Dynamics of Reef Fish Communities in Algarve's Greatest Reef: A baseline biological assessment prior to Marine Protected Area (MPA) implementation in Southern Portugal

Coastal zones are heavily impacted by human activities, leading to significant biodiversity loss and reduced fish populations. Marine Protected Areas (MPAs) are established to mitigate these impacts, but assessing their effectiveness may be complicated by environmental variability. To differentiate effects of protection measures from spatio-temporal environmental changes, it is crucial to start monitoring prospective MPAs before their establishment and carry out regular follow-up. For this purpose and in order to enable future supervision, we conducted the baseline monitoring of the coming MPA - The Marine Park of the Algarve Reef– by characterizing the demersal fish community in the different submarine areas and in a control area outside the future protected zone. The study used Stereo Baited Remote Underwater Video (SBRUV) and aims at assessing whether there are pre-existing differences in species richness, fish assemblage, abundance, biomass and habitat among the different zones (i.e., with different levels of protection), which is essential to prevent overestimation or potential errors in the assessment of the effects of the future protection measures. This study highlights the great diversity of demersal species within the MPA, identifying 43 species spread across fifteen families, including four threatened species requiring protection. Multivariate analysis revealed significant differences in abundance and biomass between future protection zones and the control zone, with some specific species contributing most to these differences, indicating potential changes in fish assemblage composition due to regulatory measures. Futhermore, the study found that habitat complexity and sampling season did not influence these differences. This study collected multi-year and multi-season data, which allows detecting changes in the structure of the fish assemblage between years and seasons. These data will be useful in determining the effectiveness of the MPA's different zones on the fish population and ecosystem after its creation.