both tuna species prefer some temperature at or 

 over 20° C, will move into adjacent water as cool 

 as 17° C. if food is more plentiful there, and will 

 not enter water below 17° C. even if food is ex- 

 tremely abundant. If so, tuna will not round Cape 

 San Lucas from the east as long as water under 

 17° C, and perhaps under 20° C, remains there ; 

 data from Griffiths (1963) indicated that 20° C. 

 was the limiting temperature in May 1960. Under 

 those circumstances their entry into the area west 

 of Baja California will be by the offshore tongue 

 of warm water, as suggested by figure 4. Tlie area 

 enclosed by the offshore 20° C. isotherm in figure 

 4 was well occupied by both species in the second 

 half of June and the first half of July, although 

 there were still no records of tuna caught between 

 that area and the coast (between long. 110° and 

 in° ^V.). Tuna were not recorded in the latter 

 area until the second half of July, when they 

 suddenly became widespread; these fish were all 

 yellowfin tuna, and probably the same aggrega- 

 tions as shown to the east of the front in figure 4. 

 Probably the 20° C. isotherm moved northward 

 from Cape San Lucas about mid-July and per- 

 mitted the yellowfin tuna to round Cape San 

 Lucas; this isotherm was located at about lat. 25° 

 N. during the last 10 days of July (Scripps In- 

 stitution of Oceanography, 1966). Thus, tuna 

 appear to follow two pathways from the tropics 

 into the area west of Baja California at the begin- 

 ning of a tuna season, both determined by the 

 distribution of surface temperature. One is from 

 the east around Cape San Lucas, and the other 

 is from the south. The former is mainly for yellow- 

 fin tuna, and the latter is for both species; skipjack 

 tuna are much less common than yellowfin tuna to 

 the east of the meridian of Cape San Lucas in most 

 years (Joseph and Calkins, 1969). 



Cruise TO-64— 1 was the only one of tliis series 

 in which tuna occurred at temperatures substan- 

 tially lower than 20° C. They probably can tolerate 

 temperatures down to 17° C. to obtain a larger 

 food supply, as indicated above. Lee (1952) found 

 that cod will enter waters over 2° colder than those 

 in which they usually occur if food is jjlentiful. 

 On cruise TO-64-1 only one small area had more 

 than 40 ml./l,000 m.^ of red crabs in water at 

 20° C. or over (station 34, with 68 ml./l,000 m.^*). 

 On all tlie later cruises, such areas were extensive 



and the concentrations of red crabs in them were 

 generally over 100 ml./l,000 m.^ 



Several tuna boats were fishing off the south 

 coast of Baja California at the time the front was 

 surveyed, and their operations were watched to see 

 if their fishing success bore any relation to the 

 position of the front. The only obvious relation 

 was that they worked mainly to the east or north 

 of the front and occasionally on its warm edge. 

 They were probably avoiding water under about 

 20° C, and the front itself, as distinct from the 

 limiting isothenns located in it, seemed to have 

 no effect. 



Various authors have suggested that tunas may 

 aggregate in fronts in response to aggregations 

 of prey organisms. Griffiths (1963, 1965) found 

 that some kinds of zooplankton were more 

 aljundant in the middle of the Cape San Lucas 

 front than on either side of it, but, on the other 

 hand, micronekton (potential tuna forage) was 

 most abundant on the warm side. On cruise TO- 

 64— 1 micronekton hauls were again made on both 

 sides of the front and in the middle, all on the 

 same night ; the highest concentration was on the 

 warm side, as in Griffiths' series, but all three con- 

 centrations were similar (16.3 ml./l,000 m.' warm 

 side; 9.1, middle; 11.8, cold side). 



All the foregoing observations were made in the 

 part of the front that is oriented parallel to the 

 south coast of Baja California. Another series 

 of seven micronekton hauls, made across the 

 stronger part of the front near Cape San Lucas on 

 cruise TO-64-1, showed highest concentrations 

 in the upwelled water on the cold side. The 

 evidence, therefore, does not support the idea of 

 a concentration of tuna forage in the Cape San 

 Lucas front. This front may have no special at- 

 traction for tunas and is probably avoided by 

 them when unsuitably cold water occurs in it. 



CRUISE TO-64-2 



The results of this cruise, whicli was made in 

 August of a rather cold year (see fig. 1), are 

 probably typical of conditions in the early part 

 of the tuna season, including July, when the fish 

 rapidly expand their range nortliward. Figures 5 

 and 6 show ci'uise coverage and property distribu- 

 tions. The isotherms in figure 6 refer to the tem- 

 perature distribution on part 1 of the cruise, Au- 

 gust 5-16, 1964. At the few stations that were re- 



158 



U.S. FISH AND WILDLIFE SEEVICB 



