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Determinants of small pelagic fish population dynamics

Last update: : Friday 5 April 2019, by Nicolas Busser

Around 2008, an ecosystem shift occurred in the Gulf of Lions, highlighted by considerable changes in biomass and fish mean weight of its two main small pelagic fish stocks (European anchovy; European sardine). Surprisingly these changes did not appear to be mediated by a decrease in fish recruitment rates (which remained high) or by a high fishing pressure (exploitation rates being extremely low). To understand these population’s dynamics, we used an integrative ecosystem approach exploring alternative hypotheses, ranging from bottom-up to top-down control, not forgetting epizootic diseases. First, the study of multiple population characteristics highlighted a decrease in body condition for both species as well as an important decrease in size resulting from both a slower growth and a progressive disappearance of older sardines. While top-down control by bluefin tuna or dolphins, emigration and disease were mostly discarded as important drivers, bottom-up control mediated by potential changes in the plankton community appeared to play an important role via a decrease in fish energy income and hence growth, condition and size. Isotopic and stomach content analyses indicated a dietary shift pre- and post-2008 and modeled mesozooplankton abundance was directly linked to fish condition. Investigating abiotic environment through satellite and in situ data, we showed a decrease in chlorophyll-a at a similar time most likely as a result of changes in the Rhône river discharge and nutrient inputs. Despite low energy reserves from 2008 onwards, sardines and anchovies maintained if not increased their reproductive investment, likely altering the life-history trade-off between reproduction and survival and resulting in higher natural mortality. Using experimental approaches, we further studied the effect of food size and quantity on sardine life-history traits to better understand the mechanisms involved in the bottom-up control of sardine populations and try and estimate natural mortality in the wild.