Thomas Trancart

Les poissons migrateurs thalassotoques réalisent des migrations pour changer de biomes à deux moments de leur vie. La première migration est une migration anadrome qui a lieu au stade juvénile. Après une phase de croissance en eau douce, ils réalisent une migration catadrome vers les habitats marins, afin de s’y reproduire.Il existe une très forte diversité, écologique et morphologique, au sein de ce groupe qui partage une stratégie d’histoire de vie commune : la catadromie. En décomposant par phases la migration anadrome de colonisation des bassins versants, cette étude avait pour objectif de mettre en évidence les tactiques déployées lors de cette colonisation, puis de les comparer afin de mettre en évidence d’éventuelles variations, appelées « variants ».Deux méthodologies principales ont été utilisées : les premières phases de colonisation ont été étudiées en structures expérimentales. Pour les individus âgés de un an et plus, une approche de suivi par télémétrie acoustique a été choisie.Un patron global de migration de colonisation des bassins versants par les thalassotoques aainsi pu être proposé. Des variants interspécifiques liés à l’écologie et à la morphologie des espèces ont aussi pu être mis en évidence, ainsi que des variations intraspécifiques. Ces variations suggèrent l’existence de différentes stratégies d’histoire de vie au sein d’une même espèce de thalassotoques.

European eel is thought to be a symbol of the effects of global change on aquatic biodiversity. The species has persisted for millions of years and faced drastic environmental fluctuations thanks to its phenotypic plasticity. However, the species has recently declined to historically low levels under synergistic human pressures. Sublethal chemical contamination has been shown to alter reproductive capacity, but the impacts and required actions are not fully addressed by conservation plans. This paper proposes a modelling approach to quantify the effects of sublethal contamination by anthropogenic pollutants on the expression of life history traits and related fitness of the critically endangered European eel. Material and Methods: We sampled female silver eels from eight different catchments across Europe previously shown to be representative of the spectrum of environmental variability and contamination. We measured 11 fitness-related life history traits within four main categories: fecundity, adaptability and plasticity, migratory readiness, and spawning potential. We used machine learning in models to explore the phenotypic reaction (expression of these life history traits) according to geographical parameters, parasite burdens (the introduced nematode Anguillicoloides crassus) and anthropogenic contaminants (persistent organic pollutants (POPs) in muscular tissue and trace elements (TEs) in gonads, livers and muscles). Finally, we simulated, the effects of two management scenarios—contamination reduction and contamination increase—on the fecundity and recruitment. Results: Contamination in our sampling was shown to have a stronger control on life history traits than do geographic and environmental factors that are currently described in the literature. We modelled different contamination scenarios to assess the benefit of mitigation: these scenarios suggest that reducing pollutants concentrations to the lowest values that occurred in our sampling design would double the fecundity of eels compared to the current situation. Discussion: Remediation of contamination could represent a viable management option for increasing the resilience of eel populations, with much more effects than solely reducing fishing mortality. More broadly, our work provides an innovative way for quantitative assessment of the reaction norms of species’ biological traits and related fecundity to contamination by organic and inorganic pollutions thus opening new management and conservation pathways to revert the erosion of biodiversity.

Changes in migration timing, resulting from the alteration in river continuity or the effect of climate change, can have major consequences on the population dynamics of diadromous fish. Forecasting the phenology of fish migration is thus critically important to implement management actions aimed at protecting fish during their migration. In this study, an 11‐year monitoring survey of Atlantic salmon smolts (Salmo salar) from the Ourthe River, Belgium, was analysed within a European Special Area of Conservation to improve the understanding of environment‐induced spring migration. A logistic model was fitted to forecast smolt migration and to calculate phenological indicators for management, i.e. the onset, end, and duration of migration, while accounting for the influence of photoperiod, water temperature, and hydrological conditions. The results indicated that the photo‐thermal units accumulated by smolts above a 7°C temperature threshold was a relevant proxy to reflect the synergistic effect between temperature and photoperiod on smolt migration. After integrating the effect of river flow pulses, the model accurately explained the inter‐annual changes in migration timing (R² = 0.95). The model predictions provide decisive management information to identify sensitive periods during which mitigation measures (e.g. hydropower turbine shutdown, river discharge management) should be conducted to promote smolt survival. The model was used to predict phenological characteristics under future scenarios of climate change. The results suggest a joint effect of hydrological alterations and water warming. Temperature increases of 1–4°C were associated with earlier initiation of migration, 6–51 days earlier, and spring flood events greatly influenced the duration of the migration period. Accordingly, the combined effects of human‐induced modifications of the hydrological regimes and increasing temperatures could result in a mismatch between the smolt and favourable survival conditions in the marine environment.