STRUCTURAL AND FUNCTIONAL RESPONSES OF FISH ASSEMBLAGES TO DIFFERENT DRIVERS OF CHANGE

Biotic and abiotic factors synergistically act on natural systems. As a result ecological communities adjust their structural and functional responses to persist in a given area. Worldwide, marine ecosystems have been changing through time with increasing anthropogenic pressure in the last centuries leading to only 13.2% of the total marine environments in a ‘pristine’ state. The consequences of direct and indirect anthropogenic pressures may have on marine organisms can largely differ, with both negative and positive effects of drivers of change of human origin. In the span of three years for my PhD thesis, I performed different experiments with the main goal of examining how direct or indirect drivers of change may affect fish assemblages. For this purpose, I focussed on positive effects of protection activities within Marine Protected Areas (MPAs) in the Mediterranean Sea and potentially detrimental impacts of increasing CO2 concentrations in Italy, Japan, and Papua New Guinea. The first part of this thesis examined ecological responses within 11 Mediterranean Marine Protected Areas (MPAs) belonging to six different countries (France, Spain, Greece, Italy, Croatia, and Slovenia). I assessed whether and how MPAs can be successful in reaching their conservation goals (higher fish diversity and biomass than outside) and in re-establishing their trophic state (higher levels of top-down control inside their boundaries), despite differing management and protection schemes. Overall, results highlighted higher biomass levels both for whole fish assemblages and for commercial species targeted by small-scale and industrial fishery inside the 11 pilot MPAs. Overall predation rates were unaffected despite a few MPAs showing higher rates inside than outside their boundaries. The second part of this thesis tackled the direct and indirect effects of ocean acidification at an ecosystem level. Ecological surveys on both benthic habitat and fish assemblages were carried out in three different volcanic seeps belonging to diverse climatic zones in the Mediterranean and the Pacific ocean, with the main aim of assessing similar response trends. Chronical exposure to elevated CO2 levels did affect benthic communities in a direct way, indirectly modifying associated fish assemblages both in terms of diversity and relative abundances. Predation rates were similar among regions and within high CO2 and reference CO2 levels. Such findings support the hypothesis of unaltered food web dynamics under OA conditions despite community shifts and fish diversity changes, therefore contrasting with previous literature documenting unbalanced food webs both for benthic and fish communities. Therefore, the present results support a high trophic resiliency of fish communities, likely as a result of functional redundancy of fish predator species. In summary, through innovative and large-scale ecological surveys this thesis contributed to assessing Mediterranean MPAs efficacy, to address possible management solutions to reduce pre-existing conflicts between MPA stakeholders and fishers, and to understand the direct and indirect ecosystem-level impacts of OA scenarios on benthic and fish assemblages.