Wikimedia Commons / Mannheim Reinhard JahnSouth American sea lion: after leaving Uruguay and reaching the waters off Rio Grande do Sul, it reproducesWikimedia Commons / Mannheim Reinhard Jahn
The study of marine mammals demands the use of expensive tracking equipment because these animals can swim long distances without approaching the coast and can stay underwater much of the time. One alternative is to survey beaches for their carcasses. While the method has its limitations – after all, not every animal that dies in the water washes up on the beach – it is very informative. In his doctoral dissertation, under the advisorship of Eduardo Secchi at the Federal University of Rio Grande (FURG), oceanographer Jonatas Prado analyzed data from coastline monitoring in the state of Rio Grande do Sul, running from Patos Lagoon to the city of Chuí and covering the period from 1976 to 2013 (PLOS One, January 27, 2016). During that time, 12,540 animals of 40 species were found. Most (97%) belonged to five of these species: the Franciscana dolphin, the common bottlenose dolphin, the South American sea lion, the South American fur seal, and the subantarctic fur seal. The most interesting finding concerned the relation between the beached mammals and their ecological context. The highest mortality of dolphins coincided with the period of heaviest fishing, when they die trapped in nets. On the other hand, seals were more often found during their post-reproductive migratory period, while the deaths of sea lions appeared to be associated with both types of events.
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