Figure 1. — Changes in latitudinal position of the 21° C s«rface isotherm and of the northern limits of commercially 

 caught yellowfin and skipjack tunas off the coast of Baja California and California, 1951-65. 



when suitable food is abundant in those waters, but 

 do occur in waters of surface temperature at and 

 above 20° C, provided suitable food is abundant. 

 "Suitable food" was defined as pelagic red crab, 

 and "abundant" was defined as a concentration at 

 or above 40 ml. displacement volume per 1,000 m.^ 

 of water. The specified temperature, prey species, 

 and concentration of prey are explained below. 



The cruises were intended also to identify 

 environmental properties that determine the dis- 

 tribution of the red crab, which is important in 

 the diet of other commercial fishes as well as 

 yellowfin and skipjack tunas (Boyd, 1967). Noth- 

 ing was known about these properties except that 

 temperature was not one of them (Longhurst, 

 1967) . I thought that the properties would include 

 standing stock of phytoplankton or of zooplank- 

 ton, depending upon whetlier the red crab was 

 predominantly herbivorous or carnivorous in the 

 area studied; in the outcome it proved, as ex- 

 pected, to be predominantly lierbivorous (Long- 

 hurst et al., 1967). Further, the cruises were 



intended to investigate possible relations between 

 the tuna-connected properties — temperature, prey, 

 food of the prey — and physical features of the 

 environment such as upwellings and fronts. The 

 results of those studies are included in this paper. 

 The range-limiting temperature was specified in 

 the hypothesis as 20° C. for both species of tuna, 

 but some deviation from it was expected. Range- 

 limiting temperatures for yellowfin and skipjack 

 have been discussed by Uda (1957), Blackburn 

 and associates (1962), Laevastu and Rosa (1963), 

 Broadhead and Barrett (1964), Blackburn (1965), 

 and others. It is evident from these papers that 

 successful commercial fishing, which requires a 

 fairly high concentration of the fish, seldom occurs 

 at temperatures below 20° C. for yellowfin tuna 

 or below 19° C. for skipjack tuna. Both species 

 can occur in waters as cool as 15° C. in some parts 

 of the world, however. In the eastern Pacific the 

 limiting temperature appears to be nearl}- always 

 close to 20° C. for commercial concentrations of 

 both species. Blackburn and associates (1962) 



DISTRIBUTION OF TROPICAL TUNAS OFF WESTERN BAJA CALIFORNIA 



149 



