DISTRIBUTION OF TUNA LARVAE (PISCES, SCOMBRIDAE) IN THE 

 NORTHWESTERN GULF OF GUINEA AND OFF SIERRA LEONE' 



William J. Richards and David C. Simmons^ 



ABSTRACT 



Investigations of tuna larvae distributions in the northwestern Gulf of Guinea and off Sierra Leone 

 were made during February-April 1964, August-October 1964, and February-April 1965. Larvae of 

 the yellowfin tuna, bigeye tuna, skipjack tuna, little tunny, and frigate mackerels were collected and 

 studied. Analyses of the data indicated that larvae of yellowfin tuna and bigeye tuna migrate to the 

 surface during the day, skipjack tuna migrate to the surface during the night, and frigate mackerels 

 do not seem to migrate at any time. Our data for little tunny were inconclusive. All species were 

 widely distributed over the area but larvae of the commercially important tunas — yellowfin, bigeye, 

 and skipjack — were restricted to waters where surface temperatures were higher than 24° C. 



The distribution of tunas varies seasonally in 

 the eastern Atlantic Ocean (Richards, 1969). 

 In 1964 and 1965, the Bureau of Commercial 

 Fisheries research vessel Geronlmo (cruises 3, 

 4, and 5) collected tuna larvae in the northwest- 

 ern Gulf of Guinea and off Sierra Leone. These 

 collections were part of extensive investigations 

 intended to relate the spatial and temporal dis- 

 tributions of tunas to the environment. Cruise 

 3 was in the northwestern Gulf of Guinea be- 

 tween 10 February and 26 April 1964, which 

 is within the winter-spring "warm season" in 

 the Gulf of Guinea, when sea-surface tempera- 

 tures are higher than during summer and fall. 

 Cruise 4 was in the northwestern Gulf of Guinea 

 between 5 August and 13 October 1964, which 

 is within the summer-fall "cool season" in the 

 Gulf of Guinea, when sea-surface temperatures 

 are lower than during winter and spring. Dur- 

 ing cruise 5. collections were made in two areas: 

 the northwestern Gulf of Guinea and off Sierra 

 Leone. The northwestern Gulf of Guinea area 

 was generally the same as that covered in 

 cruises 3 and 4 and collections were made from 

 14 March to 19 April 1965 within the winter- 

 spring "warm season." The area off Sierra 



' Contribution No. 185, National Marine Fisheries 

 Service, Tropical Atlantic Biological Laboratory, Miami, 

 Fla. 33149. 



- National Marine Fisheries Service, Tropical Atlantic 

 Biological Laboratory, Miami, Fla. 33149. 



Leone, which is immediately northwest of the 

 areas covered in cruises 3, 4, and part of 5, was 

 studied from 10 February to 2 March 1965 (see 

 Figure 1). 



Figure 1. — Reference map for the areas studied. The 

 shaded area east of long 10° W was surveyed on 

 Geronimo cruises 3, 4, and part of 5; the shaded area 

 west of long 10° W was surveyed on part of cruise 5. 



The purposes of this study are to (1) analyze 

 the time the collections were made, (2) describe 

 the distribution of the tuna larvae, and (3) dis- 

 cuss the relations of the tuna larvae to oceano- 

 graphic featui'es. In addition to the collecting 

 of larvae on each cruise, sightings of surface 



Manuscript accepted February 1971, 



FISHERY BULLETIN: VOL. 69, NO. 3, 1971. 



555 



