Table 7. — Total number of juvenile Auxis and (in parentheses) the 

 average number per tow collected by a 12 m wide by 8 m high mid- 

 water trawl during the day (1200-1800), night (2000-0200). and morn- 

 ing (0400-1000), RV Townsend Cromwell cruise 32, 12 July-25 

 September 1967. The trawl, fished at a depth of 100 m during deep 

 hauls and 20 m during shallow hauls, was towed at a speed of 1.5 

 m/s (Higgins 1970). 



'The duration of these tows was <6 h. 



young tunas collected in inshore waters off the island of 

 Oahu. Offshore, the catch rate dropped to 0.2 juvenile/ 

 tow. 



2.22 Adults 



Despite the argument that the distribution of A. 

 thazard cannot be separated from that of A. rochei 

 because of difficulties in distinguishing the two species, 

 a study of stomach contents of tunas and billfishes indi- 

 cated that there are conspicuous differences between the 

 pattern of occurrence of these two species. Watanabe 

 (1964) found that in the Banda Sea, specimens of A. 

 rochei were found in the stomach contents more fre- 

 quently than those of A. thazard, although both species 

 occurred there. This pattern of occurrence reflected the 

 difference in abundance of the two species in the Banda 

 Sea. Off the Queensland coast in Australia, all the 

 sp)ecimens collected from tuna and billfish stomachs 

 were A. rochei. 



2.3 Determinants of distribution changes 



The extreme southern boundary of the distribution of 

 Auxis in the Indian Ocean lies at about lat. 36°S which 

 is extremely close to the position of the 20°C isotherm 

 for the greater part of the year, including the southern 

 summer (Williams 1963). Off South Africa and 

 Australia, the occurrence of Auxis coincides with the 

 time of maximum water temperature. Likewise, off East 

 Africa and the Seychelles, Auxis occur in the months of 

 the northwest monsoon when the temperature is max- 

 imal at about 29°-30°C. The appearance of Auxis in 

 East African inshore waters also coincides with the time 

 of greatest fertility of the surface waters. 



Temperature, it has been shown, is clearly a highly 

 important variable in explaining the distribution of 

 Auxis larvae. Klawe et al. (1970) observed that the op- 

 timum temperature of surface waters under which larval 

 Auxis are found is between 27.0° and 27.9°C (Fig. 11). 

 Their tolerance for temperature, however, is very wide. 

 Richards and Simmons (1971) determined that Auxis 

 larvae occur in waters with surface temperatures as low 

 as 21.6°C and as high as 30.5°C, the widest range among 

 any of the tuna larvae they studied. 



The vertical distribution of larval Auxis has been 

 reported to be limited to the layer above the thermo- 

 cline. Comparing average catches of tuna (including 

 Auxis) larvae for two types of plankton tows made in the 

 eastern Pacific, Klawe (1963) found that the surface 

 tows usually caught 9.2 times as many larvae as the 

 deep tows. But comparison of catches made by surface 

 and by 140 m oblique tows showed that the former 

 caught an average of only 3.2 times as many larvae as 

 the latter. In fact, the numbers of larvae caught in the 

 simultaneous surface and oblique tows were signifi- 

 cantly correlated. Klawe found that the relative number 

 of tuna larvae taken in surface and oblique hauls ap- 

 proximates the ratio obtained from the depth of the obli- 

 que tow to the depth of the layer above the thermocline 

 thus substantiating the conclusion earlier reached by 

 others that larvae are limited to the layer above the 

 thermocline. 



Among other determinants of larval distribution that 

 have been examined are salinity, plankton, and light 

 conditions. Klawe et al. (1970) found no relationship be- 

 tween zooplankton volumes and larval catches; 

 therefore, their findings are in agreement with those of 

 Strasburg (1960) and Nakamura and Matsumoto (1967) 

 who observed this phenomenon for tuna larvae in 



190 20.0 210 220 230 24.0 250 260 270 280 29.0 



I I I I I I I I I I I 



199 209 219 229 239 249 259 269 279 289 299 



TEMPERATURE (°C) 

 Figure 11. — Average catches, log'" (number caught +1). of larval 

 Auxis grouped according to surface temperatures at the time of 

 capture (Klawe et al. 1970). 



14 



