Table 4. — Concentrations of Pleuroncodes planipes, adults and juveniles (not larvae), in ml.jlO^m} of water strained, on 



cruise TO-59-2 



[Letters 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] 



which might have indicated upwelling, was a very 

 small one off Point Tosco (21°-22° C). Tempera- 

 tures for the whole area were between 18° and 26° 

 C. on part 1 of the cruise; on part 2, temperatures 

 at reoccupied stations were only slightly (less than 

 1° C.) lower. 



The range of chlorophyll a. concentrations was 

 about the same as on cruise TO-65-1, but their 

 distribution was more uniform. Concentrations 

 were ov-er 0.1 mg./m.^ in most of the area as shown 

 in figure 14. Concentrations were over 0.2 mg./m.^ 

 at all stations north of lat. 26° 20' N., and there 

 only. Similarly, concentrations of red crabs were 

 40 ml./l,000 m.^ or higher in most parts of the 

 cruise area (see table 6 for data). Finally, the 

 recorded catches of tuna (all tliose for the period 

 November 4-21) were scattered through the large 

 area of suitable temperatures and abundant red 

 crabs. 



There was no opportunity to make a similar 

 cruise closer to tlie end of the .season for yellowfin 

 and skipjack tunas off western Baja California, say 

 in December or January. Such a cruise would 

 probably have shown that the area available for the 

 tunas had contracted because of the southward 

 movement of isotherms, and a rather featureless 

 distribution of the tunas — similar to that on cruise 

 TO-66-1, associated with relatively uniform dis- 

 tributions of red crabs and chlorojihyll a — in the 

 area of suitable temperature. 



OCEANIC PROPERTIES AND THE 

 DISTRIBUTION OF TUNAS 



Tlie results of cruises TO-64^1, TO-64-2, 6608, 

 and TO-59-2 all show the expected close agreement 

 in detail between areas of relatively cool water 

 that are attributable to upwelling and areas of 

 relatively high surface chlorophyll a. The poor 

 agreement between isograms of temperature and 

 chlorophyll a in the northwestern portion of the 

 cruise area shown in figure 6 is not necessarily an 

 exception to the preceding statement, because the 

 cool water may not represent upwelling there. The 

 results of cruise TO-65-1 show a region of rela- 

 tively high surface chlorophyll a in a locality and 

 with a shape, which suggest an origin in upwelled 

 water. No signs of upwelling appeared on cruise 

 TO-66-1. The distributions of surface isotherms in 

 space and time are consistent with previous in- 

 formation al)out upwelling off southern Baja Cali- 

 fornia. Temperatures rise and chlorophyll a 

 concentrations fall in the upwelling areas as the 

 upwelling bei-omes weaker. 



The results of all the cruises show rather close 

 agreement in detail between the areas of relatively 

 high surface chlorophyll a and the areas in wliich 

 the concentration of red crabs is more than 40 

 ml./l,000 m\ The concentration of chlorophyll a 

 whicli shows this agreement varies; on most of the 

 cruises it was 0.2 or 0.1 mg./m\, but it was higher 

 on cruise TO-64^1 and lower on cruise TO-59-2. 

 The only complete lack of agreement appears in 



DISTRIBUTION OF TROPICAL TUNAS OFF WESTERN BAJA CALIFORNIA 



167 



