SHARKS AND TUNAS: FISHERIES IMPACTS ON PREDATORS WITH CONTRASTING LIFE HISTORIES

Abstract

Large-scale pelagic fisheries exploit a diversity of apex predators with a wide range of life history strategies. Exploitation of species with different life history strategies has different population and food web consequences. We explored the changes in predation that result from exploitation of a common species with a slow growth and low fecundity life history strategy (blue shark, Prionace glauca) with those that result from exploitation of a common species with fast growth and high fecundity (yellowfin tuna, Thunnus albacares) in the central Pacific Ocean. Longline fisheries directed toward billfishes and tunas also capture blue sharks as incidental catch. Mortality rates of sharks had been relatively low prior to the recent surge in finning that has resulted in a substantial rise in mortality of adult and subadult sharks in the last decade. We estimated the magnitude of changes in predation by populations of yellowfin tuna and blue sharks in response to longline fisheries that involve shark finning. Bioenergetics models for sharks and tunas were coupled to simple population models that account for changes in size-structure in response to fishery-induced mortality regimes in order to estimate predation responses to changes in fishing intensities. Our analyses demonstrate that blue shark populations are very sensitive to low exploitation rates, while yellowfin tuna populations are extremely robust across a wide range of exploitation rates by longline fisheries. Although predation rates by yellowfin tuna are 4–5 times higher than by blue sharks, longline fisheries have substantially greater effects on shark predation than on yellowfin tuna predation at the food web scale. Expected food web responses will be strongest where the unexploited biomass of long-lived species is high and predation is relatively specialized compared with other apex predators. Our analyses suggest that active management to reduce finning mortality in sharks will play an important role toward minimizing the effects of longline fisheries on the food web structure of the pelagic Pacific Ocean.