Anadromous migratory fish populations (e.g. Atlantic salmon and sea trout) have undergone a significant decline in their abundance since the 1970s across their entire distribution range, together with changes in their life history traits (e.g., decline in the average age at reproduction). Causes of this decline are multifactorial and involve multiple stressors acting on different stages of the life cycle, in different environments (in freshwater, at sea) and at different spatial scales.
Available knowledge supports the hypothesis that these changes partly result from a population response to changes in the marine ecosystem, leading to changes in the biotic or a biotic environment of fish during marine migration. However, the marine phase of the life cycle of migratory fish remains difficult to observe and knowledge about the mechanisms involved in the response of individuals and populations to these changes remains fragmented.
The deteriorated state of these fish populations of great patrimonial interest has motivated the implementation of management measures at the regional, national and international levels. The core of those management measures are based on population dynamic models used to support scientific expertise on the status of populations and to predict their evolution (e.g. resilience). However, these models are generally based on the assumption of stable environmental conditions. In particular, they do not take into account the influence of recent and possibly rapid changes in the marine environment on the dynamics and viability of populations.
In order to improve the quality of scientific expertise and the effectiveness of management measures, it is necessary to improve the understanding of the mechanisms of the plasticity of salmonids life cycle, in particular in response to changes in the (marine) environment.
This PhD project is part of the European SAMARCH project (http://samarch.org/; Interreg FMA programme) which aims to address this challenge by building on the synergy between 10 French and English partners from both the academic and management communities.