Carcass Decomposition Conceals the True Magnitude of Small Cetacean Bycatch in Southern Brazil

ABSTRACT

Bycatch in fisheries is a primary threat to small cetaceans, but its true magnitude is often underestimated, particularly in regions with less thorough stranding response networks. Advanced decomposition of stranded carcasses often obscures evidence of fisheries interaction, masking the actual rate of anthropogenic mortality. Here, we quantify fisheries interactions affecting small cetaceans along the Southern Brazilian coast and estimate cryptic mortality masked by decomposition. We analyzed data from 321 necropsied cetaceans collected by the Santos Basin Beach Monitoring Project (PMP‐BS) from 2016 to 2019, focusing on the Franciscana dolphin ( Pontoporia blainvillei ) and the common bottlenose dolphin ( Tursiops truncatus ). A high fisheries interaction rate (68%) was observed among the 25 well‐preserved carcasses. However, most carcasses were found in an advanced state of decomposition, precluding direct analysis. Extrapolating this interaction rate to decomposed P. blainvillei carcasses increased the estimated number of bycaught individuals from 68 to 182, indicating that mortality may be severely underreported. Our results highlight that decomposition masks the actual impact of fisheries, which is likely far greater than currently recorded. This extrapolation approach provides a crucial tool for more realistic mortality assessments.