Table 6. — Concentrations of Pleuroncodes planipes, adults and juveniles {not larvae), in mLjK^m} oj water strained^ on 



cruise TO-66-1 



fLetters under kind of observation signify: M, micronekton haul; Z. zooplankton haul; H, high-speed net haul between stations; S, seen in the water. Where 



concentrations were measured or estimated by more than one method, the highest concentration, corresponding to the first letter, is listed] 



' After station 68, on northbound track between lat. 23"'40' N. and 25°30' N., at night. 

 ' Immediately after station 69, on northbound track, at night. 



tuna distribution offered in this paper is that many 

 people who are acquainted with the Baja Cali- 

 fornia fisheiy consider that yellowfin and skip- 

 jack tunas are distributed in relation to the banks 

 shown in figure 2 and that abundance is higher 

 at the banks than elsewhere, except when tempera- 

 tures are low. No publications clearly demonsti-ate 

 this relation as far as Baja California is con- 

 cerned, and no studies off Baja California or else- 

 where show conclusively the nature of any "bank 

 effect" which might be attractive to tunas. In 

 fact, only one such study has be€n attempted 

 and the results were inconclusive (Bennett and 

 Schaefer, 1960, at Shimada Bank in the eastern 

 tropical Pacific). Nevertheless, belief in some kind 

 of favorable bank effect upon tunas is so wide- 

 spread that it must be considered here. 



The tuna catches charted in figures 4, 6, 8, 10, 

 12, and l-t may be compared with the bank posi- 

 tions in figure 2. Many of the catches were made 

 at or veiT near banks; they can be classified into 

 two gi-oups. All of those for the periods of cruises 

 TO-64-2, 6608, TO-59-2, and TO-65-1 were in 

 or close to tongues or patches of upwelled water, 

 which either enveloped or touched the banks. None 

 of the catches in the periods of cruises TO-64— 1 

 and TO-66-1 were associated with upwelled water. 

 On TO-64-1 no such water was warm enough for 

 the tunas to enter, and on TO-66-1 there was none 



at all. The charts show also that many catches 

 were in areas of upwelled water which were not 

 close to banks, except on cruises TO-64— 1 and 

 TO-66-1. 



When upwelled water of coastal origin extends 

 over banks, it provides a ready explanation not 

 only for the catches of tmia on banks (and at 

 Alijos Rocks, as noted above) but also for the 

 catches made between the banks. The unspe<;ified 

 bank effect may exist independently of the upwell- 

 ing effect, but it is not required to explain the 

 tuna distributions. If a bank effect exists, it is 

 probably small in relation to the upwelling effect, 

 and the banks are probably more suitable for tuna 

 when upwelled water reaches them (providing it 

 is not too cold) than when it does not. An observa- 

 tion from Uncle Sam Bank, which is very fre- 

 quently visited by fishermen during the tuna 

 season, is pertinent. Surface temperature charts 

 for CalCOFI cruises 6007-8 (July 26 to Aug. 13, 

 1960, in the area of interest) and 6008 (Aug. 20-22, 

 1960) both show tongues of cool water (but over 

 20° C.) protruding offshore from the northern 

 upwelling area. On the earlier cruise the tongue 

 lay considerably west of Uncle Sam Bank, but on 

 the later cruise it had changed its position and 

 enveloped the bank (Scripps Institution of Ocean- 

 ography, 1962a, 1962b). No tuna catches were re- 

 corded near the bank during the first cruise period. 



DISTRIBUTION OF TROPICAL TUNAS OFF WESTERN BA.IA CALIFORNIA 



173 



