Ecological role and historical trends of large pelagic predators in a subtropical marine ecosystem of the South Atlantic

Abstract

Large pelagic predators occupy high positions in food webs and could control lower trophic level species by direct and indirect ecological interactions. In this study we aimed to test the hypotheses: (1) pelagic predators are keystone species, and their removals could trigger impacts on the food chain; (2) higher landings of pelagic predators could trigger fishing impacts with time leading to a drop in the mean trophic level of catches; and (3) recovery in the pelagic predators populations, especially for sharks, could be achieved with fishing effort reduction. We performed a food web approach using an Ecopath with Ecosim model to represent the Southeastern and Southern Brazil, a subtropical marine ecosystem, in 2001. We then calibrated the baseline model using catch and fishing effort time series from 2001 to 2012. Afterwards, we simulated the impact of fishing effort changes on species and assessed the ecological impacts on the pelagic community from 2012 to 2025. Results showed that the model was well fitted to landing data for the majority of groups. The pelagic predators species were classified as keystone species impacting mainly on pelagic community. The ecosystem was resilient and fisheries seem sustainable at that time. However, the temporal simulation, from 2001 to 2012, revealed declines in the biomass of three sharks, tuna and billfish groups. It was possible observe declines in the mean trophic level of the catch and in the mean total length of landings. Longline fisheries particularly affected the sharks, billfish and swordfish, while hammerhead sharks were mostly impacted by gillnet fishery. Model simulations showed that large sharks’ biomasses could be recovered or maintained only after strong fishing effort reduction.