FISHERY BULLETIN: VOL. 73, NO. 2 



5, 6, 7, 8; Tables 15, 16, 19). No relation appears 

 between skipjack distribution and particular 

 temperatures within the 20° to 29°C range. 



Blackburn and Laurs (1972) expected adult 

 skipjack to be distributed like their forage in 

 offshore areas where all surface temperatures are 

 suitable. This was because Blackburn (1969) found 

 such a relation for skipjack in waters of suitable 

 temperature near the coast, and Magnuson (1969) 

 found that skipjack eat the equivalent of 15% of 

 their body weight per day when fed to saturation. 

 Thus adult skipjack would probably be most 

 numerous in the latitudinally oriented zones of 

 abundant forage which occur offshore, with forage 

 concentrations comparable to those in coastal 

 waters, during their westward movement from 

 the coast to the spawning areas (Blackburn and 

 Laurs 1972). They would probably migrate slowly 

 through forage-rich zones or areas and quickly 

 through those poor in forage, and thus be more 

 abundant per unit area in the forage-rich situa- 

 tions. 



Blackburn and Laurs (1972) showed from EAS- 

 TROPAC data that the richest and most persis- 

 tent zones of skipjack forage in our study area 

 occurred a few degrees north and sometimes south 

 of the equatorial upwelling. They also recognized a 

 less conspicuous zonal forage maximum near the 

 northern boundary of the North Equatorial Coun- 

 tercurrent, probably associated with high 

 biological production over the shoal pycnocline. 

 Data from the November-December cruise show 

 the expected maxima of forage and skipjack near 

 the Equator, but do not clearly show a maximum of 

 either on the north side of the Countercurrent 

 (Figures 23, 24; Table 3). Tables 12 and 13 show two 

 statistically significant positive correlations 

 between availability of large skipjack and their 

 forage on the same cruise, although only for night 

 concentrations of forage and for data averaged 

 over a 2° zone of latitude. As mentioned earlier the 

 actual significance may be disputable for the lower 

 of these correlation coefficients, but not for the 

 higher one, taking the total number of correlations 

 in Tables 11 and 12 into account. Correlations 

 between skipjack and day forage were not sig- 

 nificant (Table 11). 



Skipjack probably do much of their feeding in 

 the daytime (Nakamura 1962) although forage is 

 much scarcer in the upper water layers by day than 

 by night. Thus the lack of relation between skip- 

 jack and day forage may seem surprising. One 

 could however interpret these results as follows. 



Spatial distributions of day and night forage 

 broadly coincide (Blackburn and Laurs 1972) 

 because they are determined by the same 

 physicochemical and basic biological features of 

 the environment. Skipjack tend to occur in broad 

 zones where both kinds of forage are initially 

 abundant, for reasons suggested above. Within 

 these zones they aggregate in the richer patches of 

 day forage and eat them down, whereby their 

 relation with the day forage will be sometimes 

 direct and sometimes inverse. If they eat the much 

 more abundant night forage they probably do not 

 so frequently reduce it to a point at which the 

 relation becomes inverse. 



The significant November-December correla- 

 tions become nonsignificant when data for skip- 

 jack < 45 cm are included (Table 12). Thus 

 juvenile skipjack may be distributed in relation to 

 a different kind of forage, or possibly to other 

 environmental properties excluding forage. 

 Blackburn and Laurs (1972) made no statement 

 about ecology of juveniles. 



The relatively sparse data for the March-April 

 cruise of 1971 show a forage maximum near the 

 Equator but not clearly elsewhere (Figures 25 and 

 26), although one may nevertheless have been 

 present near the northern edge of the Counter- 

 current, as mentioned previously. The principal 

 maximum of skipjack in March-April 1971 was 

 located slightly north of the North Equatorial 

 Countercurrent, and there was a secondary 

 maximum near the Equator (Table 4). Data on 

 skipjack and forage yielded no significant correla- 

 tions. They were probably too sparse to do so (Ta- 

 ble 12). 



Tables 3 and 4 show that skipjack were less 

 abundant in the North Equatorial Countercurrent 

 than in either of the adjacent currents, on both the 

 1970 and 1971 cruises. This was expected because 

 neither Blackburn and Laurs (1972) nor we found 

 much forage in the Countercurrent. However 

 skipjack availability was much higher in the 

 Countercurrent (lat. 6° to 8°N) than in the South 

 Equatorial Current on the 1969 cruise (Tables 15, 

 16). Forage data are lacking for the cruise, but it is 

 not likely that forage was highly abundant in the 

 North Equatorial Countercurrent. We note that 

 the large skipjack taken in October-November 

 1969 had sexually maturing or mature gonads 

 (Table 21), whereas most of those taken on the 

 other cruises had spent, spent-recovering, or rest- 

 ing gonads (Tables 8, 9). Possibly the October- 

 November fish were close to spawning, and thus 



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