Killer whale predation on bluefin tuna: exploring the hypothesis of the endurance-exhaustion technique

Killer whales Orcinus orca occur in the area of the Strait of Gibraltar, where they prey on migrating bluefin tuna Thunnus thynnus. In the spring, killer whales were observed to chase tuna for up to 30 min at a relatively high sustained speed (3.7 +/- 0.2 m s(-1)) until they captured them. Using simple models based on previous locomotor performance data on killer whales and thunnids, we investigated the hypothesis that killer whales push tuna beyond their aerobic limits to exhaust and capture them. To test this hypothesis, the endurance of bluefin tuna was estimated from data on maximum burst and aerobic swimming available for bluefin and yellowfin tuna T albacares. The endurance performance of killer whales was evaluated on the basis of the maximal rate of oxygen uptake during exercise (V-O2max). We modelled the maximum aerobic power output for a killer whale according to swimming speed using a V-O2max ranging between 20 and 30 ml O-2 kg(-1) min(-1). The output of this model was compared to the observed sustained swimming speed of killer whales chasing prey over long durations. Our results support the hypothesis that killer whales may use an endurance-exhaustion technique to catch small to medium sized (up to 0.8 to 1.5 m) bluefin tuna, while larger tuna may be inaccessible to killer whales unless they use cooperative hunting techniques or benefit through depredation of fish caught on long lines, drop lines or trap nets.