Opportunistic records and beach surveys of stranded animals are cost-effective and efficient methods that have been used for decades to assess species diversity, occurrence, distribution, and mortality patterns of marine megafauna. However, the ad hoc approach of most stranding data can make the analysis tricky and results must be interpreted carefully. For example, most studies rely on opportunistic (for example, reports from the public) or non-systematic surveys. This effort data is not associated with standardized results and can lead to biased conclusions of stranding patterns.
In this Editor's Choice article, the authors describe the temporal and spatial patterns of more than 50 000 sea turtle, seabird, and cetacean strandings observed during systematic beach surveys. For five consecutive years, and across more than 800 km of coastline in southeast Brazil, these surveys were carried out on a daily basis (totalling > 1.1 million km of beach surveyed). Survey effort was accounted for by using stranding rates (individuals/1000 km beach surveyed) for descriptive analyses and an offset term (log of km beach surveyed) in an exploratory modelling approach using generalized additive models for location, scale and shape (GAMLSS). The use of GAMLSS in ecology is relatively new, and it allows for modelling the sigma parameter (scale) of the response distribution. In addition to the usual inferences based on the mean (location), the authors explore how the stranding data variability varied over space and time.
Higher stranding numbers were observed during winter/spring for all megafauna groups. The study suggests that this temporal finding may be due to a higher occurrence/abundance of species in the region due to seasonal productivity compounded with stronger seasonal drifting forces. Most modelled species showed temporal and spatial stranding patterns related to their general occurrence/abundance cycles; however, local, small-scale mortality factors may have influenced stranding numbers of two sea turtle (Caretta caretta, Chelonia mydas) and two dolphin (Pontoporia blainvillei, Sotalia guianensis) species. For these species, other empirical studies have shown high bycatch in the areas offshore from the study sites, which is in line with higher stranding numbers found by this study in certain locations. Bycatch is a leading cause of mortality of marine megafauna and the authors suggest it should be considered as a non-trivial driver for the observed spatial patterns in mortality.
By incorporating survey effort in the analysis and covering a large spatial scale, the authors could better understand species occurrences and possible anthropogenic interactions in the studied region. Weighing stranding numbers according to survey effort helped minimise confounding factors and uncover reliable stranding patterns. This study stresses the need to account for effort data and present standardized results for stranding data, boosting hypotheses generation and testing while allowing comparisons between species and areas over time.
Read the full paper, Intensive and wide-ranging beach surveys uncover temporal and spatial stranding patterns of marine megafauna, in ICES Journal of Marine Science.
Editor's Choice articles are always free to read in ICES Journal of Marine Science.
Magellanic penguin (left), franciscana dolphin (upper right), and green turtle (lower right) found washed ashore during systematic daily beach surveys in southeast Brazil. © PMP-BS/Laguna