Maximum sustainable speeds and cost of swimming in juvenile kawakawa tuna (Euthynnus affinis) and chub mackerel (Scomber japonicus)

Tunas (Scombridae) have been assumed to be among the fastest and most effic ient swimmers because they elevate the temperature of the slow-twitch, aero bic locomotor muscle above the ambient water temperature (endothermy) and b ecause of their streamlined body shape and use of the thunniform locomotor mode. The purpose of this study was to test the hypothesis that juvenile tu nas swim both faster and more efficiently than their ectothermic relatives. The maximum sustainable swimming speed (U-max, the maximum speed attained while using a steady, continuous gait powered by the aerobic myotomal muscl e) and the net cost of transport (COTnet) were compared at 24 degreesC in s imilar-sized (116-255 mm fork length) juvenile scombrids, an endothermic tu na, the kawakawa (Euthynnus affinis) and the ectothermic chub mackerel (Sco mber japonicus), U-max and COTnet were measured by forcing individual fish to swim in a temperature-controlled, variable-speed swimming tunnel respiro meter, There were no significant interspecific differences in the relations hip between U-max and body mass or fork length or in the relationship betwe en COTnet and body mass or fork length. Muscle temperatures were elevated b y 1.0-2.3 degreesC and 0.1-0.6 degreesC above water temperature in the kawa kawa and chub mackerel, respectively. The juvenile kawakawa had significant ly higher standard metabolic rates than the chub mackerel, because the tota l rate of oxygen consumption at a given swimming speed was higher in the ka wakawa when the effects of fish size were accounted for. Thus, juvenile kaw akawa are not capable of higher sustainable swimming speeds and are not mor e efficient swimmers than juvenile chub mackerel.