Réponse des populations de salmonidés migrateurs aus changements globaux : rôle de la croissance dans les stratégies d'histoire de vie et la dynamique de population chez le saumon atlantique (Salmo salar)

This thesis aims to better understand the ecological and demographic processes determining life history traits and population structure in Altantic salmon Salmo salar populations. I test the hypothesis that growth is key to explain variations in adults returning to rivers to reproduce, with differencial responses between males and females. The project relies on the analysis of 21 to 47 years of empirical data from five monitored wild populations, and on the development of a population model with a focus on the marine phase of the life cycle. I show that high growth over first summer at sea favours early sexual maturation (thus a return to freshwater to reproduce). Females need to reach higher body size in order to mature.Individual growth over the first months of the marine migration declined over time. This temporal signal in growth over first summer at sea is common to several populations, underlying the effect of common environmental stressors. This response is combined with differences between populations, underlying that variability in age at return is partly due to factors specific to each population. Finally we show that post-smolt survival, depending on smolt size, and maturation rate, depending on sex and marine growth, ultimately condition age and sex structure in returns. However temporal variability of return rate and reproductive potential essentially relies on the survival rate of post-smolts.L’objectif de cette thèse a été d’acquérir une meilleure compréhension des mécanismes écologiques et démographiques qui déterminent les traits d’histoire de vie et la structure des populations de saumon atlantique Salmo salar. Je teste l’hypothèse que la croissance occupe une place centrale dans les variations des retours des adultes en rivière, avec des réponses différentes chez les mâles et les femelles. Le projet s’appuie sur l’analyse de 21 à 47 années de données empiriques provenant du suivi de cinq populations sauvages sud-européennes, et sur la construction d’un modèle de population centré sur la phase marine du cycle de vie. Je démontre qu’une meilleure croissance en mer pendant le premier été favorise une maturation sexuelle précoce (donc un retour en rivière pour la reproduction). Les femelles doivent atteindre une taille plus importante pour déclencherla maturation. La croissance pendant les premiers mois de la migration marine a diminué au cours du temps. Ce signal temporel dans la croissance au premier été en mer est commun à plusieurs populations, soulignant l’effet de pressions environnementales communes. Cette réponse s'accompagne de différences entre populations: la variabilité de l'âge moyen à maturation est en partie due à des facteurs spécifiques à chaque population. Enfin, nous mettons en évidence que la survie des post-smolts, dépendant de la taille des smolts, et le taux de maturation, dépendant du sexe et de la croissance en mer, conditionnent la structure d’âge et de sexe des retours. Cependant la variabilité temporelle du taux de retour et du potentiel reproducteur dépend essentiellement de la variabilité du taux de survie des post-smolts

A multi‐population approach supports common patterns in marine growth and maturation decision in Atlantic salmon ( Salmo salar L.) from southern Europe

Abstract This study provides a regional picture of long‐term changes in Atlantic salmon growth at the southern edge of their distribution, using a multi‐population approach spanning 49 years and five populations. We provide empirical evidence of salmon life history being influenced by a combination of common signals in the marine environment and population‐specific signals. We identified an abrupt decline in growth from 1976 and a more recent decline after 2005. As these declines have also been recorded in northern European populations, our study significantly expands a pattern of declining marine growth to include southern European populations, thereby revealing a large‐scale synchrony in marine growth patterns for almost five decades. Growth increments during their sea sojourn were characterized by distinct temporal dynamics. At a coarse temporal resolution, growth during the first winter at sea seemed to gradually improve over the study period. However, the analysis of finer seasonal growth patterns revealed ecological bottlenecks of salmon life histories at sea in time and space. Our study reinforces existing evidence of an impact of early marine growth on maturation decision, with small‐sized individuals at the end of the first summer at sea being more likely to delay maturation. However, each population was characterized by a specific probabilistic maturation reaction norm, and a local component of growth at sea in which some populations have better growth in some years might further amplify differences in maturation rate. Differences between populations were smaller than those between sexes, suggesting that the sex‐specific growth threshold for maturation is a well‐conserved evolutionary phenomenon in salmon. Finally, our results illustrate that although most of the gain in length occurs during the first summer at sea, the temporal variability in body length at return is buffered against the decrease in post‐smolt growth conditions. The intricate combination of growth over successive seasons, and its interplay with the maturation decision, could be regulating body length by maintaining diversity in early growth trajectories, life histories, and the composition of salmon populations