Abstract
Multiple changes have been described since 2008 in the Gulf of Lions ecosystem and particularly in small pelagic fish dynamic. In particular, the two main exploited species, i.e. anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) are now smaller and in poor condition while a third species, the sprat (Sprattus sprattus) strongly increased in terms of biomass and abundance. This PhD thesis investigated these changes through the analysis of small pelagic fish body condition (i.e individual energy stores) variations, its causes and consequences. Anchovy and sardine body condition was optimal in 2005 and 2006 and decreased after 2008 to remain steady at low level since 2010. Moreover, older sardine have particularly displayed poor body condition since 2008. A part of these changes is associated with changes in zooplankton concentration for both species, but also with both sea surface temperature and diatoms for sardine and Rhône outflow for anchovy. Thus, as bottom-up control was highlighted, the following chapter investigated potential changes in diet through isotope and stomach content analyses. Isotopic niche varied temporally and have been overlapping since 2010 for both anchovy and sardine with the sprat, resulting in a potential new trophic competition. Further, preys are also smaller and probably less energetic. Both studies may explain the poorer body condition by a reduced food supply. Although smaller energy stores are available, both species still allocate a large part of their energy to reproduction, as observed through e.g. gonado-somatic indices. This bias in energy allocation towards reproduction might impair survival and explain the disappearance of oldest mature sardine in the Gulf of Lions. Maternal effects were also underlined. Indeed, large individuals spawn more eggs while fatter individual spawn higher quality eggs. The decrease in size and condition has thus led to a lower production of eggs since 2010 for sardines while anchovy managed to maintain if not increase its egg production thanks to the expanding number of individuals and the decline of anchovy size and age at maturity. At a broader scale, we pointed out that fish body condition decreased in other Mediterranean areas such as the Adriatic Sea, the Catalan Sea or the Strait of Sicily. Nonetheless, body condition variations were not synchronous in the Mediterranean Sea, pointing out the importance of local factors in this quasi-enclosed basin. Furthermore, anchovy displays a better body condition in high energetic areas (fronts areas, local upwellings) when sardine prefers areas with high primary productivity. Those differences could be linked to the different reproductive cycles and trophic behaviours. This PhD thesis complements the previous work made on top-down processes and strengthens the bottom-up importance to determine forage fish body condition both in the Gulf of Lions and the Mediterranean Sea. Studying body condition allowed understanding the reasons of the sardine demographic truncation. Indeed, the older the individual the poorer the body condition, preventing them to achieve both reproduction and survival. This PhD thesis reaches new conclusions and improves knowledge on small pelagic fish dynamic in the Gulf of Lions. Although, further work is still necessary to estimate experimentally if a body condition threshold exists and to improve zooplanktonic monitoring to better understand the link between planktonic productivity and small pelagic fish dynamic. This will help to investigate deeply the ecological and economical consequences of small pelagic fish decreasing body condition in the Mediterranean.