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Fishery Bulletin 89(2), 1991 



best set of characters for group separation. 

 I examined gill-raker counts because this 

 meristic character appeared useful in 

 separating groups of Pacific yellowfin tuna 

 (Godsil and Byers 1944, Schaefer 1955). 

 Rather than using the term "stock(s)," since 

 it is not known whether there is a genetic 

 component to the differences observed, I 

 use the term "group(s)," as defined by Marr 

 (1957), because this avoids the technicality 

 of the degree to which genetics are involved 

 in the differences observed. 



Materials and methods 

 Sampling and data collection 



20° 100' 80° 



i t°J _!LJ D ' V -'-*■ 



100° 120° 140° 160° 160° 160° 140° 120° 100° 80° 



Yellowfin tuna were captured by baitboats, 

 trollers, or sportfishing boats during 

 January to May 1988, from five localities in 

 the Pacific Ocean: the Revillagigedo Islands, 

 Mexico; Manta, Ecuador; New South Wales, 

 Australia; Ishigaki, Japan; and Oahu, Ha- 

 waii (Fig. 1). These locations were selected 

 to optimize spatial coverage within the dis- 

 tribution of the surface and longline fish- 

 eries for yellowfin tuna in the Pacific. Sam- 

 ples ranged from 66 to 105 individuals per 

 location (Table 1), and included fish from at 

 least four schools from each area. 



Thirteen linear measurements (Fig. 2) 

 were made with calipers on each specimen 

 within 24 hours of capture, and recorded 

 to the nearest millimeter, according to 

 methods described by Marr and Schaefer 

 (1949). The number of gill rakers on the up- 

 per and lower limbs of the first left gill arch were also 

 recorded for each fish. Counts for the lower limb in- 

 cluded the single gill raker present at the angle be- 

 tween the upper and lower limbs (Collette and Nauen 

 1983). Sex was determined by examination of the 

 gonads of the fish from Ecuador and Australia, and this 

 subset of fish was used to test the hypothesis of no sex- 

 ual dimorphism in morphometric characters of yellow- 

 fin tuna. 



Statistical analyses 



Because of the variation in size of fish from different 

 areas (Table 1), morphometric data were statistically 

 adjusted to permit comparative analysis in terms of 

 shape independently of size (Gould 1966, Thorpe 1983). 

 The morphometric measurements were first trans- 

 formed to common logarithms because linearity and 

 multivariate normality are usually more closely approx- 



Figure 1 



Sampling locations in the Pacific Ocean for yellowfin tuna: (1) Revillagi- 

 gedo Islands, Mexico; (2) Manta, Ecuador; (3) New South Wales, Australia; 

 (4) Ishigaki, Japan; and (5) Oahu, Hawaii. 



imated by logarithms than by the original variables 

 (Pimentel 1979). Outliers were detected by regression 

 analyses of morphometric characters against total 

 length and by scatter plots of residuals versus predicted 

 values (Cook and Weisburg 1982). When an outlier was 

 found, all the morphometric data (but not the gill-raker 

 data) for that fish were withdrawn from further con- 

 sideration. This procedure resulted in the elimination 

 of morphometric data for 23 fish. 



Each of the morphometric characters showed a linear 

 relationship with total length (r 2 ranged from 0.95 to 

 0.99), when analyzed by geographic region. Analysis 

 of covariance (ANCOVA) was employed to test for 

 differences in allometric relationships among samples, 

 and to estimate the common within-group regression 

 slopes. Within-group regression slopes were signifi- 

 cantly different (P<0.01) for 10 of the morphometric 

 characters, and thus size adjustments were based on 

 the common within-group slopes. Coefficients from the 



