On the following subject:
"Responses of fish populations to local anthropogenic pressures
in a context of climate change".
Director: PHILIPPE CUNY, UNIVERSITY PROFESSOR
Co-director: SYLVIE GOBERT, UNIVERSITY PROFESSOR
Summary of the thesis
The Mediterranean Sea is home to significant marine biodiversity, with a high rate of dendemism. Fish represent 684 species, of which 9.2 % are endemic. In addition to the necessary role played by fish in trophic dynamics and the flow of matter and energy within marine ecosystems, human societies also benefit from numerous ecosystem services generated by fish populations. However, the biodiversity and functioning of Mediterranean coastal ecosystems are among the most affected on a global scale due to increasing anthropogenic pressures, the impacts of which are and will be amplified by the effects of climate change. Unlike the dynamics associated with global change, local anthropogenic pressures can be more easily identified and managed on a regional scale. To do this, it is necessary to quantify and assess precisely the impacts of these pressures, particularly on the fish community. In this study, the impacts on the ichthyological community of four anthropogenic pressures common to Mediterranean coasts were examined in the Bay of Calvi in Corsica (France): effluents from the wastewater treatment plant, an aquaculture farm, recreational fishing and scuba diving. The results show that discharges from the effluent treatment plant do not lead to any change in the ichthyological assemblage, despite a local enrichment of the water column in nutrients. The aquaculture studied influences the wild fish community by causing species aggregation, leading to an increase in total abundance and species richness in the vicinity of the cages. Although the aggregative effect of the cages is proven, the radius of impact is very limited spatially. The multi-approach study carried out on recreational fishing made it possible to quantify and characterise the local fishing population in Balagne for the first time. In addition, our results show that 28 tonnes of fish are taken annually by recreational boat fishermen. Our study also revealed that around 50 % of catches do not comply with the minimum authorised catch size. Finally, the work carried out on scuba diving has enabled us to quantify, for the first time, the activity in the entire Bay of Calvi, with more than 25,000 divers recorded in 2019. This pressure results in particular in pressure on the habitat of fish populations, with divers making an average of 1 contact per minute with the substrate. The factors influencing diver behaviour were also studied. In order to reduce the impact of divers on the habitat, a diver awareness measure was introduced and tested. Finally, although no major changes were noted in total abundance or diversity indices, the abundances of three species (D. dentex, E. marginatus and D. vulgaris) were significantly reduced during/after the divers' passage. In addition, the behaviour of brown grouper towards divers showed variations according to the size of the individual, with a marked flight behaviour in the case of larger individuals. Finally, all these results were discussed in the context of climate change, which will lead to major changes in Mediterranean ecosystems over the coming decades. This thesis represents a significant advance in our ability to understand, prioritise and anticipate the impacts of local anthropogenic pressures on Mediterranean coastal fish populations, and recommends management measures that will need to be adaptive to future developments in global change.