Predation of <i>Otaria flavescens</i> over artisanal fisheries in Uruguay: opportunism or prey selectivity?

Abstract

Interactions between pinnipeds and fisheries have become an increasingly important topic for fisheries and pinniped management. In particular, the predatory behavior on fisheries is a cause of concern in many places because seals frequently opt to take fish from fishers' gear rather than searching and caching their own food. Sea lion prey selectivity on artisanal fisheries catches was analyzed, with the aim of determining if the predatory behavior was opportunistic or selective. Data were collected through direct observations of sea lion predation onboard during routine fishing trips at four fishing ports on the Uruguayan coast (Buceo, Piriápolis, La Paloma and Cabo Polonio), during two time periods (winter and spring-summer) in 1997/1998. The proportion of the most consumed fish and those most caught by the fishery was analyzed. An odds ratio was calculated as a selection index, using the number of prey items consumed by sea lions, and those caught by the fishery. Two general predatory strategies are apparent, one at the two fishing ports located on La Plata River estuary (Buceo and Piriápolis) and the second at the other two localities on the Atlantic Ocean coast. In the first strategy, the most consumed prey were the same most caught by the fishery (Macrodon ancylodon and Urophycis brasiliensis), suggesting an opportunistic behavior. However, at La Paloma and Cabo Polonio sea lions preyed mostly upon species which were not the main for the fishery (Cynoscion guatucupa in La Paloma, and Mustelus schmitti in Cabo Polonio) and exhibited selections and rejections of other species. Preferences and rejections however, represented small proportions of sea lion consumption and of the fishery catch. Seasonal differences in prey consumption and catches, as well as in selections and rejections were also evident. In some cases prey selections were reversed between both time periods. There was no evidence of an important conflict between sea lions and artisanal fisheries because the most selected species were not the most important for the fishery.