Local Ecological Knowledge (LEK) and environmental DNA (eDNA) research has shown that: a) Pristis pectinata and P. pristis were formerly abundant and widely distributed in the Mexican Atlantic coast, and the latter also in the Mexican Pacific coast, and b) sawfishes (at least P. pectinata) are still found in some coastal refugia in central Veracruz state and Campeche state on the Atlantic coast. In the present research we developed improved primers that allow for better species ID resolution for Pristis pectinata and P. pristis through successful sequencing of eDNA samples by amplifying a 270 bp fragment of the CO1 gene. Our results based on the analysis of 305 eDNA samples showed an improved positive species identification rate through sequencing, demonstrate that P. pectinata occurs in other coastal refugia along the Yucatán Peninsula in the Mexican Caribbean, and suggest this species might be extirpated from southern Veracruz, while no P. pristis eDNA has been found along the Atlantic coast. [ABSTRACT FROM AUTHOR]

Information on habitat transitions is critical to understand whether efforts to manage the freshwater phase of juvenile green sturgeon (Acipenser medirostris) rearing habitats are feasible or beneficial. We implanted microacoustic transmitters in naturally produced age-0 juvenile green sturgeon in the Sacramento River to evaluate residency, general spatial distribution, movement rates, and downstream migration patterns over 300 river kilometers. Furthermore, we investigated whether changes in three environmental variables (discharge, turbidity, and water temperature) were associated with the beginning of movements resulting in the transition from riverine to brackish waters during their outmigration to the Sacramento-San Joaquin Delta Estuary. We captured 183 juvenile green sturgeon by trawl of which 88 were implanted with microacoustic tags. An additional 10 individuals were collected and tagged from the Red Bluff Diversion Dam rotary-screw traps. Linear mixed models showed that individuals caught by trawl had longer upper river residence times, post-tagging, than those caught by trap. Intergate velocity models showed that trap-caught individuals relocated at higher upper river reach velocities than trawl-caught fish, but there were no differences between trap- and trawl-caught fish within the middle or lower reach. Detections showed that juvenile green sturgeon began the downstream transition to brackish water habitat when discharge and turbidity increased. Temperature was not found to significantly influence the initiation of downstream migration. Both continuous and stepped downstream migration patterns were observed in each of the four cohorts, with smaller individuals being more likely to exhibit stepped migration. These data provide information that was previously unknown about the life history of the southern distinct population segment of the North American green sturgeon and can be utilized to assist with water resource management and recovery of this threatened fish species. [ABSTRACT FROM AUTHOR]

While haplotype‐specific genetic load shapes the evolutionary trajectory of natural and captive populations, mixed‐haplotype assembly and genotyping hindered its characterization in diploids. Herein, we produced two phased genome assemblies of the critically endangered fish Chinese Bahaba (Bahaba taipingensis, Sciaenidae, Teleostei) and resequenced 20 whole genomes to quantify population genetic load at a haplotype level. We identified frame‐shifting variants as the most deleterious type, followed by mutations in the 5′‐UTR, 3′‐UTR and missense mutations at conserved amino acids. Phased haplotypes revealed gene deletions and high‐impact deleterious variants. We estimated ~1.12% of genes missing or interrupted per haplotype, with a significant overlap of disrupted genes (30.35%) between haplotype sets. Relative proportions of deleterious variant categories differed significantly between haplotypes. Simulations suggested that purifying selection struggled to purge slightly deleterious genetic load in captive breeding compared to genotyping interventions, and that higher inter‐haplotypic variance of genetic load predicted more efficient purging by artificial selection. Combining the knowledge of haplotype‐resolved genetic load with predictive modelling will be immensely useful for understanding the evolution of deleterious variants and guiding conservation planning. [ABSTRACT FROM AUTHOR]

The brown meagre (Sciaena umbra) is an endangered species, which requires specific protection measures to ensure its conservation. These measures need to be informed by high-quality scientific knowledge on their space use patterns. Here, we used acoustic telemetry to assess its seasonal movement patterns and habitat use within a marine protected area (MPA). Our results suggested that S. umbra is a highly sedentary species (home range < 1.0 km2) and, therefore, the MPA is extensive enough to protect the local population. Their population was discretely distributed in two main areas within the MPA, which was likely a result of habitat segregation and density-dependent movements. The temporal variability of their movements further uncovered when and where spawning occurs (mainly, but probably not only, in the fully protected area in June) and indicated that spillover of this species is limited but still possible. Overall, we highlight the importance of MPAs in the recovery of S. umbra, we advocate the need to perpetuate the current national fishing bans and extend it to other countries in the Mediterranean region, and we emphasize that considering the fine-scale movements of S. umbra in future management actions is key to achieving a successful recovery of their populations. [ABSTRACT FROM AUTHOR]

Human water use combined with a recent megadrought have reduced river and stream flow through the southwest United States and led to periodic drying of formerly perennial river segments. Reductions in snowmelt runoff and increased extent of drying collectively threaten short‐lived, obligate aquatic species, including the endangered Rio Grande silvery minnow (Hybognathus amarus). This species is subject to boom‐and‐bust population dynamics, under which large fluctuations in abundance are expected to lower estimates of effective population size and erode genetic diversity over time. Rates of diversity loss are also affected by additions of hatchery‐origin fish used to supplement the wild population. We used demographic and genetic data from wild and hatchery individuals to examine the relationship of genetic diversity and effective population size to abundance over the last two decades. Genetic diversity was low during the early 2000s, but diversity and demographic metrics stabilized after the hatchery program was initiated and environmental conditions improved. Yet, from 2017 onward, allelic diversity declined (Cohen's d = 1.34) and remained low despite hatchery stocking and brief wild population recovery. Across the time series, single‐sample estimates of effective population size based on linkage disequilibrium (LD Ne) were positively associated (r = 0.53) with wild abundance and total abundance, but as the proportion of hatchery‐origin spawners increased, LD Ne declined (r = −0.55). Megadrought limited wild spawner abundance and precluded refreshment of hatchery brood stocks with wild fish; hence, we predict a riverine population increasingly dominated by hatchery‐origin individuals and accelerated loss of genetic diversity despite supplementation. We recommend an adaptive and accelerated management plan that integrates river flow management and hatchery operations to slow the pace of genetic diversity loss exacerbated by megadrought. [ABSTRACT FROM AUTHOR]

The Media Luna spring, Mexico, is the main reservoir of the endemic and endangered fish Ataeniobius toweri. In the last decades, the ecosystem has been modified by tourism, and the habitat has changed for this species. Therefore, for better conservation management of the natural fish population, it is necessary to understand its abundance status and suitable habitat conditions, in ecological and spatial scenarios, on a temporal scale. In the present study, we modeled A. toweri's ecological responses and spatial distribution for adult and juvenile life stages, in three summer periods (years 1999, 2009, and 2019). As habitat variables, we used water depth and underwater coverage. Ecological response curves were obtained from a Generalized Linear Model; distribution models were obtained with DOMAIN. In the modeling evaluation, for the Linear Regression Model, we obtained true statistical skills metric > 0.30 and, for DOMAIN, an area under the curve (AUC) > 0.70 with an AUC ratio > 1.00. In general, as the summer periods progressed, we found the highest probability of occurrence (P > 0.20) and distribution (P > 0.60) in areas with conditions of large coverage of underwater vegetation, in the first 1.5 m of depth, and near the shores of the spring. Also, the variations of relative abundance were always observed at sites with these habitat conditions. Thus, we concluded that our models had the performance to discern between suitable and unsuitable habitat conditions for A. toweri, and that areas with little or no anthropogenic pressure are more important for this species. [ABSTRACT FROM AUTHOR]

Spermatogenesis is a systematically organized process that ensures uninterrupted sperm production in which the spermatogonial stem cells (SSCs) play a crucial role. However, the existing absence of teleost-specific molecular markers for SSCs presents a notable challenge. Herein we characterized phenotypically the spermatogonial stem cells using specific molecular markers and transmission electron microscopy. Moreover, we also describe a simple method to suppress common carp spermatogenesis using the combination of Busulfan and thermo-chemical treatment, and finally, we isolate and enrich the undifferentiated spermatogonial fraction. Our results showed that C-kit, GFRα1, and POU2 proteins were expressed by germ cells, meanwhile, undifferentiated spermatogonial populations preferentially expressed GFRα1 and POU2. Moreover, the combination of high temperature (35 °C) and Busulfan (40 mg/kg/BW) effectively suppressed the spermatogenesis of common carp males. Additionally, the amh expression analysis showed differences between the control (26 °C) when compared to 35 °C with a single or two Busulfan doses, confirming that the testes were depleted by the association of Busulfan at high temperatures. In an attempt to isolate the undifferentiated spermatogonial fraction, we used the Percoll discontinuous density gradient. Thus, we successfully dissociated the carp whole testes in different cellular fractions; subsequently, we isolated and enriched the undifferentiated spermatogonial population. Therefore, our results suggest that probably both GFRα-1 and POU2 are highly conserved factors expressed in common carp germinative epithelium and that these molecules were well conserved along the evolutionary process. Furthermore, the enriched undifferentiated spermatogonial population developed here can be used in further germ cell transplantation experiments to preserve and propagate valued and endangered fish species. • GFRα-1 and POU2 exhibit a strong presence in undifferentiated spermatogonia, indicating their conservation across vertebrate evolution. • Effective suppression of spermatogenesis via busulfan treatment and temperature elevation, improving SSC transplantation effectiveness. • Establishment of an effective protocol for isolating and enriching common carp (Cyprinus carpio) SSCs. [ABSTRACT FROM AUTHOR]