Table h.—Concenlralions of Pleuroncodes planipes, adults and juveniles {not larvae), in ml./Wm? of water strained, on 



cruise TO-BS-l 



(Lettersunder kind of observation signify: M, micronekton haul; Z, zooplankton liaul; H, liigli-spced net haul between stations; S, seen in the water; P, found in 

 predator stomachs. Where concentrations were measured or estimated by more than one method, the highest concentration, corresponding to the first letter, 

 is listed) 



1 stomach contents of 3 yellowfln tuna (Thunnus albacares) at Thetis Bank. 



2 Series of night surface hauls with the large micronekton net near Uncle Sam Bank. 



3 After station 67, on northbound track between lat. 25°50' N. and 26°40' N., at night. 



ply (40 ml./l,000 m.=' or more of red crabs), for 

 all cruises except TO-64-1. On that cruise tlie 

 tunas showed a tolerance of temperatures down to 

 17° C, but no lower in the presence of a larger 

 food supply than that available to them at or over 

 20° C. Only one small area of abundant food (over 

 40 ml./l,000 m.^) and no area in which it was 

 highly abundant (over 100 ml./l,000 m.^) had 

 temperatures at and over 20° C; food was highly 

 abundant only at temperatures below 17° C. This 

 difficulty (for the tunas) did not arise on any of 

 the other cruises. On cruise TO-64-2 the edges of 

 the areas of abundant food were warm enough 

 for the tunas, which were found there and nowhere 

 else. On cruises TO-59-2, TO-65-1, and TO-66-1 

 all parts of the f ood-ricli areas were warm enough 

 for tunas, which were, accordingly, widespread in 

 them; they might have been found in still other 

 places in the food-ricli areas if the fishermen had 

 searched there. On cruise 6608 the situation was 

 partly like cruise TO-64^2 and partly like the 

 other cruises. Except on cruise TO-64^1, no tuna 

 were ever found more than 20 nautical miles (37 

 km.) from the charted boundaries of the food-rich 



areas. 



From these results I may conclude that yellow- 

 fin and skipjack tunas aggregate in the areas of 

 most abundant food where surface temperatures 

 are about 20° ( ±1°) C. or over in waters west of 

 Baja California, except at the beginning of their 



seasonal entry into those waters wlien they may 

 occur at temperatures down to 17° C. Temperature 

 determines range limits (penetration northward 

 and toward the coast) , and food supply determines 

 distribution within the range limits. Because the 

 principal food in this area is the red crab which 

 occurs in areas rich in phytoplankton, tunas gen- 

 erally aggregate in or near the areas of highest 

 surface chlorophyll a, provided that temperatures 

 are suitable. 



Because the distributions of temperature, cliloro- 

 phyll a, and red crabs are all partly controlled by 

 the seasonal coastal upwelling, at least through 

 September, the same is true of the distribution of 

 the tunas. Concentrations of tuna prey are much 

 higher off western Baja California than anywhere 

 else in the eastern tropical Pacific, where they are 

 generally less than 10 ml./l,000 m.^' (Blackburn, 

 1968). These high values are a consequence of the 

 upwelling and of the unusually short tuna food 

 chain. Tlie upwelling, however, furnislies an envi- 

 ronment that tends to be too cold for yellowfin and 

 skipjack tunas in spite of its biological richness, 

 and this physical feature is decisive. The tunas do 

 not enter the area and exploit the rich food supply 

 until temi>eratures begin to rise. They then aggre- 

 gate around the edges of the large tongues or 

 patches of food and gradually penetrate into the 

 cores of those areas as they become warm. Later 

 the distribution of both the food and the tunas 



170 



U.S. FISH AND WILDLIFE SERVICE 



