Broken expectations: Population decline of a key grouper species within a 30-year-old no-take MPA in the Southwestern Atlantic.

Understanding the impacts of human actions on marine species and developing mitigation strategies are crucial conservation challenges. Many fish species, especially marine top predators, are already highly depleted and rely on population monitoring and conservation strategies for their maintenance. In this context, Marine Protected Areas can help recover target species and provide crucial information about population trends when human activities are restricted or managed. Here, we used the longest and most complete temporal series of reef fish data in the Southwestern Atlantic, comprising 16 years of underwater visual census sampling. We evaluated temporal trends in the density and biomass of the dusky grouper (Epinephelus marginatus), both inside and outside a no-take and no-entry Marine Reserve. Our findings reveal a consistent decline in density and biomass inside and outside the MPA over time, reaching alarmingly low densities and experiencing a steeper biomass decline in the last five years. Our results contradict global expectations of positive population recovery trends within MPAs. We highlight the adverse impacts of human activities on the dusky grouper population, including illegal fishing and habitat degradation. Additionally, we address the limited understanding of species' life stages, underscoring that their occurrence outside protected boundaries may render the entire population vulnerable to human pressures. Thus, to guarantee population recovery, stakeholders and researchers must take immediate action to increase knowledge of all life stages and improve ongoing strategies. • Despite conservation efforts over decades, the dusky grouper population is declining. • The population is declining in one of the oldest No-Take areas in the Southwestern Atlantic. • The situation worsened in the last 5 years of the 16-year time series studied. • The Reserve might be underperforming due to the disregard of local factors. • Current conservation strategies must be urgently reviewed. [ABSTRACT FROM AUTHOR]