FISHERY BULLETIN: VOL. 73, NO. 2 



LATITUDE 



5N 



100 





S 200 



I 

 I- 



0. 



iij 

 a 



300 



300 



400 



500 



Figure 16.— Dissolved oxygen content (ml/liter), Part I transect, cruise Cromwell 51, 1-7 November 1970. 



their distributions are charted here (Figures 19 to 

 22). Catches by the 1-m net are more likely to be 

 related to skipjack tuna than those by the 0.5-m 

 net, because more skipjack forage organisms occur 

 in them. These relationships are investigated sta- 

 tistically in the next section. 



The main features of the day and night 

 zooplankton distributions in November-December 

 1970 are a maximum at about lat. 1° to 2°N, a 

 minimum in the extreme north of the area, and a 

 secondary minimum at about lat. 3° to 5°N. 

 Maxima occurred in March-April 1971 at about lat. 

 3° to 6°N and 9° to 10°N; elsewhere at that period 

 the catches were moderate with no conspicuous 

 spatial minima except in the extreme north. Com- 

 parisons between Figures 19 to 22, and Figures 5 

 to 8, made by overlay, show no obvious relation 

 between distributions of zooplankton and skip- 

 jack. 



Skipjack Forage 

 (and Zooplankton in Part) 



One of the objectives of the cruises was to see if 

 availability of offshore skipjack varied with the 



forage, as presumed by Blackburn (1965, 1969) and 

 Blackburn and Laurs (1972); or with zooplankton, 

 as suggested by Schaefer (1961); or with the 

 arithmetic product of forage and zooplankton, as 

 suggested by Riley (1963). 



Charts of skipjack forage (in milliters/ 1,000 m'), 

 both day and night data, are shown in Figures 23 

 and 24 for both parts of the November-December 

 1970 cruise, and similarly in Figures 25 and 26 for 

 the March- April 1971 cruise. Corresponding charts 

 of total micronekton were very similar and are not 

 given here. Data are available at Southwest 

 Fisheries Center, National Marine Fisheries Ser- 

 vice, La JoUa, Calif. 



Measurements of day concentration of forage 

 were more numerous than those of night concen- 

 tration. Thus Figures 23 and 25 show more detail 

 than Figures 24 and 26. Blackburn and Laurs 

 (1972) showed that day and night distributions of 

 forage were broadly similar in the area on a given 

 cruise as far as locations of maxima and minima 

 were concerned, although the night concentrations 

 were about 10 times higher than day concentra- 

 tions. Contours for day and night distributions 

 were therefore drawn to agree with each other as 



396 



