The Mediterranean Sea is home to a significant marine biodiversity with a high rate of endemism (around 25% of species). The fish population (Osteichthyans, Chondrichthyans and Agnatha) is composed of 684 species, of which 9.2% are endemic. Beyond the fundamental role played by fish populations in the trophic dynamics and flows of matter and energy within marine ecosystems, they also provide human societies with many ecosystem services (e.g., source of protein, regulation of food web dynamics, aesthetic and recreational values generating important revenues from tourism and recreational activities). However, biodiversity and, particularly at a more global level, the functioning of coastal ecosystems, are deeply and increasingly affected by anthropic pressures, which are and will be amplified by the effects of climate change, particularly in the semi-closed Mediterranean Sea. In contrast with the complex dynamics of global change at a larger scale, local anthropogenic pressures can be identified and managed more easily at the regional scale. To do this, the quantification and evaluation of the consequences of these pressures are necessary, especially regarding the fish community.
In this study, we assessed the impacts of four anthropogenic pressures common to the Mediterranean coasts on the ichthyological community present in the Bay of Calvi in Corsica (France). The anthropogenic pressures included effluents from the outfall of the Calvi wastewater treatment plant, an aquaculture farm, recreational fishing activities and scuba diving. The results indicated that the discharges from the outfall do not lead to any modification of the ichthyological assemblage despite a local enrichment of the water column in nutrients. The research also revealed that the small-scale production aquaculture influences the fish community by causing an aggregation of omnivorous or carnivorous species, leading to an increase in total abundance and species richness in the vicinity of the cages. Although the aggregative effect of the cages is demonstrated, the radius of impact is very limited in space and the observed aggregation remains minimal compared to other larger fish farms. The multi-approach study carried out on recreational fishing made it possible to quantify and characterise the population of local fisher in Balagne for the first time, as well as to establish the characteristics of boat fishing, which is predominantly practiced in this region. This assessment revealed that recreational boat fishers catch 28 tonnes of fish annually, equivalent to 55 % of the total production of professional fisher in Balagne. In addition, our study has shown that around 50 % of catches did not meet the minimum authorised in terms of mesh size. Finally, the study carried out on SCUBA diving enabled the first quantification of the activity in the entire Bay of Calvi, with more than 25,000 divers recorded in 2019. This pressure has specific impacts on fish populations’ habitat, with on average 1 contact per minute between divers and the substrate. The results of our study also showed that non-experienced divers are responsible for most of the involuntary contacts, while experienced divers and/or photographers are involved in most of the voluntary contacts. In order to reduce the impact of divers on fish habitat, we implemented and tested a measure to raise awareness about the effects of repeated diver contacts on rocky habitats. As a result, divers who benefitted from information to raise awareness about their impact before diving had significantly less voluntary and involuntary contacts while diving. Lastly, the disturbance of fish by divers did not have the same effects depending on the species considered or size, within the same species. Although there were no major changes in the total abundance or diversity index, the abundance of three species (Dentex dentex (Linnaeus, 1758), Epinephelus marginatus (Lowe, 1834) and Diplodus vulgaris (Geoffroy Saint Hilaire, 1817)) was found to be significantly reduced during/after the divers’ passage. Furthermore, the behaviour of brown grouper (E. marginatus) towards divers showed variations based on the size of the individual considered, while noting a tendency to flee among larger individuals (> 70 cm).
The above-mentioned results were discussed in the context of global change and particularly of climate change, which will induce a major evolution of Mediterranean ecosystems in the upcoming decades.
This thesis shows the significant progress made in our ability to understand, prioritise and anticipate the impacts of local anthropogenic pressures on Mediterranean coastal fish populations. It further provides management recommendations that will need to be adapted to the human-environment dynamics in the context of global change.