BLACKBURN and WILLIAMS: DISTRIBUTION AND ECOLOGY OF SKIPJACK TUNA 



offshore areas by the U.S. surface tuna fishery has 

 been very limited, especially for skipjack. 

 Coverage by the Japanese subsurface tuna fishery 

 has been greater, but still poor for skipjack 

 (Miyake 1968). The regulation of yellowfin tuna, 

 Thunnus albacares, in the eastern Pacific is ex- 

 pected to increase the need for information on 

 skipjack in the offshore waters. 



Work in coastal waters has shown that adult 

 skipjack are most numerous when temperature is 

 in the range 20° to 29°C (Williams 1970) and 

 standing stock of skipjack forage is high (Black- 

 burn 1965, 1969). Sea surface temperatures are in 

 the suitable range for most of the year throughout 

 the offshore eastern tropical Pacific (Wyrtki 1964; 

 La Violette and Seim 1969; Love 1971b, 1972a, b, in 

 prep.). Thus distribution of forage seemed likely to 

 be a main factor in determining distribution of 

 adult skipjack in offshore waters. The distribution 

 of forage in that region was described from data 

 of EASTROPAC Expedition (1967-68) by Black- 

 burn and Laurs (1972), who expected that adult 

 skipjack would prove to be distributed in the same 

 way. One of the purposes of the present study was 

 to test that expectation. The forage concentra- 

 tions on EASTROPAC were characteristically 

 high in certain zones of latitude and low in others, 

 broadly corresponding to distributions of phy- 

 toplankton, zooplankton, and total micronekton 

 (Love 1970, 1971a, in prep.; Blackburn et al. 1970; 

 Owen and Zeitzschel 1970a). 



Evidence, summarized by Williams (1972), in- 

 dicates that most exploited skipjack in the eastern 

 Pacific have a spawning origin in the central 

 Pacific west of long. 130°W. It also suggests that 

 the majority of the skipjack enter the present 

 fishery, which is concentrated near tropical 

 American coasts, during only 1 yr of their life 

 history. They are then relatively small (average 

 length about 50 to 55 cm, average weight 3 to 3.5 

 kg) and sexually immature. Thus it is probable 

 that migration pathways exist for skipjack, both 

 below and above these sizes, across the offshore 

 eastern tropical Pacific. 



Such pathways might occur at particular lati- 

 tudes since forage concentrations and many other 

 ocean conditions, including surface currents, are 

 zonally oriented. On this basis Williams (1972) 

 presented three qualitative models of the migra- 

 tion of young (recruit) skipjack from the central 

 Pacific into the eastern Pacific fishing areas. The 

 data from the present cruises may be useful for 

 testing and modification of the models, and incor- 



poration with other models of skipjack movement 

 such as that of Seckel (1972). 



PLAN OF 

 THE INVESTIGATIONS 



The inward skipjack migration models of 

 Williams (1972) assumed that the routes were 

 principally zonal across the offshore eastern 

 Pacific. Mechanisms and timing of the migrations 

 are probably dependent on oceanographic condi- 

 tions and events in this region. The strategy for 

 the present investigations was to have latitudinal 

 sampling of fish and environmental parameters in 

 meridional areas considered critical to the migra- 

 tions. The area discussed in this paper includes the 

 meridian of 119°W. It is important because the 

 surface North Equatorial Countercurrent nor- 

 mally becomes intermittent or absent east of this 

 meridian from January to May. 



Each cruise had two parts (Williams 1971). Part 

 I was a rapid meridional transect of the area along 

 long. 119°W to monitor ocean conditions and com- 

 pare them with previous data. These results 

 showed the positions of zonal surface current 

 boundaries and forage bands, and hence the lati- 

 tudinal zones that were to be fished for skipjack. 



In Part II detailed fisheries operations were 

 carried out in the selected zones to a standardized 

 plan based on a "unit area" of 2° latitude by 2° 

 longitude, together with supporting environmen- 

 tal observations. Fishing was by multiple trolling 

 during daylight. Figure 2 shows schematically the 

 track and scheduled observations. The work time 

 for each unit area, including entry and exit, was 96 

 h. Coverage in a zone of latitude could consist of 

 any multiple of unit areas or fractions thereof 

 (quadrants or 1° x 1° areas). 



The first cruise in November-December 1970 

 utilized the vessels Townsend Cromwell (Cruise C 

 51) (R. Uchida, Chief Scientist) and David Starr 

 Jordan (J 57) (F. Williams, Chief Scientist). The 

 second cruise in March-April 1971, was made with 

 only the Jordan (J 60) (M. Blackburn, Chief 

 Scientist). The Jordan 60 cruise was severely cur- 

 tailed due to illness of a crew member. On the first 

 cruise the Part I transect was completed by the 

 Cromwell; and data were sent by radio to the Jor- 

 dan; subsequent Part II operations were carried 

 out by both vessels. 



This paper also discusses data from a cruise 

 made by National Marine Fisheries Service, 

 Hawaii, to the same area in October-November 



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