FISHERY BULLETIN: VOL 76, NO. 3 



with the habitat volume shown on the maps. If, 

 under normal oceanographic conditions, they 

 occur in significant numbers well outside of the 

 mapped habitat volume the hypothesis is invalid, 

 and we should look for evidence of adaptations 

 (behavioral, physiological, and genetic) which 

 were not evident in our work with captive fish. If 

 the distribution of skipjack tuna is more restricted 

 than our habitat maps would indicate, we should 

 look for factors in addition to temperature and 

 dissolved oxygen (e.g., turbidity) which might 

 further limit the habitat. Salinity is not apt to be 

 one of these factors, however, since captive skip- 

 jack tuna do not respond to rather drastic changes 

 in this variable (Dizon 1977). 



More immediate and definitive tests of the 

 hypothesis can be made by tagging wild skipjack 

 tuna of various sizes with tags capable of sensing 

 and telemetering depth, temperature, and dis- 

 solved oxygen. 



TEMPERATURE AND 

 OXYGEN REQUIREMENTS 



Lower Temperature Limit 



Dizon et al. (1977) determined that 15° C was 

 the lower lethal temperature limit for skipjack 

 tuna. They subjected recently caught, apparently 

 healthy individual fish to water temperatures 

 which decreased 1° or 5°C/day, starting at 

 ambient temperature (about 24° C), with other 

 skipjack tuna held continuously at ambient temper- 

 ature as controls. All seven of the test animals 

 survived and fed until temperature had declined 

 to 18° C; at 17° C all but one fish stopped eating, 

 and one fish died. None of the fish survived 15° C 

 for more than a few hours. Accordingly, we have 

 selected 18° C as the lowest water temperature 

 which Hawaii skipjack tuna can withstand for pro- 

 longed periods of time without significant in- 

 creases in mortality. 



This preliminary estimate of the lower tempera- 

 ture limit is essentially identical with the value 

 obtained by Williams ( 1970) from a comparison of 

 fishery catch data with sea-surface temperatures 

 in the eastern Pacific. Williams found that most 

 skipjack tuna were caught in water with tempera- 

 tures between 29° and 20° C, with diminishing 

 catches down to temperatures between 18° and 

 17° C, and no catches in colder waters. 



Three large (ca. 70 cm) Hawaiian skipjack tuna 

 released with tags that telemetered swimming 



654 



depth were found to spend >85'7f of their time in 

 water warmer than 20° C and < 10% in water col- 

 der than 18° C (Dizon et al. in press). 



In the western South Pacific, skipjack tuna have 

 been caught in water near 15° C, off Tasmania 

 (Robins 1952) and off eastern Australia (G. I. 

 Murphy, Division of Fisheries and Oceanography, 

 CSIRO, New South Wales, Australia, 1977, pers. 

 commun.). These fish probably belong to a differ- 

 ent subpopulation (Fujino 1972) than fish found in 

 Hawaii. 



Lower Dissolved Oxygen Limit 



Gooding and Neill (see footnote 4) examined the 

 effects of low dissolved oxygen concentrations on 

 skipjack tuna. Their animals, habituated in open 

 tanks with circulating, essentially saturated sea- 

 water (4.5 ml 0.>/l, or 6.4ppm), were transferred to 

 tanks in which the concentration of dissolved oxy- 

 gen could be maintained at a preselected constant 

 subsaturation level. Temperatures in both sets of 

 tanks were ambient, 23° to 24° C. Dissolved oxy- 

 gen concentrations down to 1.0 ml/1 ( 1.4 ppm) were 

 used. Resistance times and swimming speeds were 

 measured, and general behavior was observed for 

 up to 4 h in each experiment. Under their experi- 

 mental conditions, Gooding and Neill concluded 

 that hypoxic stress was first manifest, through 

 changes in swimming behavior and speed, at 

 about 2.8 ml/1 (4.0 ppm), a value fairly typical for 

 fish. Lethal oxygen levels, leading to death in 4 h 

 or less, were found to be higher than those for any 

 other freshwater or marine fish thus far studied. 

 Only one fish (out of six) survived 4 h at 2.5 ml/1 

 (3.5 ppm), and none survived as long as 2 h at still 

 lower concentrations. At higher oxygen values, 

 above 2.5 ml/1, all skipjack tuna tested in this 

 study survived at least 4 h. 



Because we sought to estimate the lowest dis- 

 solved oxygen concentrations that skipjack tuna 

 can tolerate indefinitely without significant 

 stress, we have chosen a conservative value of 3.5 

 ml/1 (5 ppm) as the lower limit to the skipjack 

 tuna's habitat, where temperature and other vari- 

 ables are not limiting. 



Upper Temperature Limit 



The case for an upper temperature limit to the 

 skipjack tuna's habitat is somewhat less direct. 

 Three small (30-35 cm) individual skipjack tuna 

 maintained in water warmed l°C/day survived 



