The body temperatures (Tb) of nine yellowfin tuna (Thunnus albacares) were monitored while fish swam in a large water tunnel at controlled velocities (U) and ambient temperatures (Ta). Monitoring Tb during step changes in Ta at constant U permitted estimation of the thermal rate coefficient (k), an index of heat transfer. In the yellowfin, k is dependent on both Ta and the direction of the thermal gradient (i.e. whether Ta is greater or less than Tb). Modulation of k in response to Ta was further demonstrated during tests in which U was varied; the elevation of Tb in response to equal increases in U was 3&shy;4 times less at 30 C than at 25 and 20 C. These experiments demonstrate that the yellowfin tuna can modulate heat transfer. This ability could prevent overheating during intense activity, retard heat loss during a descent into cool water and permit increased heat gain upon returning to warm surface waters (i.e. when Tb<Ta).

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