interest since it corresponds with the central equatorial 

 spawning grounds from which the skipjack tuna taken in 

 the eastern Pacific fishery is assumed to originate (Ka- 

 wasaki 1965; Rothschild 1965; Williams 1972). 



Adult-Larvae Relationship in 

 the Equatorial Pacific 



As discussed previously, the tuna longline tends to 

 catch the larger tunas, and so a major portion of the 

 skipjack tuna taken by this gear consists of the larger, 

 more mature fish. Consequently, in areas where the 

 longline CPUE is high, especially in waters suitable for 

 spawning, the concentrations of larvae also should be 

 high. To confirm this probable relationship and to delin- 

 eate the spawning area, the relative abundance of adults 

 in 20° sectors along the equator was compared with 

 that of the larvae. 



Numerous larval net tows have been made in the 

 Pacific over the past 25-30 yr, but this coverage of the 

 ocean with respect to time and space has not been as 

 intensive as that by the longline. Moreover, larval net 

 tows made in all sections of the Pacific were not 

 uniform with respect to towing methods and net sizes. 

 In the eastern Pacific, where net tows were made 

 principally by the Inter-American Tropical Tuna Com- 

 mission, the Scripps Institution of Oceanography, and the 

 National Marine Fisheries Service, La Jolla Laboratory, 

 30-min surface tows and oblique tows to 140, 300, and 

 400 m were made with a 1-m net equipped with a 

 flowmeter (Klawe 1963); in the central Pacific, where 

 net tows were made by the National Marine Fisheries 

 Service, Honolulu Laboratory, 30-min surface tows and 

 oblique tows to 60, 140, and 200 m were made with a 

 similar net (Matsumoto 1958; Strasburg 1960); and in 

 the western Pacific, where net tows were made by the 

 Far Seas Fisheries Research Laboratory, a 2-m net 

 without flowmeter was towed horizontally for 20 min at 

 the surface and at depths of 20-30 and 50 m at vessel 

 speeds of 1.03 m/s or 2 knots (Ueyanagi 1969). 



Of all the tows made, only the surface tow was 



commonly used in all three areas. Consequently, abun- 

 dance of skipjack tuna larvae across the Pacific was 

 determined only from catches in surface tows after the 

 following adjustments had been made: 



1. To ehminate variability resulting from diel vertical 

 migration, a prominent characteristic of the skipjack 

 tuna larvae (Matsumoto 1958; Ueyangi 1969), only sur- 

 face night tows were used. 



2. To obtain comparable coverage across the ocean, 

 the areas from which tows were compared were limited 

 to 20° longitudinal sectors along the equator between 

 lat. 00" and 20°N in the western; between lat. 10°S and 

 20°N in the central; between lat. 07°S and 15"'N in the 

 east-central; and between lat. 04° and 15°N in the 

 eastern Pacific. 



3. To reduce seasonal variability in the catches, only 

 tows made in spawning months were considered. These 

 included April through September in areas between lat. 

 10° and 20°N and all months of the year in areas 

 between lat. 10°N and 10°S (Matsumoto 1958; Strasburg 

 1960; Klawe 1963; Ueyanagi 1969). 



4. To compensate for differences in net size and dura- 

 tion of tow, the catches in all areas were adjusted to a 

 standard tow that strained 1,454 m^ of water, a volume 

 equivalent to that strained by a 1-m net towed at 1.03 

 m/s for 30 min. The adjustment was made by apply- 

 ing a conversion factor (i.e., the ratio of standard 

 volume to average volume of water strained in the 

 central Pacific do not constitute a single, homogeneous 

 volume to estimated volume of water strained in the 

 western Pacific) to the catches in the three areas. 

 Although the adjustment is rough, the adjusted catch 

 values for all quarters for the years 1950-67 (Table 3) 

 appear reasonable. 



A plot of adjusted catch rates (Fig. 9) indicates that 

 between lat. 20°N and 10°S, skipjack tuna larvae were 

 most abundant in the central Pacific between long. 

 140°W and 160°E, with the peak in the sector between 

 long. 160°W and 180°. Larvae were less abundant in the 

 areas east of long. 140°W and west of long. 160°E. The 



Table 3. — Catch rates of larval skipjack tuna in night surface tows across the Pacific Ocean. 



'Conversion factor is the ratio of standard to average volume of water strained. Standard volume used was 1,454 m' 

 'Computed from average volume of 792 m'from 62 tows in the eastern Pacific. (DaU from Klawe 1963.) 

 'Computed from average volume of 1,759 m' from 289 tows in the central Pacific. (DaU from BCF Biological Laboratory, Honolulu files and 

 Strasburg 1960.) 



"Computed from estimated volume of 3,878 m\ the volume strained at a given towing speed of 1.03 m/s (2 knots) in the western Pacific. (Catch 



data from Ueyanagi 1969.) 



18 



