FISHERY BULLETIN: VOL. 69, NO. 3 



Auxis larvae wei-e equally abundant in day 

 and night tows, indicating that this species does 

 not migrate to the surface. The indication of 

 net avoidance during the day had no detectable 

 effect on apparent abundance, but if Anxis 

 larvae were more abundant at the surface dur- 

 ing the day and net avoidance had a significant 

 effect on abundance, the same results could be 

 obtained. Larval Auxis were almost equally 

 abundant at the surface in day and night col- 

 lections according to Wade (1951). Strasburg 

 (1960) captured more Auxis larvae in to 60 m 

 tows at night and stated that Matsumoto (1958) 

 also captured more specimens in night surface 

 tows. Klawe (1963) reported greater success 

 in catching Auxis larvae at night in surface and 

 300-m oblique tows but not in 140-m oblique 

 tows; he suggested that net avoidance may be 

 primarily responsible for decreased day catches. 

 In a more recent study, Klawe, Pella, and Leet 

 (1970) concluded that Auxis larvae did not ex- 

 hibit a diel vertical movement; they also found 

 no indication of net avoidance. 



DISTRIBUTION OF LARVAE 



Because all our collections were made by 

 surface tows, it was not possible to directly com- 

 pare our totals with the number of larvae col- 

 lected during the Equalant surveys (Richards, 

 1967, 1969). The two multiship Equalant sur- 

 veys covered most of the tropical Atlantic Ocean. 

 Equalant I took place at the same time of year 

 as Geronimo cruises 3 and 5 ("warm season"), 

 Equalant II corresponded to the time of Gero- 

 nimo cruise 4 ("cool season"). The average 

 number of tuna larvae collected per 1,000 m^ of 

 water strained on each Geronimo cruise herein 

 discussed and the average under 1 are (100 m-) 

 of sea surface for Equalant I and Equalant II 

 (Richards, 1969) are shown in Table 3. The 

 average numbers of larvae collected on the 

 Geronimo cruises were corrected for diel var- 

 iations in abundance. This was computed by 

 the following formula: 



where a = total number of standardized 

 day-caught larvae 

 a' = total number of standardized 



night-caught larvae 

 h = total number of day tows 

 h' — total number of night tows. 

 The correction was applied to all species except 

 Auxis because that species was equally abun- 

 dant in day and night collections. The averages 

 for Auxis were obtained by dividing the total 

 number of standardized larvae by the total num- 

 ber of tows. The Equalant averages were not 

 corrected for diel variations in abundance be- 

 cause most of the collections were oblique and 

 sampled the entire vertical range of all tuna 

 larvae. Calculations for the average number of 

 larvae collected were similar to those used for 

 Auxis but were expressed as the number under 

 1 are of sea surface. In the following separate 

 accounts we report on our detailed findings con- 

 cerning each species of larval tuna. 



Table 3. — The average number of tuna larvae col- 

 lected on Geronimo cruises 3, 4, and 5 and the two 

 Equalant surveys. 



a/h 



/6' 



1 14 March to 19 April 1965 in northwestern Gulf of Guinea. 

 - 10 February to 2 March 1965 off Sierra Leone. 



YELLOWFIN TUNA LARVAE 



The distribution of yellowfin tuna larvae in 

 the northwestern Gulf of Guinea is shown in 

 Figure 7. During Geronimo cruise 3, yellowfin 

 tuna larvae were common throughout most of 

 the area, averaging 11.4' larvae per 1000 m^ of 

 water strained. During cruise 5 (in the Gulf of 

 Guinea a year later), a smaller area was sam- 

 pled and an average of 1.1 larvae was collected 

 per 1000 m'' of water strained. During Equalant 

 I, no larvae were found north of about lat 2° N in 

 the same area, in contrast to the distribution 

 found during Geronimo cruise 3. We presume 



560 



