Congratulations to Chia-Ting CHEN (EMBIO team) who defended her thesis on Monday, November 25, 2019
On the following subject:
"Feeding planktonophagous fish from the Marseille region (northwestern Western Mediterranean)"
All MIO staff warmly congratulate her
Feeding planktonophagous fish in the Marseille region (North-Western Western Mediterranean)
Thesis Director: François Carlotti, CNRS Research Director
Co-Director: Danièla Banaru, AMU Senior Lecturer
This thesis analyses the temporal variations in the diet and relative body condition of 8 species of plankton fish caught in the Bay of Marseille (northwestern Mediterranean), in relation to their trophic environment during a 20-month observation period.
The analysis highlights the seasonality and interannual differences in environmental parameters and the cascading mechanisms of their impacts in planktonic food webs to plankton-phagous predators. This study starts from a discrimination of the different sources of organic matter, and analyses the share of climatic and anthropogenic forcings in these contributions. Stable isotopic tracers of carbon (δ13C) and nitrogen (δ15N) demonstrate how this material is transferred between the different planktonic trophic scales, and finally identify the scales used as a resource for planktonophagous fish. It is also shown that variations in inputs are partially mitigated, or at least temporarily displaced in fish, with an impact that can be both quantitative and qualitative.
This study shows that the diet and relative body condition of planktonophagous fish are directly dependent on the most energetic planktonic species. Analysis of the taxonomy of prey in stomach contents shows that different plankton species consume different main prey in the 500-2000 µm range, and therefore share the same resource, which is also shown by the analysis of stable isotopes. The relative body condition of fish varies seasonally with prey dynamics, but also with the size and sex of individuals for some species. For sardines, whose study was the most complete, the best condition was observed in spring-summer, in relation to more energy-rich prey, while in autumn-winter, their condition was low, also related to the investment of energy for egg laying. The link with plankton was demonstrated by taking into account a time lag, for a duration 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 food bowl were richer in protein.
This study also addresses the results obtained in the Bay of Marseille in the broader context of the Gulf of Lion, which has seen a very marked change in the population dynamics of small pelagic fish since 2007, both in terms of dominant species, stocks and the quality of individuals caught. Sardines and anchovies from the Bay of Marseille were larger with a better condition than in the Gulf of Lions. This work tends to demonstrate that a detailed temporal observation of the condition of fish in relation to the trophic environment is necessary, because it makes it possible to take into account the reaction times of the various components of the trophic chains up to the fish. In general, the coupling of the methods used in this thesis has led to a significant advance in understanding the complex dynamics of the links between the environment, plankton and plankton fish in the Bay of Marseille. This will provide a better understanding of the ecological and economic impacts of changes in the planktonic food web in the Gulf of Lions.
Keywords: plankton fish, plankton, relative body condition, time variations, diet, biochemical composition