Filter-feeding and cruising swimming speeds of basking sharks compared with optimal models: they filter-feed slower than predicted for their size

Movements of six basking sharks (4.0-6.5 m total body length, L-T) swimming at the surface were tracked and horizontal velocities determined. Sharks w ere tracked for between 1.8 and 55 min with between 4 and 21 mean speed det erminations per shark track. The mean filter-feeding swimming speed was 0.8 5 m s(-1) (+/-0.05 S.E., n = 49 determinations) compared to the nonfeeding (cruising) mean speed of 1.08 m s(-1) (+/-0.03 S.E., n = 21 determinations) . Both absolute (m s(-1)) and specific (L s(-1)) swimming speeds during fil ter-feeding were significantly lower than when cruise swimming with the mou th closed, indicating basking sharks select speeds approximately 24% lower when engaged in filter-feeding. This reduction in speed during filter-feedi ng could be a behavioural response to avoid increased drag-induced energy c osts associated with feeding at higher speeds. Non-feeding basking sharks ( 4 m L,) cruised at speeds close to, but slightly faster (similar to 18%) th an the optimum speed predicted by the Weihs (1977) [Weihs, D., 1977. Effect s of size on the sustained swimming speeds of aquatic organisms. In: Pedley , T.J. (Ed.), Scale Effects in Animal Locomotion. Academic Press, London, p p. 333-338.] optimal cruising speed model. In contrast, filter-feeding bask ing sharks swam between 29 and 39% slower than the speed predicted by the W eihs and Webb (1983) [Weihs, D., Webb, PW., 1983. Optimization of locomotio n. In: Webb, PW., Weihs, D. (Eds.), Fish Biomechanics. Praeger, New York, p p. 339-371.] optimal filter-feeding model. This significant under-estimatio n in observed feeding speed compared to model predictions was most likely a ccounted for by surface drag effects reducing optimum speeds of tracked sha rks, together with inaccurate parameter estimates used in the general model to predict optimal speeds of basking sharks from body size extrapolations. (C) 2000 Elsevier Science B.V. All rights reserved.