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

Empirical and theoretical determinations of minimum threshold prey densitie s for filter-feeding basking sharks Cetorhinus maximus were used to test th e idea that threshold foraging behaviour could provide a means for estimati ng oxygen consumption (a proxy for metabolic rate). The threshold leeding l evel represents the prey density at which there will be no net energy gain (energy intake equals expenditure). Basking sharks were observed to cease f eeding at their theoretical threshold; thus, the assumption underpinning th e concept presented here was that over the narrow range of zooplankton prey densities that induce 'switching' between feeding and non-feeding in baski ng sharks, the energetic value of the minimum threshold prey density is equ ivalent to the shark's instantaneous level of energy expenditure. Four inde pendent estimates of the lower threshold prey density obtained for C, maxim us in the English Channel were converted to equivalent rates of oxygen cons umption. 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, p roportion of prey filtered, buccal flow velocity, prey energy content and e nergy 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 compari son, 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 equat ion 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 relati onships 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 e xpected oxygen consumption values although derived from different methods s how 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 doe s actual metabolism.