BLACKBURN and WILLIAMS: DISTRIBUTION AND ECOLOGY OF SKIPJACK TUNA 



this could indicate that they were southern 

 spawners. All troll-caught yellowfin tuna were 

 immature, sex indeterminate, as were the bigeye 

 except for one immature male of 59 cm. 



DISCUSSION AND CONCLUSIONS 



Gulland (1971) reviewed research findings which 

 indicate that skipjack is the most abundant tuna in 

 the Pacific, except possibly for frigate mackerel 

 which is a small and presently valueless species. 

 Our results and those of Hida (1970) show that 

 each of the three cruises yielded many more skip- 

 jack than all other species of fish combined, 

 including nontunas and unidentified fish (Tables 2, 

 15, 18, 19). On each cruise some skipjack were ob- 

 tained in almost every part of the area in which 

 fishing was done. Occurrences of other tunas 

 (yellowfin, bigeye, and frigate mackerel) were 

 much fewer and more localized (Tables 2, 3, 4, 15, 

 18, 19). 



Our results also support the general hypothesis 

 of Rothschild (1965), Williams (1972), and others 

 that skipjack migrate as juveniles from central 

 Pacific spawning areas towards the American 

 coast, spend part of their adolescent life near the 

 coast, and then return to the central Pacific. The 

 present study area lies between the spawning 

 areas and the coast. Thus one would expect the 

 skipjack in that area to include, at times, in- 

 dividuals both smaller and larger than those 



Table 21. -Number of skipjack gonads in each maturity stage by 

 size of fish, study area, Cruise Gilbert 116, October-November 



1969. 



Totals: males (M) 132, females (F) 117. 



Table 22. -Number of skipjack gonads in each maturity stage by 

 size of fish, outward track, Cruise Gilbert 116, October 1969. 



Totals: males (M) 10; females (F) 10. 



typical of coastal waters. We have demonstrated 

 their occurrence (Tables 6, 7, 20; Figures 9, 30). 



Matsumoto (1966), Ueyanagi (1969), and Love 

 (1970, 1971a, in prep.: EASTROPAC data) indicate 

 that skipjack larvae are rare east of long. 130° W in 

 the tropical Pacific, but increase rapidly west of 

 that meridian. Thus our study area is close to a 

 spawning region. One would then expect some of 

 the large skipjack in the study area to have ma- 

 turing, spent, or spent-recovering gonads at times, 

 and this condition was found (Tables 8, 9, 10, 21, 

 22). The occurrence of spent-recovering and rest- 

 ing gonads in November-December suggest prin- 

 cipally northern summer spawning, especially 

 since no spawned-out fish were taken south of lat. 

 3°N. The presence of spent-recovering fish in 

 March-April perhaps indicates southern summer 

 spawners (northern winter); however, the oc- 

 currence of skipjack with maturing gonads at this 

 time may also signify northern summer spawners. 

 Fish taken on the two cruises may be of two 

 spawning groups, northern and southern (see 

 Williams 1972). 



The juvenile skipjack ( < 45 cm) constituted a 

 small proportion, 13% or less on each cruise, of the 

 total skipjack caught in the study area. Their dis- 

 tribution varied spatially and temporally (Tables 

 6, 7, 20; text p. 389). On the cruise of October- 

 November 1969 they were found only in the South 

 Equatorial Current, but the North Equatorial 

 Current was not sampled. In November-December 

 1970 they were found principally in the North 

 Equatorial Current and sparsely in the South 

 Equatorial Current. In March-April 1971 they 

 were very scarce or absent in all parts of the study 

 area. West of long. 125°W in October-November 

 1969, some juveniles were taken in the North 

 Equatorial Countercurrent as well as in the other 

 two currents. It does not seem possible from these 

 data to make a choice among any of the three 

 models of coastward migration of juveniles 

 proposed by Williams (1972), or to eliminate any of 

 them from consideration. Data from other periods 

 of the year are desirable. 



From previous studies by Williams (1970), adult 

 skipjack (> 45 cm) were expected to occur in 

 waters of surface temperature 20° to 29°C, but not 

 preferentially at particular temperatures within 

 that range. All waters of the study area had such 

 temperatures on all cruises (Figures 10, 11, 12, 13, 

 28, 29). Thus they were all suitable for skipjack as 

 far as temperature was concerned, and skipjack 

 occurred to some extent in most of them (Figures 



407 



