Intensive and wide-ranging beach surveys uncover temporal and spatial stranding patterns of marine megafauna

Abstract

Abstract We describe the temporal and spatial patterns of three tetrapod groups (sea turtles, seabirds, and cetaceans) stranded in southeast Brazil, based on daily beach surveys of over >800 km of coastline between 2015 and 2020. Patterns were analysed by stranding rates (individuals/1000 km of coastline) of groups and species; for the 14 most numerous species, we also fitted generalized additive models for location, scale, and shape (GAMLSS) using temporal and spatial explanatory variables. We recorded 53850 animals (22738 sea turtles, 28155 birds, and 2957 cetaceans) of 78 species. Higher stranding numbers were observed during winter/spring for all groups, and could be an effect of a higher occurrence/abundance at the region, driven by productive waters, jointly with higher stranding probabilities due to stronger drifting forces. Most modelled species showed temporal and spatial stranding patterns most likely related to their general occurrence/abundance cycles in southeast Brazil, but for Caretta caretta, Chelonia mydas, Pontoporia blainvillei, and Sotalia guianensis local mortality factors may be influencing stranding numbers. By using intensive, wide ranging beach surveys and including survey effort in the analysis, we provide a comprehensive baseline for stranding numbers in the region, and open a pathway for future hypothesis-testing studies to investigate its drivers.