Exceptionally high but still growing predatory reef fish biomass after 23 years of protection in a Marine Protected Area

Abstract

Marine Protected Areas (MPAs) help replenish fish assemblages, though different trophic levels may show diverse recovery patterns. Long-term protection is required to achieve total recovery but poaching events may prevent the achievement of full carrying capacity. Here, we have analysed the effect of long-term protection on the entire reef fish community and the different trophic levels in the Cabo de Palos-Islas Hormigas MPA (SE Spain; SW Mediterranean Sea) in order to assess their recovery patterns after 23 years of protection. We compared the values for carrying capacity obtained with the maximum values achieved at regional scale, and we assessed the effect of a reduction in the surveillance over a few years, during which poaching events increased, on the recovery patterns. We found that, overall, biomass of fishes increased with time while density diminished. In particular, piscivorous and macro-invertivore fish increased while the other trophic groups remained constant or declined, suggesting top-down processes. For the entire study period, those trophic groups were approaching carrying capacity; however, when accounting only for the period in which enforcement was high and constant, they grew exponentially, indicating that full carrying capacity may have not been achieved yet. When compared to other Mediterranean MPAs, the Cabo de Palos-Islas Hormigas MPA showed values for biomass that were disproportionately higher, suggesting that local factors, such as habitat structure and associated oceanographic processes, may be responsible for the dynamics found. Our results help to understand the potential trajectories of fish assemblages over a consolidated MPA and highlight empirically how the reduction of surveillance in a period may change the recovery patterns.

Fig 1. Study area.

Study area showing the Cabo de Palos-Islas Hormigas MPA and the Cabo Cope unprotected area and their location in the SE of Spain, indicating the protection levels of the MPA.

Fig 2. Population trajectories for the descriptors related to the whole fish community in the Cabo de Palos-Islas Hormigas MPA and in the Cabo Cope unprotected area.

Population trajectories are shown for the biomass (expressed as g · 250 m^-2) and density (number of individuals · 250 m^-2) response variables (± SE) for the Cabo de Palos MPA (black circles) and the Cabo Cope unprotected area (white circles). The dashed line indicates the model selected when the whole period of study was considered. The black line indicates the population trajectory during the first period in which surveillance was high and constant. The light grey line indicates the trajectory of the fish after the reestablishment of the surveillance.

Fig 3. Population trajectories for the trophic descriptors in the Cabo de Palos-Islas Hormigas MPA and in the Cabo Cope unprotected area.

Population trajectories are shown for the biomass (expressed as g 250 m^-2) and density (individuals 250 m^-2) response variables (± SE) for the Cabo de Palos MPA (black circles) and the Cabo Cope unprotected area (white circles). The dashed line indicates the model selected when the whole period of study was considered. The black line indicates the population trajectory during the first period in which surveillance was high and constant. The light grey line indicates the trajectory of the fish after the reestablishment of the surveillance.

Fig 4. Abundance of predatory species in the Cabo de Palos-Islas Hormigas MPA.

a) A large group of Dentex dentex interspersed with individuals of Epinephelus costae; photo: Javi Ferrer; b) Individuals of the species Epinephelus costae, Epinephelus marginatus and Mycteroperca rubra; photo: Johann Mourier.