Salmonids as trout (Salmo trutta) are characterized by a diversity of strategies that can be presented as a gradient of strategies ranging from “resident trout” which grow and reproduce in their natal river, to “sea trout” which migrate to grow at sea for several months or years and come back in their natal river to reproduce. Different tradeoffs between survival, growth and reproduction may be responsible for the diversity of these strategies. Depending on the relative importance of each trait, it is predicted that different strategies may be differently impacted by environmental changes. For instance, a degradation of the freshwater environment, as a density increase, may have a strong impact on resident trout through reduced survival and growth. In contrast, strategies allowing individuals to temporarily emigrate out of a limiting environment should be favored as it may offer greater fitness opportunities. To study the differential impact of density increase on strategies traits and improve our understanding of mechanisms underlying temporal changes in strategies, we used a unique individual-based long term monitoring (1997-2015) of a trout population where a gradient of strategies co-exists. In this French river, a multi-event capture recapture approach allowed us to compare key transitions of life cycle.
Results/Conclusions
Our results highlight different responses to density change in resident and sea trout, respectively. The probability for a juvenile to follow a given strategy changed with density. The resident strategy was dominant in the population throughout the study, but the probability for a juvenile to stay in the river decreased as the density increased, thus the probability to emigrate increased. This suggests that the median strategy has changed and moved along the gradient of life history strategies. Moreover, life history traits were influenced by density and these responses depended on the strategy. Resident individuals showed a decrease of survival and body size that were significantly correlated to density increase. In contrast, sea trout traits did not appear to vary according to density. This environmental degradation seemed to favor emigration as it may offer greater fitness opportunities compared to the resident strategy. Consequently under increasing density, the predominant life history strategy of a population can change, with expected consequences on population dynamics. Therefore, this result highlights the importance of considering jointly the different responses of each life history strategy to improve our understanding and management of natural population dynamic.