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

A 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+/-20degreesC. The bonito rate of oxygen uptake ((V) over dot (O2))-speed function is U-shaped with a minimum (V) over dot (O2), at 60 cm s-1, an exponential increase in (V) over dot (O2), with increased speed, and an elevated increase in (V) over dot (O2), 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 (U-opt) for the bonito at 18 +/-2degreesC is approximately 70 cm s(-1) (1.4 FL s(-1)) and the gross cost of transport at U-opt is 0.27 J N-1 m(-1). The mean standard metabolic rate (SMR), determined by extrapolating swimming (V) over dot (O2), to zero speed, is 107+/-22 mg O-2 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.