FISHERY BULLETIN: VOL. 70, NO. 3 



adoption of the deep-swimming habit is age-spe- 

 cific rather than size-specific and that the smaller 

 fish are scarce in eastern longline catches because 

 they are not old enough to adopt the deep-swim- 

 ming habit. This proposal is not as farfetched 

 as it might seem, for albacore appear to adopt 

 the deep-swimming habit at a size (about 

 13.6 kg) at which yellowfin tuna still school at 

 the surface. Furthermore, the fact that the size 

 at which males dominate the catches increases, 

 as the average size of the individuals in the deep- 

 swimming population increases (Figure 14) ar- 

 gues against migration and suggests that at 

 least some physiological changes are age-specific. 

 Behavior changes might easily be among these 

 age-specific attributes. 



In summary, there are east-west clines in 

 morphological and other biological characters of 

 the catches. These clines can be explained by 

 migration, if one chooses to ignore the evidence 

 of morphometry and the changes of size at the 



onset of male dominance. If the clines are at- 

 tributed to genotypic or phenotypic changes in 

 relatively sedentary stocks, all available evi- 

 dence can be accepted, but additional assump- 

 tions such as slower growth rate in the west 

 and age-specific behavior changes must be in- 

 troduced. 



MERIDIONAL DISTRIBUTION 



In a preceding section we examined the ocean 

 and the fish populations in plane view and con- 

 centrated on the descriptive aspects. Most of 

 the prominent features of the environment and 

 the fish populations are zonally aligned, and the 

 relations among the several fields can be most 

 effectively visualized when their properties are 

 examined in a meridional section. In presenting 

 the details of the oceanography, the tuna distri- 

 bution, and the tuna-oceanography interrelation- 



s'* 8" 

 LATITUDE 



10" iZ" 14° 16" 18" N 



Figure 15. — Meridional temperature section based on bathythermograms, Hugh M. Smith cruise 11 



along long 150° W (Austin, 1954b). 



886 



