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

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Abstract
The minimum threshold foraging response of basking sharks has not been determined despite the widely held view that has been perpetuated in the literature 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 theoretical estimate suggested basking sharks would expend more energy collecting zooplankton at concentrations less than 1.36 g m−3 than could be obtained from it. This led to the claim that basking sharks will feed at an energetic loss for much of the annual cycle as zooplankton abundance outside summer months 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 prey density of between 0.55 and 0.74 g m−3 and an observed foraging threshold of between 0.48 and 0.70 g m−3 (mean = 0.62 g m−3). The close agreement between theoretical and empirical threshold values suggests basking sharks can achieve net energy gain in much lower zooplankton densities than previously thought. The findings imply that this species may not be reliant upon the ‘migration–hibernation’ energy conservation strategy it is purported to exhibit when seasonal zooplankton abundance decreases below 1.36 g m−3.