Migratory Fish Research Articles

Swimming behaviour of downstream migrating carp in accelerating flows

The downstream migration of fish is impacted by accelerating flow. This study coupled computational fluid dynamics (CFD) modelling and laboratory observations to investigate the effects of two different accelerating flow hydrodynamics on the swimming behaviours of two fish species. The silver carp (Hypophthalmichthys molitrix) and bighead carp (Hypophthalmichthys nobilis) were chosen as target fish species and four hydrodynamic parameters were selected as main indicators to affect fish swimming behaviours. Four specific swimming behaviours were classified in the contraction flume based on fish downstream trajectories. The results indicated that high accelerating flow, low turbulent kinetic energy (Tke ) and shear stress generated the symmetric distribution of fish along two boundaries of the contraction flume. It also indicated that discharge increase would increase the fish entrainment risk from intakes to diversion pipe, while two fish species exhibited different swimming behaviour patterns. Velocity and velocity gradient had more significant effects than other hydrodynamic parameters on fish swimming strategies. There was mutual compensation among fish tail-beat frequency, angle and amplitude. The results illustrated the relationship between the swimming behaviours and hydrodynamics, which could contribute to understanding the fish downstream migration behaviours in accelerating flow.

Enhancing migratory potential in fish passes: The role of pier shape in minimizing debris accumulation

Biodiversity preservation is a primary challenge of the 21st century, focusing on restoring unobstructed river flows and mitigating the effects of barriers, supported by European biodiversity strategies up to 2030. Maintaining ecological continuity, such as unblocking fishways clogged by floating debris disrupting natural fish migration paths, remains a challenge despite conventional protective methods. This study, taking a vertical slot fish pass in Wrocław on the Odra River as a case study and based on research on bridge piers, suggests modifying pier shapes from rectangular to rounded in order to reduce debris accumulation. Field studies, utilizing an OTT MF Pro flow meter, were conducted to validate the numerical model. The measured flow rate in the field was 3.15 [m³·s−1], while the numerical modeling yielded a flow of 3.19 [m³·s−1]. Focusing on optimizing the shape of cross-wall piers to enhance fish migration conditions, the study examined six different pier configurations, analyzing flow speed in the main slot, crucial for migration. Using 2D hydraulic modeling with Iber, it assessed the migratory potential of different pier designs by analyzing hydraulic conditions and comparing them with the swimming capabilities of fish species native to the Odra River. Results indicate that rounding the pier edges positively affects flow speeds in the main slot, enhancing fish migration possibilities, contributing to fish pass functionality improvement and supporting broader biodiversity and ecosystem health goals.

Unveiling the gut bacteriome diversity and distribution in the national fish hilsa (Tenualosa ilisha) of Bangladesh.

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

How fish populations in Lake Bafa (Western Anatolia) respond to ecological shifts

Abstract Long‐term biodiversity monitoring is crucial for freshwater ecosystems as it enables the detection of even subtle changes and biodiversity trends, guiding conservation efforts and ensuring the sustainability of these vital habitats. Despite becoming more commonly considered in the field of freshwater ecology and biological invasions, studies using long‐term time series from Türkiye's freshwater resources have remained scarce. To assess the availability of data and ultimately present a baseline for future efforts, we combined published museum records and samples from recent field studies from the highly anthropogenically‐altered Lake Bafa in Western Anatolia covering the period 1958–2019. Lake Bafa has a very diverse aquatic ecosystem, providing habitat for both freshwater and saltwater species, and is one of the allottees in Türkiye's inland fish production. In the current study, we investigated how fish populations in Lake Bafa were affected by environmental changes and examined changes in taxonomic and functional diversity of non‐native species over time. The analyses revealed—concomitant to an increase in native marine and freshwater species richness—an increase in non‐native species richness over time. Non‐native species did not interfere with native species' niche space, whereas applied models indicate that in this highly altered ecosystem, foremost temperature and salinity shaped the fish community over time, limiting the impacts of non‐native species. These results have implications for the fishery of the lake, which includes highly valuable catadromous fish species, highlighting the value and importance of collecting long‐term data in Türkiye to better understand both invasion dynamics and changes in the naturality of Turkish ecosystems. These findings further underline the importance of long‐term data to create new management strategies for the lake and to start restoration processes, thus improving fisheries management.

Ecological stoichiometric of C, N and P of Coilia species

Fish of the Coilia genus play a crucial ecological role by serving as a vital link between marine and freshwater ecosystems. To gain a comprehensive understanding of the ecological patterns, intra-interspecific variations, and potential determinants of Coilia from an ecological stoichiometry perspective, analyzed the concentrations of C, N, and P elements in various types of Coilia, including C. nasus, C. mystus, C. grayii, C. brachygnathus, and C. nasus taihuensis, across the Yangtze River basin. The result shows that: 1) A broad spectrum of eco-stoichiometric patterns within Coilia species, facilitating their adaptation to diverse ecosystems, including seawater and freshwater. Specifically, the C content in Coilia species from 40% to 70%, while N% and P% range between 4%–12% and 0.5%–3%, respectively. 2) Intriguingly, interspecific variations in eco-stoichiometric patterns are predominantly driven by anadromous populations. The anadromous Coilia species, including C. nasus, C. mystus, and C. grayii, exhibit significantly higher C contents and C/N, C/P, and N/P ratios compared to their landlocked and freshwater counterparts (C. nasus taihuensis and C. brachygnathus). However, the trends in N and P content are reversed. 3) Intraspecific differences vary among Coilia species, notably observed in C. nasus taihuensis and C. brachygnathus, where there's minimal correlation with body size. Conversely, C. nasus and C. mystus demonstrate a substantial influence of body size, with increasing fish size associated with higher C content, C/N, C/P, and N/P ratios, while N and P content decrease. This suggests that larger fishes store more energy (C%) in preparation for reproductive migration. In conclusion, this study provides a scientific foundation for comprehending the ecological patterns and intra-interspecific variations in Coilia. Furthermore, it contributes to the application of ecological chemometrics in the study of migratory fish, enriching our understanding of the intricate dynamics within this ecologically significant species.

Otolith biogeochemistry reveals possible impacts of extreme climate events on population connectivity of a highly migratory fish, Japanese Spanish mackerel Scomberomorus niphonius

Otolith biogeochemistry reveals possible impacts of extreme climate events on population connectivity of a highly migratory fish, Japanese Spanish mackerel Scomberomorus niphonius

Assessment of the impact of chemical pollution on endangered migratory fish in two major rivers of France, including spawning grounds

Water pollution is a one of the most contributors to aquatic biodiversity decline. Consequently, ecological risk assessment methods have been developed to investigate the effects of existing stresses on the environment, including the toxic effects of chemicals. One of the existing approaches to quantify toxic risks is called “Potentially Affected Fraction of species” (PAF), which estimates the potential loss of species within a group of species studied. In this study, the PAF method was applied to the Garonne catchment (southwest France) due to the limited information available on the involvement of water pollution in the decline of diadromous fish populations. This approach was used to quantify the potential toxic risk associated with chemical contamination of water for fish species. The objectives were to quantify this risk (1) in the Garonne and Dordogne rivers and (2) in the spawning grounds of two endangered anadromous fish species: the allis shad and the European sturgeon during the development period of their early life stages. Environmental pollution data was provided for 21 sites within the Garonne catchment between 2007 and 2022, and toxicity data was obtained specifically from freshwater toxicity tests on fish species. Then, for each site and each year, the potential toxic risk for a single substance (ssPAF) and for a mixture of substances (msPAF) was calculated and classified as high (>5 %), moderate (>1 % and < 5 %) or low (<1 %). Potential toxic risks were mostly moderate and mainly associated with: metals > other industrial pollutants and hygiene and care products > agrochemicals. In summary, this study highlights the probable involvement of water contamination on the decline, fate and restoration of diadromous fish populations in the Garonne catchment, focusing notably on the toxic effects on early life stages, a previously understudied topic.

Habitat use of barbel (Barbus barbus) in a restored urban Danube tributary

Tributaries play a vital role in fish spawning and recruitment, significantly influencing mainstem river fish populations. However, in the Anthropocene era, tributaries within river networks suffered ecological degradation due to fragmentation and channelization. This has led to reduced distribution ranges and declining populations of various riverine species, underscoring the urgency of conservation and rehabilitation efforts. Our study explores the potential for reintroducing fish from the mainstem Danube River into the Wien River, an urban tributary that has undergone partial rehabilitation. We assessed habitat use and movement patterns of 20 adult barbel (Barbus barbus), a species classified as ‘near threatened’ in Austria, collected from the Danube River during the spawning season. These barbel were tagged with radio telemetry tags and relocated into the upper reaches of the Wien River, surmounting several artificial barriers in the lower sections. Although spawning activity was not observed among the barbel, possibly due to prevailing water temperatures, our data suggest that the Wien River could function as a viable temporary habitat. Barbel were notably inclined to inhabit deeper pool and run habitats. Their daily movements and home ranges were relatively limited, ranging from 0.0 to 1.1 km and 100 m to 2.9 km, respectively. One contributing factor to restricted movement was the presence of beaver and knotweed dams, which created temporary migration obstacles. On average, the barbel remained within the system for more than a month. Most of these fish migrated downstream at the onset of a significant high-flow event. Our assessment suggests that while the Wien River may not serve as a permanent habitat, it could function as a temporary habitat for migratory fish. However, to ensure accurate assessments of the restored Wien River as a spawning habitat, it is imperative to reevaluate the findings under stable spawning conditions and gather comprehensive data on relevant abiotic factors. This study advocates for the restoration of longitudinal connectivity between tributaries and mainstem rivers as a means to counteract biodiversity loss in Anthropocene river ecosystems.

Atlantic mackerel population structure does not support genetically distinct spawning components.

The Atlantic mackerel, Scomber scombrus (Linnaeus, 1758) is a commercially valuable migratory pelagic fish inhabiting the northern Atlantic Ocean and the Mediterranean Sea. Given its highly migratory behaviour for feeding and spawning, several studies have been conducted to assess differentiation among spawning components to better define management units, as well as to investigate possible adaptations to comprehend and predict recent range expansion northwards. Here, a high-quality genome of S. scombrus was sequenced and annotated, as an increasing number of population genetic studies have proven the relevance of reference genomes to investigate genomic markers/regions potentially linked to differences at finer scale. Such reference genome was used to map Restriction-site-associated sequencing (RAD-seq) reads for SNP discovery and genotyping in more than 500 samples distributed along the species range. The resulting genotyping tables have been used to perform connectivity and adaptation analyses. The assembly of the reference genome for S. scombrus resulted in a high-quality genome of 741 Mb. Our population genetic results show that the Atlantic mackerel consist of three previously known genetically isolated units (Northwest Atlantic, Northeast Atlantic, Mediterranean), and provide no evidence for genetically distinct spawning components within the Northwest or Northeast Atlantic. Therefore, our findings resolved previous uncertainties by confirming the absence of genetically isolated spawning components in each side of the northern Atlantic, thus rejecting homing behaviour and the need to redefine management boundaries in this species. In addition, no further genetic signs of ongoing adaptation were detected in this species.

Research on the effects of ocean current on nuclear wastewater in Japan

According to reports, the Fukushima Daiichi Nuclear Power Station nuclear pollution water drainage project has been completed. Subsequently, Tokyo Electric Power will directly discharge nuclear wastewater into the ocean through a subsea tunnel connecting the land. These nuclear wastewater will be discharged to the waters near the Tropic of Cancer, which is greatly affected by the Japanese warm current, and nuclear wastewater will be taken around the world through ocean currents. German research institutions point out that within a month from the date of discharge of Fukushima nuclear wastewater, its radioactive material will spread to most of the Pacific Ocean, which will seriously affect global fish migration, ocean fisheries, human health and ecological security. Due to the impact of the global ocean current, China's East China Sea, the Yellow Sea and the Bohai Sea will also be affected by nuclear waste water pollution. Based on this, this text will start from the impact of the El Nino warm current on the flow of nuclear wastewater, and will soon explain the impact of Japan's dumping of nuclear wastewater on global security, and from a personal perspective on the current view of environmental protection and governance.

Exploring tradeoffs in southeast United States marine fisheries management using management strategy evaluation

Recreational fishing is the fastest growing sector in industrialized nations and can have substantial impacts on marine fish populations and ecosystems. Commercial objectives are typically characterized by maximizing yield and fishery stability whereas recreational anglers generally prefer sustained access to fishing and the availability of larger fish. Achieving these objectives while balancing tradeoffs between recreational and commercial fishing is essential to effective recreational and mixed-use fisheries management. Balancing multiple sector objectives is of particular concern in the southeast United States (US), where recreational fishing is the dominant source of mortality for marine fish stocks. We developed and applied a size-structured management strategy evaluation (MSE) tool, individually, to two stocks in southeast US Atlantic waters, black sea bass (Centropristis striata), a sedentary reef fish in overfished condition, and cobia (Rachycentron canadum), a migratory coastal pelagic fish, to evaluate the performance and tradeoffs of mixed management procedures against a variety of objectives. Several recreational and commercial objectives were aligned: fishers sought to conserve the stock and maximize catch, which simplified the evaluation of tradeoffs. We tested management procedures over alternate states of recruitment for black sea bass, and natural mortality for cobia. Management procedures that allowed harvest for black sea bass resulted in or risked an overfished status if the current weak recruitment regime continues, but could rebuild with no fishing regardless of recruitment state. Although results were sensitive to uncertainty in recruitment, no management procedures could achieve historic landings for either sector, and only management procedures that reduced effort consistently resulted in a reduced number of dead discards. No management procedures resulted in an overfished status for cobia. Management procedures for cobia generally achieved objectives and were robust to uncertainty in natural mortality. In both case studies, tradeoffs occurred between maximizing catch and season length and maintaining a stronger size structure in the population. This study resulted in a flexible MSE tool with strong potential utility for scoping management procedures that explicitly consider recreational fishing objectives.

Reintroducing Atlantic salmon in the river Rhine for decades: Why did it not result in the return of a viable population?

AbstractFreshwater species biodiversity is under threat. The average global decline for migratory fish species is estimated to be more than 75% since 1970. Atlantic salmon is one of these species with a steep decline in north‐western Europe and it even went extinct in the river Rhine in the 1950s. The causes for this decline have been posted to habitat loss, pollution, climate change and overfishing. Annual stocking in the Rhine since the late 1980s resulted in an initial increase in the Atlantic salmon numbers after which numbers collapsed again. In this paper, we lay out the recent decline, estimate losses of smolts and adults at different sections in the freshwater habitat and elaborate on potential causes of the recent decline and these losses. We found that the salmon population of the river Rhine has declined rapidly over the past two decades, with a current estimated spawning population of only ~350–800 individuals. The percentage of salmon smolts returning as adults to spawning grounds is estimated at 0.5%–0.6%, well below the 3% supposedly needed to maintain a self‐sustaining population. Many individuals disappear during their migrations, with the highest percentage of smolts disappearing in the German tributaries (44%) and the Dutch lower Rhine (71%), while the percentage of disappearing adults is highest in both the Dutch (74%) and the German (78%) Rhine. Causes for the losses per river section remain unclear and possible threats, some specific to the river Rhine, are being discussed. The large losses of smolts and adults in inland waters, compared with open sea losses, indicate that restocking the river Rhine might only result in a self‐sustaining population with more ecological restoration than carried out so far along the intensively shipped and highly regulated river course and associated high levels of predation, and might be increasingly limited by future climate change.

Analysis of mesoscale frontal zones of the Norwegian Sea based on satellite observations and reanalysis data in May 2011–2020

The aim of this study is to compare the horizontal temperature gradients calculated based on satellite observations and reanalysis data in the area of mesoscale frontal zones’ surface manifestations, both for the entire Norwegian Sea and during the onset of pelagic fish spawning migrations in May from 2011 to 2020. Using monthly average temperature data from MODIS/Aqua, GHRSST OSTIA, and CMEMS GLORYS12v1, the fields of monthly and decade-long horizontal gradients on the surface of the Norwegian Sea were derived. A comparison was made between the decade-long temperature gradient estimates and their mean climatic values. The majority of the data show surface manifestations of the main frontal zones in the Norwegian Sea. Validation of temperature fields in the area of the Iceland-Faroe Frontal Zone was conducted based on unique in situ observations. It was demonstrated that for the analysis of the frontal zones in the Norwegian Sea, using the temperature fields of GHRSST OSTIA is most preferable. The obtained physico-geographical characteristics of mesoscale frontal zones could be used to assess their relationship with pelagic fish migrations in the Norwegian Sea.

Genomic insights into the seawater adaptation in Cyprinidae

BackgroundCyprinidae, the largest fish family, encompasses approximately 367 genera and 3006 species. While they exhibit remarkable adaptability to diverse aquatic environments, it is exceptionally rare to find them in seawater, with the Far Eastern daces being of few exceptions. Therefore, the Far Eastern daces serve as a valuable model for studying the genetic mechanisms underlying seawater adaptation in Cyprinidae.ResultsHere, we sequenced the chromosome-level genomes of two Far Eastern daces (Pseudaspius brandtii and P. hakonensis), the two known cyprinid fishes found in seawater, and performed comparative genomic analyses to investigate their genetic mechanism of seawater adaptation. Demographic history reconstruction of the two species reveals that their population dynamics are correlated with the glacial-interglacial cycles and sea level changes. Genomic analyses identified Pseudaspius-specific genetic innovations related to seawater adaptation, including positively selected genes, rapidly evolving genes, and conserved non-coding elements (CNEs). Functional assays of Pseudaspius-specific variants of the prolactin (prl) gene showed enhanced cell adaptation to greater osmolarity. Functional assays of Pseudaspius specific CNEs near atg7 and usp45 genes suggest that they exhibit higher promoter activity and significantly induced at high osmolarity.ConclusionsOur results reveal the genome-wide evidence for the evolutionary adaptation of cyprinid fishes to seawater, offering valuable insights into the molecular mechanisms supporting the survival of migratory fish in marine environments. These findings are significant as they contribute to our understanding of how cyprinid fishes navigate and thrive in diverse aquatic habitats, providing useful implications for the conservation and management of marine ecosystems.

Ensuring that ingested fishbones that migrate to the neck are located, diagnosed, and removed early.

The aim of this retrospective study was to explore the clinical characteristics of and diagnostic and therapeutic strategies for the removal of fish bones that migrate to the neck. We reviewed the clinical data of 30 patients over the past 12years who underwent neck surgery in our otorhinolaryngology department for the migration of fish bones from the throat. The location of fish bones and the positivity rate of different examination methods (neck CT and B-ultrasound) were evaluated statistically. The diagnosis and treatment strategy for fish bone migration to the neck was also summarized. A total of 24 patients had a history of foreign body ingestion. The duration from foreign body ingestion to the appearance of symptoms in the neck ranged from 26 to 151 days, with a median of 50days (interquartile range, 32-86days). Among the 24 patients with fish bones located in the front or side of the neck, 50% (12/24) and 100% (24/24) of whom had positive neck CT and B-ultrasound results, respectively. Additionally, for 6 patients with fish bones in the retropharyngeal space, the positive rate for neck CT was 100%, whereas neck B-ultrasound showed negative results due to the air and depth in the trachea and esophagus. A strong correlation was observed between the length of fish bones detected by B-ultrasound and CT and the actual length. Indeed, no significant difference was observed between the length of fish bone determined by B-ultrasound and the actual length. In patients with fish bones located in the anterior and lateral neck regions, the foreign bodies were successfully removed by a lateral cervical approach operation (23/24). For the 6 cases with fish bones located in the retropharyngeal space, all (6/6) were removed by incising the posterior pharyngeal wall with assistance from transoral endoscopy. The techniques of B-ultrasound and CT have advantages for the diagnosis of migratory foreign bodies in the neck. Although B-ultrasound is more accurate for estimating the length of migratory fish bones in the neck, a combination of both methods can improve the preoperative positive rate of diagnosis. Therefore, a variety of surgical approaches should be employed to manage the different locations of cervical foreign bodies.

Archaeozoology Supports a Holistic View on Fish Assessments in Large Rivers—A Case Study from the Volga River: From Quantitative Data and Ancient DNA to Biodiversity Analysis

Knowledge of the historical and present dynamics of populations of migratory fish can promote our understanding of factors affecting their recruitment and abundance. Taxonomic identification of 23,802 bone remains and 13,539 scales of fish from 30 archaeological sites along Volga River revealed that they belonged to 41 different fish species. These data allow for retrospective comparisons and highlight the potential of archaeozoology in conservation biology. Sturgeons and salmonids are vulnerable to the impacts of fishery and climatic change. The sharp decline in the numbers of Starry sturgeon (Acipenser stellatus), Caspian trout (Salmo caspius), and Caspian Inconnu (Stenodus leucichthys) from the Volga in the 17th and 18th centuries was likely related to a cooling period (“Little Ice Age”). At present, the population numbers of all anadromous sturgeons and salmonids of the Volga River are critically low. In the Volga basin over the past two millennia, the sterlet (Acipenser ruthenus) has had a very large population number, high genetic (haplogroups and haplotypes) diversity, and large body sizes. Genetic analysis (aDNA and eDNA) have great potential to expand the knowledge of fish populations along large rivers and to improve long-term biomonitoring. Therefore, analyses of historical data, conventional surveys, as well as the inclusion of genetic approaches complement each other in the development of effective conservation strategies.

A delayed and unsynchronized ovary development as revealed by transcriptome of brain and pituitary of Coilia nasus.

Coilia nasus is an anadromous fish that has been successfully domesticated in the last decade due to its high economic value. The fish exhibits a delayed ovary development during the reproductive season, despite breeding and selection for five to six offspring. The molecular mechanism of the delayed ovary development is still unknown, so the obstacles have not been removed in the large-scale breeding program. This study aims to investigate the key genes regulating ovarian development by comparing the transcriptomes of ovarian-stage IV and stage II brain/pituitary of Coilia nasus. Ovarian stages were validated by histological sections. A total of 75,097,641 and 66,735,592 high-quality reads were obtained from brain and pituitary transcriptomes, respectively, and alternatively spliced transcripts associated with gonadal development were detected. Compared to ovarian Ⅱ- brain, 515 differentially expressed genes (DEGs) were upregulated and 535 DEGs were downregulated in ovarian Ⅳ- brain, whereas 470 DEGs were upregulated and 483 DEGs were downregulated in ovarian Ⅳ- pituitary compared to ovarian Ⅱ- pituitary. DEGs involved in hormone synthesis and secretion and in the GnRH signaling pathway were screened. Weighted gene co-expression network analysis identified gene co-expression modules that were positively correlated with ovarian phenotypic traits. The hub genes Smad4 and TRPC4 in the modules were co-expressed with DEGs including Kiss1 receptor and JUNB, suggesting that ovarian development is controlled by a hypothalamic-pituitary-gonadal axis. Our results have provided new insights that advance our understanding of the molecular mechanism of C. nasus reproductive functions and will be useful for future breeding.

Evaluating streamflow and temperature effects on Bull Trout migration and survival with linear spatial capture–recapture models

AbstractObjectiveIn the U.S. Pacific Northwest, climate change is increasing air temperatures, decreasing warm season (April–September) streamflow, and increasing cool season (October–March) streamflow. Warmer water temperatures may alter conditions for migratory coldwater fishes like the Bull Trout Salvelinus confluentus. Consequently, an understanding of Bull Trout migration and survival is critical for species conservation and restoration. In the Salmon River basin, Idaho, 1992 and 1993 transpired to be two of the most opposing extreme years among the past three decades for warm season water temperature and streamflow. These extremes provided a unique opportunity to retrospectively compare Bull Trout survival and migration under potential climate change scenarios.MethodsWe evaluated prespawning and postspawning migrations and survival of fluvial Bull Trout that were radio‐tagged and tracked from 1992 to 1994. We used a Cormack–Jolly–Seber linear spatial capture–recapture model to simultaneously model the migration and survival of radio‐tagged prespawn (n = 58) and postspawn (n = 23) Bull Trout among weeks and river reaches with streamflow, water temperature, and habitat covariates.ResultMost individual prespawning migrations were similar among tagged fish, whereas postspawn fish adopted multiple migration and overwintering strategies. Movements of prespawn Bull Trout were larger when (1) weekly average daily maximum streamflow increased and (2) weekly average daily maximum water temperature increased. The model estimated that at least 52% of spawners survived to spawning, and mean weekly prespawning apparent survival was higher in the low‐streamflow year (1992) than in the year with higher and more variable streamflow (1993). Survival of 1992–1994 fish during the 38‐week postspawning period was intermediate to that in the prespawning period. Detections of prespawn Bull Trout were generally higher at sites with more complex habitats, less large woody debris, and fewer undercut banks.ConclusionWe found that the prespawn life stage can represent a shorter time frame (14–18 weeks) with increased mortality compared to the longer postspawning period (38 weeks). Bull Trout apparent survival increased with lower streamflow variability, indicating that expected future changes in climate may adversely affect Bull Trout.

Hypoxia also occurs in small highly turbid estuaries: the example of the Charente (Bay of Biscay)

Abstract. The French coast facing the Bay of Biscay (North-East Atlantic) is characterised by the presence of small macrotidal and turbid estuaries, including the Charente, which is geographically located between the two large estuaries of the Gironde and the Loire (south-west France). Multi-year, multi-site and high-frequency water quality surveys have shown that the Loire and, to a lesser extent, the Gironde suffer from summer hypoxia. These observations raised the question of the possible occurrence of hypoxia, particularly in one of these small estuaries, the Charente, which flows into the Bay of Marennes-Oléron, the first oyster-farming area in France. Unlike its two large neighbours, the Charente estuary is not continuously monitored, although it is subject to similar climatic changes and anthropogenic pressures, making it impossible to assess potential risks to the ecosystem. Here we present a first study of dissolved oxygen in the Charente estuary based on a combination of longitudinal studies along the estuary axis and instrumented sites to determine the intensity and spatial extent of deoxygenation. Temperature, dissolved oxygen and conductivity sensors were deployed at several sites during the summers of 2018, 2019 and 2020 to record temperature, salinity and dissolved oxygen every 15 min. The high-frequency dataset is compared with a long-term low-frequency dataset (1975–2022; 8–12 measurements per year) to determine whether or not there is a deterioration in the oxygenation of the Charente estuary. The high-frequency dataset shows a high variability in dissolved oxygen (DO) with a clear influence of the tidal cycle. During summer, DO concentrations are often below 5 mg L−1 and sometimes even below 2 mg L−1, indicating the presence of a summer estuarine oxygen minimum zone with an extension along the estuarine axis of about 20–25 km. Temperature is the main factor controlling DO in the Charente estuary, which limits preventive management strategies and, in the context of global warming, raises questions about the long-term suitability of estuarine conditions for the needs of the biological communities, particularly migratory fish.

A new possible breeding site of &lt;i&gt;Alosa fallax&lt;/i&gt; (Lacépède 1803) (Actinopterygii: Clupeiformes: Alosidae) on the Tyrrhenian coast of Central Italy

The River Mignone (Tarquinia, Viterbo, central Italy) is one of the best preserved watercourses in Lazio. Here, in April and May of the years 2020-2023, a conspicuous contingent of Alosa fallax (Lacépède 1803) was reported. The frenetic behaviour of the individuals observed, some of which showed swollen bellies and considerable size, suggested that this watercourse may represent a potential new breeding station for this threatened migratory euryhaline fish species. The correct recognition of the sex of A. fallax in the field, problematic during the breeding season, is crucial for understanding its population structure as indicators of possible reproduction. To this end, statistical analysis (Mann-Whitney U-test) of 20 morphometric measurements made on individuals captured and subsequently released revealed significant differences between the sexes for seven morphometric characters, confirming a sex ratio compatible with reproduction. However, the presence of a transverse dam with no fish ladder and the dispersion of biocides from farmland in the final stretch may represent a serious threat to effective reproduction.