Can threshold foraging responses of basking sharks be used to estimate their metabolic rate?

Empirical and theoretical determinations of minimum threshold prey densities for filter-feeding basking sharks Cetorhinus maximus were used to test the idea that threshold foraging behaviour could provide a means for estimating oxygen consumption (a proxy for metabolic rate). The threshold leeding level represents the prey density at which there will be no net energy gain (energy intake equals expenditure). Basking sharks were observed to cease feeding at their theoretical threshold; thus, the assumption underpinning the concept presented here was that over the narrow range of zooplankton prey densities that induce 'switching' between feeding and non-feeding in basking sharks, the energetic value of the minimum threshold prey density is equivalent to the shark's instantaneous level of energy expenditure. Four independent estimates of the lower threshold prey density obtained for C, maximus in the English Channel were converted to equivalent rates of oxygen consumption. Best estimates ranged from 62.5 to 91.1 mg O-2 kg(-1) h(-1) (mean, 80.7 mg O-2 kg(-1) h(-1), +/- 20.1 [95 %, confidence interval, CI]) for a shark of 5 m total body length (L-T) weighing 1000 kg. Sensitivity analysis using 'low' and 'high' possible values in the model for mouth gape area, proportion of prey filtered, buccal flow velocity, prey energy content and energy absorption, yielded low and high rates of 23.2 and 192.1 mg O-2 kg(-1) h(-1), respectively. Varying estimated body mass of 1000 kg in the model by +/- 200 kg gave an oxygen consumption range of 67.2 to 100.8 mg O-2 kg(-1) h(-1); a range within the 95 % CI of the best estimate mean. For comparison, a new routine oxygen consumption-body mass relationship was determined for sharks body mass range, 0.35 to 140 kg) and was described by the equation VO2 = 0.30M(0.84), where VO2 is oxygen consumption in mg O-2 h(-1) and M is mass in grams. When corrected for likely energy costs associated with filter-feeding, this relationship and 2 other metabolic rate scaling relationships in the Literature gave expected rates between 52.0 and 99.2 mg O-2 kg(-1) h(-1) for a fish of 1000 kg body mass. The threshold-converted and expected oxygen consumption values although derived from different methods show good agreement, an observation that warrants further investigation. To verify the concept it will be necessary to obtain threshold-converted rates of oxygen consumption from a wide size range of basking sharks (1.5 to 10.0 m L-T) to determine whether rates scale predictably with body mass as does actual metabolism.