Threshold foraging behaviour of basking sharks on zooplankton: life on an energetic knife-edge?

The minimum threshold foraging response of basking sharks has not been dete rmined despite the widely held view that has been perpetuated in the litera ture for the past 45 years that this species cannot use low prey densities for net energy gain and so lives on an energetic 'knife-edge'. An early the oretical estimate suggested basking sharks would expend more energy collect ing zooplankton at concentrations < 1.36 g m(-3) than could be obtained fro m it. This led to the claim that basking sharks will feed at an energetic l oss for much of the annual cycle as zooplankton abundance outside summer mo nths is too low for net energy gain to occur. Here I show from theoretical calculations and behavioural studies on individual and group-feeding sharks in the English Channel that basking sharks have a theoretical threshold pr ey density of between 0.55 and 0.74 g m(-3) and an observed foraging thresh old of between 0.48 and 0.70 g m(-3) (mean = 0.62 g m(-3)). The close agree ment between theoretical and empirical threshold values suggests basking sh arks can achieve net energy gain in much lower zooplankton densities than p reviously thought. The findings imply that this species may not be reliant upon the 'migration-hibernation' energy conservation strategy it is purport ed to exhibit when seasonal zooplankton abundance decreases below 1.36 g m( -3).