FISHERY BULLETIN: VOL. 74, NO. 1 



first extends from the coastline to just outside the 

 reef, a distance of about 4 km, and the second 

 extends from 4 to 37 km. Combined and called 

 inshore for this study, these two areas are made 

 up of relatively small statistical areas of unequal 

 sizes. It has been estimated that about 80% of the 

 effort and 75% of the skipjack tuna catch are con- 

 centrated within these areas (Uchida 1967). 

 Beyond 37 km is the third area, called offshore 

 here; the statistical divisions within it are large 

 and nearly equal in size. 



The inshore fishing ground, restricted to waters 

 within 37 km of the coastline, covered roughly 

 69,000 km^. The offshore ground, on the other 

 hand, was restricted only by the range of the ves- 

 sels, and varied from year to year. In 1948-65, the 

 vessels covered 111,000 km^ in their offshore 

 fishing, but many distant offshore areas were vis- 

 ited in only 1 or 2 yr over this period. The offshore 

 areas visited most frequently totaled roughly 

 69,600 km2. 



Comparison of Catch Per Effective Trip 

 and Catch Per Day Fished 



The monthly catches of skipjack tuna in 1965- 

 70, separated into inshore and offshore areas 

 within each vessel size class, were divided by two 

 different units of effort. One was the number of 

 days with catches, which was assumed to be equiv- 

 alent to effective trips; and the index derived 

 was CIET. The other v/as the total number of 

 days fished, which included days of fishing with 

 and without catches; and the index was CIDF. 

 The assumption that days with catches was equiv- 

 alent to effective trips appears justified; Uchida 

 (1967) showed that 96% of the effective trips 

 lasted 1 day. 



Figure 1 illustrates the relationship of the 

 monthly CIDF (Y) against CIET (X) calculated 

 for class 1 and class 2 vessels fishing the inshore 

 and offshore areas in 1965. The least squares re- 

 gression of y on X resulted in a close linear fit 

 with the regression line having an angle of 45°. 



A good fit between CIET and CIDF can be ex- 

 pected because both indexes are small when 

 fishing is poor and large when fishing is good. In 

 Hawaiian waters, periods of high tuna apparent 

 abundance are characterized by the presence of 

 larger schools and more frequent encounters be- 

 tween vessels and fish schools (Uchida and 

 Sumida 1971). 



o 

 a. 



^ 3 



o 



5 2 



2 3 4 5 6 



CATCH /EFFECTIVE TRIP ( METRIC TONS ) 



Figure l. — Relationship between catch per effective trip and 

 catch per day fished of Hawaiian skipjack tuna vessels, by 

 areas fished, January-December 1965. 



Homogeneity of Data 



At the outset of the study, it was decided that 

 one regression equation should be calculated for 

 each area within the size classes. The resulting 

 equations could then be used to estimate CIDF 

 from CIET for 1948-64. The decision to calculate 

 one equation for each area by pooling the data for 

 1965-70 is appropriate, because the data included 

 those years for which skipjack tuna catches from 

 Hawaiian waters were the lowest (1969) and 

 highest (1965) on record. Including data from 

 these 2 yr should provide sufficient low and high 

 values to determine accurately the slope and 

 level of each regression line. 



Pooling is appropriate when the samples are 

 homogeneous; therefore, it was necessary to test 

 the hypothesis of homogeneity. Statistical testing 

 of the data, discussed in the following sections, 

 was confined to only one index, CIET, because of 

 the close association between CIET and CIDF. 



The tests for homogeneity showed that yearly 

 variances of inshore CIET among class 2 vessels 

 differed significantly (x^ = 11.92; df = 5;P<0.05). 

 A plot of the yearly means and standard devia- 

 tions, shown in Figure 2A, indicated that they 

 were significantly correlated (r = 0.883; df = 22; 

 P<0.01). Furthermore, the distribution of CIET 

 was skewed because of many low and few high 



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