Le programme de repeuplement de l’anguille : bilan de trois années de suivi

The European eel is a species now considered as critically endangered. It is the subject of a European restoration plan since 2007, whereas it has long been considered in France as a pest. The restoration plan include a restocking program, which started in 2011 in France and is monitored for the first time in this study. This program consists in catching glass eels in estuarine waters, to reintroduce them on sites considered as favorable. The aim is to increase the breeding population migrating to the spawning sites. Over the period 2011-2013 have been released nearly 6.7 tonnes of glass eels, which represent nearly 20 million individuals on 28 release sites in North and Western France. Considering that this study occurs 6 months to 3 years after the stocking, it is not possible yet to conclude on the final silver eel production. Thus, other approaches have been chosen to bring the first clues necessary to answer to the question of the efficiency of this program. First, the factors influencing the short-term mortality were identified and their effects quantified in a GLM binomial model explaining almost 52% of the deviance. Then, it was shown that stocked individuals are still present in the environment, at least over the period of the study. These individuals shows recapture rates consistent with what was expected and grows, although a bit lower than what is reported in the literature. One of the major findings of this study tackles the subject of the marking used for the scientific monitoring. The latter generates an additional mortality (29%) and a lower growth (-9.9%) and should be limited as much as possible.L'anguille européenne est une espèce aujourd'hui considérée comme en danger critique d'extinction. Elle fait l'objet depuis 2007 d'un plan européen de restauration, alors qu'elle a longtemps été considérée en France comme une espèce nuisible. Ce plan de restauration inclut un programme de repeuplement, débuté en 2011 en France, dont la présente étude réalise pour la première fois le bilan. Ce programme consiste en la capture de civelles au niveau estuarien, puis à leur réintroduction sur des sites jugés favorables. L’objectif étant d’augmenter la population de reproducteurs lors de la migration vers les sites de pontes. Sur la période 2011-2013 ont ainsi été déversées près de 6,7 tonnes de civelles, soit près de 20 millions d’individus sur 28 sites du Nord et de l’Ouest de la France. Cette étude intervenant de 6 mois à 3 ans après les alevinages, il est encore impossible de conclure sur la production finale d’anguilles argentées. D’autres approches ont donc été choisies pour apporter de premiers éléments de réponses quant au succès de ce programme. Dans un premier temps, les facteurs influençant la mortalité à court terme ont été identifiés et leurs effets quantifiés, dans un modèle GLM binomial expliquant près de 52% de la déviance. Puis, il a été montré que les individus alevinés sont bien présents dans le milieu, au moins sur la période de l’étude. Ces individus présentent des taux de recaptures conformes à ce qui était attendu et grandissent, bien que sur des échelles moindres que ce qui est indiqué dans la littérature. Un des résultats important de cette étude est relatif au marquage employé pour le suivi scientifique. Ce dernier engendre une mortalité supplémentaire (+29%), ainsi qu’une moindre croissance (-9,9%) et doit donc être limité autant que possible

Changes in length-at-first return of a sea trout (Salmo trutta) population in northern France

The resilience of sea trout populations is increasingly concerning, with evidence of major demographic changes in some populations. Based on trapping data and related scale collection, we analysed long-term changes in body length of a sea trout population in the Bresle River, France. From 1984-2022, the length of first-time returning individuals decreased by 1.73 mm.year-1 (SD = 0.08), which resulted in the loss of c. 12.3% of mean body length. This decrease results from a decrease in the age at first return, with a gradual loss of the oldest individuals and an increase in the proportion of the youngest. Timing of the return migration advanced drastically, although shorter sea sojourn had little influence on body length. We found little evidence of a decrease in length-at-age, to the exception of the oldest age class, suggesting that growth conditions at sea might not have deteriorated greatly during the study period

The complete mitochondrial genome of a new invader fish in France, the pink salmon Oncorhynchus gorbuscha (Walbaum, 1792) (Teleostei, Salmonidae)

International audienceThe pink salmon Oncorhynchus gorbuscha (Teleostei, Salmonidae) is a North Pacific species. It arrived in Northern Europe through freshwater migration each odd year since 2017, creating management issues. In this study, we sequenced a complete mitochondrial genome for a pink salmon caught in the Bresle River (France) with a museum voucher. The sequence is 16,695 bp in length and is similar to previous mitogenomes published within this genus. Length heteroplasmy on the control region for this species was detected, as described in previous molecular studies. We tested several primers commonly used for metabarcoding studies on four markers (12S, 16S, COI and Cytb), and all allow the discrimination of this species. This mitogenome is a reference for molecular identification, for instance in like environmental DNA studies

Does global change increase the risk of maladaptation of Atlantic salmon migration through joint modifications of river temperature and discharge?

International audienceIn freshwater ecosystems, water temperature and discharge are two intrinsically associated triggers of key events in the life cycle of aquatic organisms such as the migration of diadromous fishes. However, global changes have already profoundly altered the thermal and hydrological regimes of rivers, affecting the timing of fish migration as well as the environmental conditions under which it occurs. In this study, we focused on Atlantic salmon ( Salmo salar ), an iconic diadromous species whose individuals migrate between marine nursery areas and continental spawning grounds. An innovative multivariate method was developed to analyse long-term datasets of daily water temperature, discharge and both salmon juvenile downstream and adult upstream migrations in three French rivers (the Bresle, Oir and Nivelle rivers). While all three rivers have gradually warmed over the last 35 years, changes in discharge have been very heterogeneous. Juveniles more frequently used warmer temperatures to migrate. Adults migrating a few weeks before spawning more frequently used warm temperatures associated with high discharges. This has already led to modifications in preferential niches of both life stages and suggests a potential mismatch between these populations' ecological preference and changes in their local environment due to global change

Environmental influences on life history strategies in partially anadromous brown trout (Salmo trutta, Salmonidae)

This paper reviews the life history of brown trout and factors influencing decisions to migrate. Decisions that maximize fitness appear dependent on size at age. In partly anadromous populations, individuals that attain maturity at the parr stage typically become freshwater resident. For individual fish, the life history is not genetically fixed and can be modified by the previous growth history and energetic state in early life. This phenotypic plasticity may be influenced by epigenetic modifications of the genome. Thus, factors influencing survival and growth determine life‐history decisions. These are intra‐ and interspecific competition, feeding and shelter opportunities in freshwater and salt water, temperature in alternative habitats and flow conditions in running water. Male trout exhibit alternative mating strategies and can spawn as a subordinate sneaker or a dominant competitor. Females do not exhibit alternative mating behaviour. The relationship between growth, size and reproductive success differs between sexes in that females exhibit a higher tendency to migrate than males. Southern populations are sensitive to global warming. In addition, fisheries, aquaculture with increased spreading of salmon lice, introduction of new species, weirs and river regulation, poor water quality and coastal developments all threaten trout populations. The paper summarizes life‐history data from six populations across Europe and ends by presenting new research questions and directions for future research. freshwater resident, habitat, migration, partial anadromy, Salmonidae, sex rati

Environmental influences on life history strategies in partially anadromous brown trout (Salmo trutta, Salmonidae)

This paper reviews the life history of brown trout and factors influencing decisions to migrate. Decisions that maximize fitness appear dependent on size at age. In partly anadromous populations, individuals that attain maturity at the parr stage typically become freshwater resident. For individual fish, the life history is not genetically fixed and can be modified by the previous growth history and energetic state in early life. This phenotypic plasticity may be influenced by epigenetic modifications of the genome. Thus, factors influencing survival and growth determine life‐history decisions. These are intra‐ and interspecific competition, feeding and shelter opportunities in freshwater and salt water, temperature in alternative habitats and flow conditions in running water. Male trout exhibit alternative mating strategies and can spawn as a subordinate sneaker or a dominant competitor. Females do not exhibit alternative mating behaviour. The relationship between growth, size and reproductive success differs between sexes in that females exhibit a higher tendency to migrate than males. Southern populations are sensitive to global warming. In addition, fisheries, aquaculture with increased spreading of salmon lice, introduction of new species, weirs and river regulation, poor water quality and coastal developments all threaten trout populations. The paper summarizes life‐history data from six populations across Europe and ends by presenting new research questions and directions for future research. freshwater resident, habitat, migration, partial anadromy, Salmonidae, sex rati