CRUISE TO-65-1 

 SEPTEMBER 1965 



<21 



lllllll 



YELLOWFIN TUNA 



SKIPJACK TUNA 



BOTH TUNA SPP. 



SURFACE TEMPERATURE, °C. 



RED CRAB, > 40 Ml./lO^M.^ 



SURFACE CHLOROPHYLL, > 0.2 MG./M.' 



IIB-W. 



28" 

 N. 



2T 



26- 



25° 



24" 



23" 



22" 



riGUBE 12. — Distributions of surface temjierature, surface chlorophyll a, and red crabs for cruise TO-65-1 



and locations of contemporaneous tuna catches. 



numerous in inshore waters off the west coast of 

 southern Baja California before and during the 

 upwelling season, and benthic adults probably pro- 

 duce many of them (Longhurst, 1968a) . The main- 

 tenance of pelagic aggregations is probably served 

 also by the formation of inshore eddies after June 

 (Wyllie, 1966). I assume that these processes, to- 

 gether with the animal's own appreciable mobility, 

 maintain high concentrations in inshore areas 

 where food (phytoplankton) is abimdant; when 

 food is scarce inshore red crabs presumably sink. 



disperse, or die. 



The presence of red crab concentrations in cool 

 water on some of the cruises results from the 

 relation between chlorophyll a and upwelling, and 

 is, therefore, not apparent on cruises TO-65-1 and 

 TO-66-1. Longhurst (1967) showed that red crabs 

 are practically eurythermal between about 9° and 

 28° C. 



The data on tuna occurrence are all consistent 

 with the hypothesized association with surface 

 temperature (about 20° C. or more) and food sup- 



DISTRIBUTION OF TROPICAL TUNAS OFF WESTERN BAJA CALIFORNIA 



169 



