Populations of freshwater dolphins in the Amazon basin are in steep decline, dropping by half about every decade at current rates, according to a study published May 2, 2018 in the open-access journal PLOS ONE by Vera da Silva from Instituto Nacional de Pesquisas da Amazônia, Manaus, Amazonas, Brazil, and colleagues.

The Amazon basin is home to two species of river dolphins -- the boto (Inia geoffrensis) and the tucuxi (Sotalia fluviatilis) -- that were once considered to be abundant in some places. However, the development of dolphin hunting to provide fish bait at around the beginning of this millennium and a Data Deficient IUCN Red List conservation status suggest that more information is needed about their current population numbers and trends. To assess the status of these freshwater dolphins, da Silva and colleagues analyzed data from 22 years of surveys in the Mamirauá Reserve, Brazil. The dolphins were surveyed by boat monthly from 1994 to 2017.

The analysis revealed that populations of both boto and tucuxi Amazonian dolphins are dropping rapidly. At current rates, boto populations are halving every 10 years, and tucuxi populations are halving every nine years. These rates of decline are some of the most severe of those known for cetaceans since the early years of modern whaling. The researchers conclude that if IUCN Red List criteria were applied based on this work, both species would be classified as Critically Endangered. River dolphins are legally protected in the Amazon basin, and given these findings, the researchers call for greater enforcement of these laws.