o 

 o 



(r 



UJ 



a. 



120° 100° W 



LONGITUDE 



Figure 9. — Catch rates of skipjack tuna larvae and adults 

 across the Pacific Ocean between lat. 10°S and 20°N. 



extremely low catch rate in the eastern Pacific may be 

 due partly to incomplete sampling coverage near the 

 equator and to the lack of sampling south of the 

 equator. Even with comparable sampling in these areas, 

 however, it seems unlikely that the catch rate would 

 have been as high as in the central Pacific. 



The catch rates (catch per 1,000 hooks fished) of 

 adult skipjack tuna taken by the longline in areas and 

 quarters similar to those of the larvae showed a com- 

 parable trend (Fig. 9). The catch rates were low in the 

 eastern and western Pacific and high in the central 

 Pacific. The only apparent difference between the two 

 plots was the slight dislocation of the peaks. The lack of 

 larval net tows for the years 1964-67 across the Pacific, 

 however, does not permit statistical testing of the catch 

 trends. Nevertheless, the similarity between the high 

 CPUE of adult skipjack tuna and the high larval density 

 in the central equatorial Pacific, seems to support the 

 assumptions by Kawaski (1965), Rothschild (1965), and 

 Williams (1972) that the central equatorial spawning 

 ground is the source of the skipjack tuna taken in the 

 eastern Pacific fishery. 



MOVEMENT OF SKIPJACK TUNA IN 

 THE PACIFIC 



Past Studies on Movemnt of Skipjack Tuna 



A number of studies have been made on the popula- 

 tion structure and movement of skipjack tuna in various 

 sections of the Pacific. Fujino (1970a, 1970b) has pro- 

 posed the existence of two genetically distinct subpopu- 

 lations of skipjack tuna in the western North Pacific, 

 separated by a line passing through the Caroline-Mari- 

 ana-Bonin archipelagoes. In a more recent study, Fujino 

 (1972) has extended this boundary line southward along 

 long. 170°E from the Equatorial Countercurrent to the 

 area east of New Caledonia and through the Tasman 

 Sea and has proposed a model of population structure 



and migration for the entire area (Fig. 10). He proposed 

 1) that fish of the western Pacific subpopulation in the 

 northern hemisphere remain within the Philippine Sea 

 and areas of the Caroline Islands in the northern winter 

 and range eastward to about long. 165°E in the north- 

 ern summer; 2) that in both northern and southern 

 hemispheres there are two spawning groups (summer, 

 A; winter, B) which behave differently from each other 

 without diversification in genetic composition; and 3) 

 that the two groups (A and B) migrate in different 

 proportions by age and area, with intermingling taking 

 place between northern and southern A groups and 

 between northern and southern B groups at the time of 

 spawning and during the larval stages. 



Kawasaki (1965) proposed that the skipjack tuna in 

 the eastern and western North Pacific fishery originated 

 from fish spawned in the central equatorial region be- 

 tween long. 160°E and 140° W. He (1972) subsequently 

 modified this view by extending the principal spawning 

 area westward to long. 120°E and, as a result of 

 Fujino's work, recognized the possibility of a separate 

 western Pacific subpopulation. 



Rothschild (1965) suggested that skipjack tuna in the 

 central Pacific do not constitute a single, homogeneous 

 population unit; that they originate in three possible 

 zones— Hawaiian, equatorial, and Marquesan; that the 

 Marquesan zone does not contribute significant numbers 

 to the skipjack tuna taken in the eastern Pacific; and 

 that the eastern Pacific skipjack tuna originate either in 

 the Hawaiian or equatorial zone. He further proposed 

 that the potential recruits to the eastern Pacific are 

 "split" into a northern group that enters the Mexican 

 fishery and a southern group that enters the South 

 American fishery, citing as a possible splitting mecha- 

 nism the warm-water cell (surface temperature >28°C) in 

 the vicinity of lat. 15°N off the central American coast. 



Fink and Bayliff (1970) discussed the migration of 

 skipjack tuna in the eastern Pacific based on tagging 

 results. Their findings indicated that fish in the northern 

 group (north of lat. 15°N) entered the fishery in April 

 through the Revilla Gigedo Islands and moved inshore 

 and northward along the coast of Baja California gen- 

 erally to lat. 30°N in the third quarter. The fish then 

 moved southward, most of them likely returning to the 

 central Pacific and some to the fishery the following 

 year (see their figure 89). The migrations of fish in the 

 southern group appeared more complex, and the in- 

 shore-offshore movements were not well defined owing 

 to inadequate data in the offshore areas. Nevertheless, 

 from the large number of skipjack tuna tagged in the 

 Gulf of Panama in 1959 and 1961, many went north and 

 west to Central America and many went south to 

 Ecuador-Peru. At least one tagged fish moved from 

 Ecuador and one from the Galapagos Islands to Peru. 



Seckel (1972) proposed a drift model showing how 

 ocean currents could contribute to the travel of skipjack 

 tuna from the eastern North Pacific to Hawaii. From an 

 earlier study (Seckel 1968) he noted that a pronounced 

 salinity gradient, which implied a strong convergence, 

 existed at the boundary of the North Pacific Central 

 Water. Using the geostrophic and wind-driven current 

 speeds he calculated the monthly drift displacement of 

 11 objects hypothetically placed between lat. 10° and 

 20°N along long. 120°W at monthly intervals. The re- 

 sults of his calculations show that objects initially locat- 



19 



