On the following subject:
"Feeding of planktivorous fish in the Marseilles region (north-western Western Mediterranean)".
All the MIO staff warmly congratulate her.
Abstract
Diet of planktivorous fish in the Marseilles region (north-western Western Mediterranean)
Thesis supervisor: François Carlotti, CNRS Research Director
Co-director: Danièla Banaru, Senior Lecturer, AMU
This thesis analyses temporal variations in the diet and relative body condition of 8 species of planktivorous fish caught in the Bay of Marseille (north-western Mediterranean), in relation to their trophic environment over a 20-month observation period.
The analysis highlights the seasonality and inter-annual differences of environmental parameters and the cascading mechanisms of their impacts in planktonic food webs right down to planktivorous predators. This study begins by identifying the different sources of organic matter and analysing the contribution of climatic and anthropogenic forcings to these inputs. Using stable isotopic tracers of carbon (δ13C) and nitrogen (δ15N), we demonstrate how this matter is transferred between the different planktonic trophic levels, and finally identify the levels that serve as a resource for planktivorous fish. We also show that variations in inputs are partly attenuated, or at least temporarily delayed at the level of the fish, with an impact that can be quantitative and qualitative.
This study shows that the diet and relative body condition of planktivorous fish depend directly on the most energetic planktonic species. Analysis of the taxonomy of prey in stomach contents shows that the different planktivorous species consume different main prey in the range 500-2000 µm, and therefore share the same resource, which is also shown by stable isotope analysis. The relative body condition of fish varies seasonally with the dynamics of prey, but also with the size and sex of individuals for certain species. For sardines, the most comprehensive study, the best condition was observed in spring-summer, linked to more energy-rich prey, while in autumn-winter their condition was low, also linked to the investment of energy for egg-laying. The link with plankton was highlighted by taking into account a time lag, for a period corresponding to the integration of organic matter into fish muscles. In the study of the biochemical quality of plankton, it was observed that phytoplankton were richer in lipids, while the zooplankton in the environment and the plankton in the sardine's food bolus were richer in proteins.
This study also looks at the results obtained in the Bay of Marseille in the wider context of the Gulf of Lion, which since 2007 has seen a very marked change in the population dynamics of small pelagic fish, in terms of dominant species, stocks and the quality of the individuals caught. Sardine and anchovy in the Bay of Marseille were larger and in better condition than in the Gulf of Lion. This work tends to demonstrate that a fine temporal observation of the condition of fish in relation to the trophic environment is necessary, as it allows us to take into account the reaction times of the various components of the trophic chains down to the fish. Overall, the combination of methods used in this thesis has enabled us to make significant progress in understanding the complex dynamics of the links between the environment, plankton and planktivorous fish in the Bay of Marseille. This will enable us to better understand the ecological and economic repercussions of changes in the planktonic food web in the Gulf of Lion.
Key words: planktivorous fish, plankton, relative body condition, temporal variations, diet, biochemical composition