FISHERY BULLETIN: VOL. 70, NO. 3 



landing size of 4V^ lb (;::;45-50 cm) , but in South 

 American landings by local boats (Manta) the 

 smallest fish are about 36 cm.] 



The length-frequency data in general show 

 that the smallest skipjack in the northern fishery 

 are found offshore at the Revillagigedo Islands 

 from March to June. The modal size of these fish 

 is <50 cm, and often between 40 and 45 cm. 

 From March to May some similar sized fish are 

 also seen in catches from adjacent inshore areas 

 between the southern Gulf of California and 

 Cape Corrientes (lat25° to20°N). Most of these 

 small fish subsequently migrate into the skipjack 

 fishery off western Baja California. In October 

 (± 1 month) of most years there appears to be 

 a small entry of fish, modal length ^45 cm, at 

 the Revillagigedo Islands, which is often reflected 

 in the length-frequency distributions for Novem- 

 ber in the Baja California area. 



In the offshore areas of the southern fishery 

 a main influx of small fish, modal lengths 

 <50 cm, appears to be at the Galapagos Islands 

 from November to April with the peak in Jan- 

 uary and February. Data from the adjacent in- 

 shore area off Ecuador subsequently seems to re- 

 flect this entry of small fish. In addition, small 

 numbers of skipjack of modal lengths ^45 cm 

 often occur in August (± 1 month) off Ecuador, 

 as well as in Peru-northern Chile catches in ab- 

 normally warm oceanographic years. In the 

 Cocos Island area fish of modal lengths ^45 cm 

 are found in January and February (there is 

 some evidence this entry of small fish starts in 

 November or December and lasts into March). 

 Fish in this same size range subsequently occur 

 in some years in the Gulf of Panama from April 

 to June. 



The apparent times and places of entry of 

 small skipjack into the eastern Pacific fisheries 

 are summarized below: 



Cocos Is. 



November-March 

 (peak January- 

 February) 



Fishery 

 Northern 



Southern 



Area of entry 



Revillagigedo Is. 



Ecuador 



? fJalapagos Is. 



Galapagos Is. 



Time of entry 



j March-June 



I October ( it 1 month) 



r August (±: 1 month) 



November-April 

 (peak January- 

 February) 



These findings are generally consistent with 

 those of Fink and BayliflF (1970) for the north- 

 ern fishery and expand those for the southern 

 fishery. 



Work on the growth of central-east Pacific 

 skipjack using length-frequencies has been com- 

 plicated by the problem of size-specific move- 

 ments through the fisheries. Additionally, there 

 is as yet no secondary age estimation method for 

 the species. Recent work by Yoshida (1971), 

 on young skipjack from the stomachs of long- 

 line-caught billfish in the central Pacific, has in- 

 dicated that a 1-year-old skipjack may be about 

 31 cm. This is a similar length to that deduced 

 from tagging data (averaged) for eastern Pa- 

 cific fish by Joseph and Calkins (1969), who dis- 

 cuss the various deduced growth rates for the 

 species, whereas Rothschild (1967) , on the basis 

 of short-term tag returns, indicated about 44 cm 

 for a 1-year-old fish. Depending on the accepted 

 growth rates, recruits to the eastern Pacific 

 fisheries might thus be between 8 and 15 or 12 

 and 24 months old on entry and between 15 and 

 24 or 24 and 36 months old when departing the 

 region for first spawning in the central Pacific. 



CIRCULATION IN THE EASTERN 

 TROPICAL PACIFIC 



The oceanography of the eastern equatorial 

 Pacific was reviewed by Wyrtki (1966, 1967). 

 The circulation of intertropical surface waters 

 shows the west-flowing North and South Equa- 

 torial Currents (NEC, SEC) with between them 

 the relatively narrow (180-360 miles) east-flow- 

 ing North Equatorial Countercurrent (NECC) 

 at about lat 5°-10°N (Figure 3). At or just 

 south of the equator below a depth of 20-50 m 

 is the Equatorial Undercurrent (EUC) (Crom- 

 well Current) v/ith eastward flow. 



However, Wyrtki (1965, 1966) indicated 

 marked seasonal fluctuations in surface currents 

 in the eastern Pacific. From June to December 

 the NECC is fully developed through to the cen- 

 tral American coast, while during January and 

 May it is intermittent, and from February to 



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