FISHERY BULLETIN.- VOL. 69. NO. 3 



schools of tuna were recorded and several ocean- 

 ographic features — temperature, salinity, and 

 dissolved oxygen — were measured. The distri- 

 butions of these oceanographic features were 

 published in a series of atlases (Goulet and 

 Ingham, 1968; Ingham, Goulet, and Brucks, 

 1968; Brucks, Ingham, and Leming, 1968a, 

 1968b). 



The northwestern Gulf of Guinea is affected 

 by the general meteorological and oceanic condi- 

 tions of the Gulf of Guinea and by some unique 

 local features. Ingham (1970) concluded that 

 two types of upwelling occur in this region — 

 a seasonal wind-driven upwelling (.July through 

 October) and a current-induced upwelling that 

 is present most of the time. The mi.xed surface 

 layer is rather thin in the coastal area (less than 

 10 m near the coast, grading to 30 to 40 m off- 

 shore) and is influenced by current-induced up- 

 welling, wind-driven upwelling, and advection. 

 Ingham (1970) reported that during the period 

 of Geronimo cruises 3, 4, and 5, advection was 

 the most effective of the three factors. 



The species collected were the yellowfin tuna, 

 Thunnus albacares (Bonnaterre) ; the bigeye 

 tuna, Thunnus obesiis Lowe; the bluefin tuna, 

 Thunniis thynmis (Linnaeus); the skipjack 

 tuna, KatsMivonus pelamis (Linnaeus) ; the little 

 tunny, Euthynnus cdletterattts (Rafinesque) ; and 

 the frigate mackerel, Aiixis sp. Larvae of the al- 

 bacore, Thunnus alalunga (Bonnaterre) were 

 not collected. Numbers of larvae, their location, 

 and the methods used to collect, sort, identify, 

 and compute the numbers of larvae have been 

 treated by Richards et ai. (1969a, 1969b, 1970) 

 for each cruise. Larvae were collected by an 

 ICITA (International Cooperative Investiga- 

 tions of the Tropical Atlantic) 1-m plankton net. 

 towed at the surface. 



ANALYSIS OF COLLECTION TIME 



The relative apparent abundance of some fish 

 larvae is complicated by diel variations. Oblique 

 plankton collections that samjiie the entire ver- 

 tical distribution of a species tend to catch fewer 

 fish larvae during the day (Ahlstrom, 19.'59), 

 presumably a result of increased net avoidance. 

 In sui'face collections, such as those taken dur- 



ing Geronimo 3, 4, and 5, diel vertical migrations 

 also could be an important factor in abundance 

 variations. 



We used the Mann-Whitney U test (Siegel, 

 1956) to determine the probability of equal 

 catches of tuna larvae in day and night surface 

 tows. A ranked test such as this should min- 

 imize the effects of patchiness. Tows with local 

 apparent midtimes from 0600 through 1759 hr 

 were designated as day tows and those with local 

 a]3parent midtimes from 1800 through 0559 hr 

 as night tows. Included in our calculations were 

 all successful tows (those that captured tuna 

 larvae) and unsuccessful tows (those that did 

 not capture tuna larvae) , except those unsuccess- 

 ful tows outside the temperature-salinity ranges 

 of the species (Table 1). These temperature-sa- 

 linity ranges are a composite from Richards 

 (1969) and the present study and should not 

 be considered absolute. The unsuccessful tows 

 were included because of the implication that 

 larvae were not cajjtured for some reason other 

 than intolerance to temperature or .salinity. In 

 calculating the statistics, a correction for the 

 tied (equally ranked) unsuccessful tows was 

 used (Siegel, 1956). 



Table 1. — Temperature-salinity ranges for larvae of 

 yellowfin tuna, bigeye tuna, skipjack tuna, little tunny, 

 and Auxis sp. These data are a composite from Richards 

 (1969) and the present study. 



The resulting probabilities (Table 2) indicate 

 that yellowfin and bigeye tunas were collected 

 more often at the surface during the day, and 

 skijijack tuna and little tunny more often at the 

 surface at night. No difference was ai^parent 

 between day and night tows for Auxis. 



Also analyzed was whether tuna larvae were 

 better able to dodge the plankton net during the 

 day than at night. The question was considered 

 because we naturally assumed that tuna larvae 

 should be able to see a plankton net more clearly 

 during the day and therefore avoid it more 



556 



