Area-dependent environmental effects shape physiological characteristics in three small pelagic fish populations

Identifying and monitoring the processes underlying changes in individual life-history traits and population size are essential to better predict natural populations' responses to environmental changes and sustainably manage natural resources. In marine ecosystems, the global decline in small pelagic fish (SPF) growth and survival capacities was linked to overfishing and climate change, but the underlying mechanisms remain elusive, although SPF play a central ecological role and sustain large fisheries. To address this issue, we combined a set of physiological markers (assessing digestive enzyme activity, oxidative, chronic, and nutritional stress) and environmental variables for sardine (Sardina pilchardus) in three environmentally contrasted areas surrounding the French coast. Overall, we found sardines with higher chronic stress, lower content in eicosapentaenoic acid (EPA), and lower digestive enzyme activity in the Gulf of Lions, an area marked by a 30% decline in sardine size-at-age and a strong adult overmortality. In contrast, sardines from the English Channel, where no such demographic decline is observed, exhibited better physiological conditions. Sardines appeared to be in poorer physiological health under warmer sea surface temperatures and lower phytoplankton concentrations. However, links between physiology and environmental variability were less clear in the Bay of Biscay, an area with similar growth and survival declines than in the Gulf of Lions. These findings highlight the critical impact of environmental factors on the physiological status and overall health of sardine populations but with global and local patterns of response, emphasizing the unique challenges faced by each population of this species.