commercially marketable species. However, 

 the fishery for skipjack has not been able to 

 extract more than 265 million pounds ( 120,- 

 200 metric tons ) from the coastal areas and 

 the neighboring islands in any year in the pe- 

 riod since the ycllovvfin fishery was first regu- 

 lated in 1966, and the yearly catch from those 

 areas has been as low as 127 million pounds 

 (57,600 metric tons) (Inter-American Tropi- 

 cal Tuna Commission, 1970). 



Thus, new fishing grounds for skipjack are 

 needed. Attention has been focused on the 

 possibility of extending the fishery from the 

 coastal areas, where present effort is mostly 

 confined, out into the offshore areas to the 

 west, where the abundance of skipjack is un- 

 known. Because of their proximity to the 

 spawning grounds, some of these offshore areas 

 may yield larger fish than are taken near shore, 

 and the larger fish arc better suited for canning. 

 Miyake ( 1968) showed that skipjack, includ- 

 ing some of large size, occur throughout the 

 tropical Pacific. It would aid the U.S. tuna fleet 

 to know where in the unfished offshore regions 

 of the eastern tropical Pacific skipjack are most 

 abundant, and which areas offer the most po- 

 tential as a commercial skipjack fishery. 



A first step in determining the distribution of 

 these offshore skipjack was to identify areas 

 where environmental conditions are most suit- 

 able for them. The cooperative EASTROPAC 

 oceanographic expedition of 1967-1968, which 

 was coordinated by the National Marine Fish- 

 eries Service ( former Bureau of Commercial 

 Fisheries), was designed to accomplish this 

 and other objectives. 



Previous investigations indicate that adult 

 tunas aggregate where food is abundant In 

 waters that are within certain ranges of tem- 



perature, which vary from species to species 

 (Blackburn, 1965). Specifically, this has been 

 confirmed for skipjack and yellowfin tuna in 

 the fishery area off Baja California in the east- 

 ern Pacific (Blackburn, 1969). In the eastern 

 Pacific fishery region, most skipjack aggrega- 

 tions occur in water between approximately 

 20 and 29 C ( Williams, 1970 ). Skipjack are 

 fished by U.S. fishermen at or near the sea 

 surface, so surface temperatures are sufficient 

 to identify areas where temperature is suitable. 

 Charts of mean monthly surface temperature 

 show that most parts of the eastern tropical 

 Pacific have temperatures suitable for skipjack 

 nearly all the year (Wyrtki, 1964), so that a 

 knowledge of temperature distribution, in 

 itself, would not show us where skipjack are 

 concentrated. However, consideration of tem- 

 perature information in conjunction with the 

 distribution of skipjack forage organisms might 

 help identify areas of potential skipjack abun- 

 dance. 



Alverson (1963) and Waldron and King 

 (1963) listed the prey organisms of skipjack 

 tuna in the eastern tropical Pacific and central 

 Pacific, respectively. Nakamura (1965) gave 

 similar information for skipjack of the Mar- 

 quesas and Tuamotu Islands, between about 

 long 135 and 155 W in the south Pacific. 

 Most of the skipjack stomachs that Alverson 

 (1963) studied were collected within about 

 500 miles of the tropical American Pacific 

 coast. The stomachs studied by Waldron and 

 King (1963) were collected broadly between 

 long 150 and 180 W. The results of skipjack 

 stomach content studies are summarized in 

 Table 1. Alverson (1963) and Nakamura 

 (1965) noted a tendency for skipjack over 60 

 cm long to have a smaller proportion of crus- 



Table 1. Sutiiiiiao' uf results of stomach content analyses of skipjack tuna. 



Alverson (1903) 2317 



Waldron and King (19(i3) 707 



Nakamura (1965) (i03 



