The use of species distribution models (SDMs) in paleoecology enhances its contribution to ecology and conservation biology, such as estimating climate change impacts and informing conservation efforts. Acipenser sturio and A. oxyrinchus are closely related sturgeon species currently arranged in two disjunct distributions across the Atlantic Ocean. However, paleogenetic findings highlighted that both species were present in European waters over at least five millennia. Their joint European history during the Holocene period might have been driven by the thermal hypothesis, with A. oxyrinchus being a cooler-water species than A. sturio. Given their current poor state of conservation, quantifying their environmental preferences at a time when the two species were more abundant could help shape future conservation/restoration measures. Paleo-species distribution models were built for each species based on paleoclimate variables and archeozoological remains, most of which were genetically identified at the species level. Response curves for air temperature and precipitation were then applied to estimate relative continental paleohabitat suitability (HSI) in watersheds across four regions (i.e., the Baltic Sea, the Northern Seas, the South European Atlantic shelf, and the Mediterranean Sea) throughout the Holocene. The estimated niches confirmed the thermal sensitivity difference between the two species, with A. oxyrinchus having a narrower thermal range towards colder annual temperatures. Comparing HSI suitability between the two species, the four regions were globally all suitable for A. sturio but not for A. oxyrinchus, which showed much greater variability. Catchments with outlets to the Mediterranean Sea and the South European Atlantic shelf were mainly suitable for A. sturio. In contrast, those with an outlet in the Baltic Sea were slightly more suitable for A. oxyrinchus than for A. sturio. Catchments with outlets to the Northern Seas showed comparable suitability for both species. This superposition in near-past suitable habitats implied that the two species can potentially exist under the same environmental conditions and as such suggested future risks of biotic interactions which should be further acknowledged in potential stocking practices.

The common carp Cyprinus carpio (Linnaeus, 1758) is a key species in the Danube River, both ecologically and economically. The carp population in the Danube is affected by a range of factors such as overfishing, loss of habitats, pollution, anthropogenic disturbance and alien species. Climate change is putting an additional strain on these fish populations, altering parameters such as length and sex structure, Fulton's condition factor. This study examines changes in these parameters within carp populations in the Danube based on data collected from 2021 to 2024. By comparing biometric and demographic indicators, this research identifies hydrological trends driven by climatic variations, and how the sex structure was also affected, with sex ratio depending on region and local climatic conditions. Fulton's condition factor was calculated to assess the nutritional status and overall health of individuals in the population. This study highlights the urgent need for sustainable fisheries management and appropriate conservation strategies to address the challenges posed by climate change.

Basé sur une analyse approfondie de la littérature internationale sur le sujet, cette note expose et illustre avec des exemples français cinq critères dont les différentes combinaisons permettent de qualifier une espèce de migratrice amphihaline. Une première liste des espèces susceptibles de pouvoir être considérées ainsi est proposée pour la métropole et les outremers. Enfin, les principales approches permettant de valider ces critères sont présentées brièvement. Ce travail devra se poursuivre en confrontant les différentes espèces mentionnées ici à ces critères éventuellement précisés.

In this study, the role of water quality has been studied to assess its suitability for the allis shad (Alosa alosa), a diadromous fish for which abundance has drastically declined at the beginning of 2000 and is remaining critically low in the Garonne basin (France). The water quality regarding contamination at spawning grounds and its toxicity for embryos and larvae is one possible explanatory factor for this decline, which has not been investigated. Early life stages of fish are particularly sensitive to environmental conditions, including water contamination, making them useful for environmental monitoring. The ex situ biomonitoring method allows continuous exposure of organisms to river water while controlling for confounding factors such as temperature, oxygen, and photoperiod. Three ex situ exposures of allis shad embryos to the Garonne water were conducted during its spawning season, in spring 2022 and 2023, under optimal temperature and oxygen saturation rate. Survival in the control groups demonstrates the value of the ex situ approach, allowing for the exposure of early life stages in their natural environment. Results indicated a high mortality rate in embryos exposed to the Garonne water compared to control, with mortality occurring during organogenesis, which is a particularly critical period of development. These findings raise concerns about the suitability of the Garonne water for the early development of allis shad and consequently for the population dynamics of this species in the Garonne basin. Moreover, these results confirm previous approaches of risk assessment and experimentations, suggesting that water quality and contamination could have an impact on other fish species.

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.

In the context of increasing pollution pressure on aquatic ecosystems, it is essential to improve our knowledge of habitat quality and its suitability for organisms. It is particularly relevant to better integrate early life stages of fish into pollution biomonitoring programs, as they are reliable indicators of ecosystem integrity and because of their high sensitivity to pollutants. To avoid the influence of environmental parameters on their development, a lab-on-field approach, called the ex-situ exposure method, was developed. Aquatic organisms were exposed to a continuous flux of water under semi-controlled temperature, oxygen, and photoperiod conditions to avoid the influence of these confounding factors when interpreting the results. To investigate the potential role of water contamination, this active biomonitoring method was applied to the Garonne River (Southwest France), where migratory fish populations have declined. Two model species from different taxa were used: embryos of the Japanese medaka (Oryzias latipes) and adults of the crustacean Gammarus fossarum. The results showed a significant impact of water quality on embryo mortality and early hatching in two separate experiments on Japanese medaka. In addition, an induction of feeding rate was observed in exposed gammarids, but no impact on their embryo survival, suggesting differences in sensitivity between the two species selected. Chemical and biological analyses did not identify trace metals, pesticides, or microorganisms as potential sources of toxicity in medaka embryos or G. fossarum. These results raise concerns about the quality of the water in the Garonne River and its toxicity to aquatic organisms.

Species Distribution Models (SDM) are useful tools providing results that can be extrapolated to anticipate species range shifts, under climate change scenarios. SDM studies integrating spatial constraints are significantly lacking in the marine environment, leading to optimistic predictions. This is particularly true for anadromous species in which marine distributions can be driven by their affinity to their natal rivers. Acipenser sturio is a critically endangered anadromous fish for which two stocked populations are currently maintained in the Gironde-Garonne-Dordogne (France) and Elbe (Germany) river systems. Benefiting from bycatch reports of A. sturio, we applied a SDM process that explicitly considers distance to home when evaluating habitat suitability. More precisely, we included the variable ‘distance to mouth of the natal river system’ into SDM inputs to test and characterize its influence on the marine distribution of A. sturio. We used this model to obtain the marine distribution under current climatic conditions with the two source populations and under population recovery scenarios (functional populations hypothesized to exist in ten currently unoccupied river systems). We projected the model under future conditions with two climatic scenarios (RCP 4.5 and 8.5) and three time slices over the 2023–2099 period. Constrained-ranges of both existing and hypothetical populations are projected to expand in the future. We observed an overall increase of habitat suitability, with new suitable sectors localized further from natal river mouths. By informing on the suitable marine surface that each hypothetical population holds and adds to the existing ones, our approach aims at informing about the feasibility of species recovery and marine habitats protection strategies. Our findings highlight the need for including dispersal information in marine SDM. The application of our dispersal-constrained approach may be considered for other less-well-known species for which dispersal point sources are identifiable, such as other diadromous species in different study areas.

Water pollution is a significant threat to aquatic ecosystems. Various methods of monitoring, such as in situ approaches, are currently available to assess its impact. In this paper we examine the use of fish in active biomonitoring to study contamination and toxicity of surface waters. We analysed 148 previous studies conducted between 2005 and 2022, including both marine and freshwater environments, focusing on the characteristics of the organisms used as well as the principal goals of these studies. The main conclusions we drew are that a wide range of protocols and organisms have been used but there is no standardised method for assessing the quality of aquatic ecosystems on a more global scale. Additionally, the most commonly used developmental stages have been juveniles and adults. At these stages, the most frequently used species were the fathead minnow (Pimephales promelas) and two salmonids: rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta). Few studies used earlier stages of development (embryos or larvae), mostly due to the difficulty of obtaining fish embryos and caging them in the field. Finally, we identified research gaps in active biomonitoring for water quality assessment which could indicate useful directions for future research and development.

The European sturgeon (Acipenser sturio) is a critically endangered anadromous fish species with the last remaining population living in the Gironde estuary, thanks to restocking programs. Between 2010 and 2018, trawling surveys (1022 trawl tows) in the estuary caught 452 sturgeons (fork length (FL) from 25.5 cm to 154 cm). Based on previous knowledge about the species ecology, individuals have been categorized into two groups, Estuarine Dwellers (ED; FL below 68 cm) that are using mainly the estuary, and Sea Explorers (SE; FL equal or above 68 cm) that could accomplish migration at sea. ED and SE were found in the estuary at all seasons with densities being the highest in autumn for SE and in all season except spring for ED. Hotspot analyses were made at the seasonal scale to localise ED and SE concentrations. Differences were analysed according to environmental variables (temperature, water column height, salinity and concentration in suspended matters) extracted from a hydrodynamic model (MARS3D), which all contribute to fish localisation at all seasons. In all seasons, both groups were using common areas located downstream (overlap from 26 to 33%) except in autumn, when different areas were used (12% overlap). SE were encountered downstream in deeper areas with higher salinity and lower temperature (except in winter). For this group, temperature seems to be a limiting factor. ED occupied downstream as well as upstream areas with lower salinity and higher temperature in summer, but there was no habitat selection linked to simulated environmental variables in spring and autumn. Since ED are mainly using the estuary they are probably accustomed to the range of values of the abiotic variables characterising the estuary and other factors are probably at play in spring and autumn, such as prey distribution. Comparison with the location of ancient wild cohorts highlights common hotspot areas downstream with our stocked population but the disappearance of a former upstream hotspot. We explained this difference by the environmental changes witnessed by the estuarine environment in the last decades. Our results highlights important areas for both groups that could help the design of conservation measures.

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.

From experimentations in optimal conditions, we identified the main features of embryogenesis of allis shad Alosa alosa. We propose to use this outcome as endpoints to evaluate the impact of stressors on A. alosa embryogenesis in future studies. Eggs are spherical in shape, transparent and non-adhesive. Mass hatching occurs between 120 and 144 hpf at an incubation temperature of 20°C. Abnormalities of larvae were minor (1.10% of hatched individuals).

The last remaining population of European sturgeon (Acipenser sturio) lives in the Gironde-Garonne-Dordogne (France) catchment (GGD). Captive young individuals are released into the GGD hydrosystem each year, as part of a restocking programme. This study aims to assess the health status of juveniles A. sturio to current conditions in the GGD hydrosystem, to evaluate their capacity to survive and grow in a moderately anthropized ecosystems. 3-month-old farmed sturgeons were exposed for one month in experimental conditions that mimic the environmental conditions in the Garonne and Dordogne rivers, followed by five months of depuration. After one month of exposure, fish exposed to Dordogne and Garonne waters bioaccumulated higher levels of metals and persistent organic pollutants, displayed a reduced hepato-somatic index, and had depleted levels of lipids and glycogen content in their liver, when compared with the Reference group. However, metabolic and swimming performance, as well as the costs of swimming were not impaired. After the 5 months depuration, a significant decrease of K was observed for all exposure conditions. HSI also decreased with time. The overall health status and adaptive capacity of juvenile A. sturio appeared to be maintained over the experimental 6 months' period. Juveniles of A. sturio seem to have the adaptive capacity to survive and grow in the GGD hydrosystem, after being released as part of a restocking programme.

Perfluorinated and polyfluorinated substances (PFASs) are widely found in freshwater ecosystems because of their resistance to degradation and their ability to accumulate in aquatic organisms. While water temperature controls many physiological processes in fish, knowledge of the effects of this factor on PFAS toxicokinetic is still limited. This study presents experimental results of internal distribution and elimination rates of two perfluorinated acid compounds, namely perfluorooctane sulfonate (PFOS) and perfluorohexane sulfonate (PFHxS) in adult rainbow trout (Oncorhynchus mykiss) exposed to three temperatures. Dietary exposure experiments were conducted at 7 °C, 11 °C, and 19 °C and liver, blood, muscle, brain, and kidney were sampled for analysis. PFOS concentrations were comparable to or exceeded those of PFHxS, while PFHxS was eliminated faster than PFOS, whatever the temperature. Internal distribution changed significantly for both substances when fish were exposed to a range of temperatures from 7 to 19 °C. Indeed, PFOS and PFHxS relative distribution increased in blood, liver, and brain while they decreased in muscle when the water temperature rose. The water temperature variation affected the elimination half-lives, depending on the substances and organs.

The European sturgeon, Acipenser sturio , is a highly endangered species that almost disappeared in the last decades. Thanks to yearly restocking of the population, this species is still found in the Gironde estuary (France), where juveniles grow during several years before leaving to the ocean. The aims of this study were to evaluate the pressure exerted on these fish by known organic and inorganic contaminants during their stay at the Gironde estuary, and to get information on the fish’s health in this context. Monthly captures over the year 2014 provided 87 fish from the cohorts 2012 and 2013 mainly, and from cohorts 2008, 2009, and 2011, all fish born in hatchery. We report the very first analyses of contaminant levels and of biological markers measured in the blood of these fish. Low inorganic contamination was found, composed of seven metals mainly Zn (< 5 μg mL −1 ), Fe (< 1.5 μg mL −1 ), Cu (< 0.8 μg mL −1 ), Se (< 0.8 μg mL −1 ), As (< 0.25 μg mL −1 ), Co (< 0.14 μg mL −1 ), and Mn (< 0.03 μg mL −1 ). Concerning persistent organic contaminants, the sum of seven PCBs varied from 1 to 10 ng g −1 plasma, that of eight OCPs from 0.1 to 1 ng g −1 , and that of eight PBDEs from 10 to 100 pg g −1 . Higher levels of contaminants were measured during spring as compared to summer. The sex steroid hormone plasma levels (estradiol, testosterone, and 11-ketotestosterone) were quite low, which was predictable for juveniles. The transcription of reproduction-involved genes ( EstR , AR , LHR , sox9 ) in blood cells was demonstrated for the first time. Some of them were correlated with organic contaminant levels PCBs and OCPs. Other gene transcriptions ( sodCu and bax ) were correlated with PCBs and OCPs. However, the DNA damage level measured here as comet tail DNA and micronuclei ratio in red blood cells were in the very low range of the values commonly obtained in fish from pristine areas. The data presented here can serve as a reference base for future monitoring of this population of sturgeons.

European sturgeon Acipenser sturio is an anadromous fish species being classified “critically endangered” with only one remaining population in the Gironde-Garonne-Dordogne basin (France). In the global warming context, this paper aims to determine the sensitivity of A. sturio early life phases to temperature and oxygen saturation. Embryos were experimentally exposed to a combination of temperature (12 to 30 °C) and oxygen (30 to 90% O2 saturation) conditions. Lethal and sublethal effects were evaluated using embryonic mortality, hatching success, malformation rate, yolk sac resorption, tissue development and swimming speed. Embryonic survival peaked at 20 °C and no survival was recorded at 30 °C regardless of the associated oxygen saturation. No hatching occurred at 50% O2 sat or below regardless of temperature. Malformation frequency appeared to be minimum at 20 °C and 90% O2 sat. Swimming speed peaked at 16 °C. The temperature optimum of early life phases of A. sturio was determined to be close to 20 °C. Its upper tolerance limit is between 26 and 30 °C and its lower tolerance limit is below 12 °C. Oxygen depletion induces sublethal effects at 70% O2 sat and lethal effects at 50% O2 sat. Within the spawning period in the Gironde-Garonne-Dordogne basin, we identified yearly favourable oxythermal windows. Consequences of climate change would depend of the phenological adaptation of the species for its spawning period.