BARKLEY ET AT: SKIPJACK TUNA HABITAT 



until the temperature reached 33 °C, when two 

 died; the other lived until the water reached 34° C 

 (Dizon et al. 1977). Skipjack tuna have a high 

 metabolic rate and a countercurrent heat ex- 

 changer in their circulatory system which dramat- 

 ically restricts heat loss through the gills. This 

 accounts for the fact that freshly caught wild skip- 

 jack tuna can have red muscle core temperatures 

 as high as 11° C above that of the surrounding 

 water (Stevens and Fry 1971). 



Temperature excesses of this magnitude could 

 lead to dangerously high muscle temperatures if 

 they occur in the warmer parts of the ocean. To 

 examine this possibility we use a heat balance 

 model developed for skipjack tuna by Neill et al. 

 (1976) which yields an estimate of temperature 

 excess in the red muscle core as a function of size 

 and metabolic activity (Figure la). Actual muscle 

 core temperatures are found by adding the values 

 shown in this figure to the temperature of the 

 surrounding water, for any given size fish. 

 Clearly, large skipjack tuna in surface waters of 

 the tropics must either tolerate high muscle core 

 temperatures, or reduce their metabolic activity 

 substantially below the 3 mg Og g"^ h-^ level. 



But skipjack tuna appear to avoid heating their 

 muscle tissue much above 35° C (Stevens and Fry 

 1971 ). This upper limit must place a similar upper 



limit on the water temperatures which skipjack 

 tuna can inhabit, unless they can thermoregulate 

 physiologically or behaviorally. In Figure lb, 

 35°C is taken as the upper limit for the red muscle 

 core, and temperature excesses of Figure la are 

 subtracted from that value to arrive at an estimate 

 of the upper temperature limits for the habitat of 

 skipjack tuna, as a function of size. If the values 

 thus obtained are valid, these fish should be able to 

 live anywhere in the ocean when they are small, 

 but they should be limited to lower and lower 

 environmental temperatures as they grow. The 

 largest known skipjack tuna, weighing approxi- 

 mately 16 kg, would — if active enough — be 

 confined to water temperature near 18°C, which is 

 also their approximate lower limit. 



SKIPJACK TUNA HABITAT 

 HYPOTHESIS 



We hypothesize that skipjack tuna of the central 

 and eastern Pacific Ocean occupy a primary 

 habitat — a volume of water whose properties they 

 can tolerate indefinitely — which is 18°C or 

 warmer, but cooler than the upper limits for nor- 

 mally active animals shown in Figure lb, provided 

 that the dissolved oxygen concentration is at least 

 3.4 ml/I (5 ppm). Skipjack tuna can presumably 



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WEIGHT OF SKIPJACK TUNA (Kg) 



20 



Figure la. — Calculated excess of internal temperature, over that of the surrounding water, in red muscle for skipjack tuna of all 

 known sizes. Values are shown for a measured minimum (anesthetized) level of metabolic activity (lower line) and our estimate of the 

 mean metabolic activity for normally active animals (upper line), triple the minimum level. (From Neill et al. 1976.) 



655 



