Distribution, population size and IUCN Red Listing of an isolated population of the threatened franciscana

Abstract

The franciscanaPontoporia blainvilleiis the only extant member of the family Pontoporiidae, and its occurrence is restricted to coastal waters from Brazil to Argentina. The species is one of the most threatened cetaceans in the southwestern Atlantic Ocean, due to unsustainable bycatch levels. A total of 11 Franciscana Management Areas (FMAs) have been defined throughout the species’ range. FMA Ia represents the northernmost franciscana population, demographically isolated on the north coast of Espírito Santo State, Brazil. In March 2018, aerial surveys were conducted to assess the distribution and estimate the abundance of franciscanas in FMA Ia. A total of 2986 km of survey effort was conducted, and 27 groups were seen (average group size: 2.52 ind., coefficient of variation [CV] = 0.50) in coastal habitats (average distance from the shore: 3.3 km, CV = 0.70). Abundance, corrected for visibility bias and group size bias, was estimated at 1183 (CV = 0.76) individuals, and the potential biological removal was computed at 1. Results suggest that at least during the summer, franciscanas in ES are distributed in coastal habitats between Conceição da Barra and Santa Cruz, with a high-density area observed near the estuary mouth of the Doce River. This is one of the smallest of all franciscana populations and one with a restricted range. The estimated abundance indicates that the ES population qualifies for listing as Endangered under IUCN Red List criterion C2a(ii). In order to reduce threats to this population, management actions are urgently needed.