SEPULVEDA, Chugey A.; GRAHAM, Jeffrey B.: The swimming energetics of the eastern Pacific bonito (Sarda chiliensis): One step closer to understanding the tuna-bonito relationship

Fishes in the family Scombridae have morphological adaptations for fast, efficient locomotion. The most derived scombrids, the tunas (tribe Thunnini), appear to further increase swimming efficiency by elevating aerobic locomotor muscle temperature above that of the ambient water (endothermy). Despite apparent adaptations for efficient locomotion, tunas have maintenance costs several times higher than those of other teleosts. Studies to determine the role of tuna endothermy in swimming efficiency and the basis of the tuna’s high standard metabolic rate (SMR), require comparative studies on the sister taxa to the tunas, the ectothermic bonitos (tribe Sardini). A swimming tunnel respirometer was used to estimate the swimming energetics of the eastern Pacific bonito (Sarda chiliensis) and test the hypothesis that endothermy increases tuna swimming efficiency (despite their high maintenance metabolism). Oxygen consumption rates (VO₂), SMR, and the incremental cost of locomotion, or net cost of transport (COT_net) were determined for 12 eastern Pacific bonito at several sustainable swimming velocities. The mean (+/-SEM) COT_net at 18&deg C was 2.36 + 0.37 J g^-1 km^-1 and the mean SMR was 0.097 + 0.017 mgO₂ g^-1 h^-1. When adjusted for temperature differences and compared to published data for similar-sized tunas, the eastern Pacific bonito appears to have a similar COT_net and a lower SMR. This does not support the hypothesis that endothermy increases swimming efficiency. Rather the data suggest that bonitos swim as efficiently as do tunas with a lower overall metabolic expenditure.