1. Alimentation des poissons planctonophages de la région marseillaise (Nord-Ouest de la Méditerranée Occidentale).
- Author
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Chia-Ting CHEN
- Subjects
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TRACERS (Chemistry) , *STABLE isotope analysis , *GASTROINTESTINAL content analysis , *CARBON isotopes , *FOOD chains , *FISH anatomy , *FISH morphology - Abstract
This thesis analyses temporal variations in diet and the relative body condition of 8 species of planktivorous fish caught in the Bay of Marseilles in relation to their trophic environment during a 20-month observation period (October 2016-May 2018). The analysis highlights seasonality and inter-annual differences in environmental parameters and their impacts in terms of cascading in planktonic food webs to planktivorous predators. This study is based on a discrimination of the different sources of organic matter and analyses the impact of climatic and anthropogenic forcing on the organic matter. How this organic matter is transferred between the different planktonic trophic levels was demonstrated through the stable isotopic tracers of carbon (d13C) and nitrogen (d15N). Organic matter that serves as the resource for planktivorous fishes was identified. The variations of the inputs of organic matter are partially attenuated, or with different time lags for the fish, with an impact which can be quantitative and qualitative. The diet and relative body condition of planktivorous fish depend directly on the most energetic planktonic species that are rich in proteins and lipids. Results of stomach content analyses showed that different planktivorous species consumed different plankton prey with their size ranging from 500 to 2000 µm. They thus share the same resources, which was also identified with the stable isotope analysis. The relative body condition of the fish varies seasonally with the prey dynamics but also with the size and sex of the individuals for some species. For example, for sardines, the best relative body condition was observed in spring-summer, in relation to more energy-rich prey, while in autumn-winter, their condition was low due to the energy investment for laying. The link with plankton has been highlighted by taking into account a time lag, for a duration corresponding to the integration of organic matter in the fish muscles. This work tends to show that a finer seasonal investigation of the condition of fish in relation to the trophic environment is necessary because it allows to take into account the reaction times of the different components of trophic chains to fish. In general, the coupling of the methods used in this thesis has led to a significant advance in the understanding of the complex dynamics of the links between the environment, plankton and planktivorous fish in the Bay of Marseilles. This will make it possible to better understand the ecological and economic repercussions of the planktonic food web changes in the Gulf of Lion. [ABSTRACT FROM AUTHOR]
- Published
- 2020
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