Swimming performance studies on the eastern Pacific bonitoSarda chiliensis, a close relative of the tunas (family Scombridae) I. Energetics

SUMMARYA large swim tunnel respirometer was used to quantify the swimming energetics of the eastern Pacific bonito Sarda chiliensis (tribe Sardini) (45–50 cm fork length, FL) at speeds between 50 and 120 cm s-1 and at 18±2°C. The bonito rate of oxygen uptake(V̇O2)–speed function is U-shaped with a minimum V̇O2 at 60 cm s-1, an exponential increase in V̇O2 with increased speed, and an elevated increase in V̇O2 at 50 cm s-1 where bonito swimming is unstable. The onset of unstable swimming occurs at speeds predicted by calculation of the minimum speed for bonito hydrostatic equilibrium (1.2 FL s-1). The optimum swimming speed (Uopt) for the bonito at 18±2°C is approximately 70 cm s-1 (1.4 FL s-1) and the gross cost of transport at Uopt is 0.27 J N-1m-1. The mean standard metabolic rate (SMR), determined by extrapolating swimming V̇O2 to zero speed, is 107±22 mg O2 kg-1 h-1. Plasma lactate determinations at different phases of the experiment showed that capture and handling increased anaerobic metabolism, but plasma lactate concentration returned to pre-experiment levels over the course of the swimming tests. When adjustments are made for differences in temperature,bonito net swimming costs are similar to those of similar-sized yellowfin tuna Thunnus albacares (tribe Thunnini), but the bonito has a significantly lower SMR. Because bonitos are the sister group to tunas, this finding suggests that the elevated SMR of the tunas is an autapomorphic trait of the Thunnini.