has been concentrated in the inshore grounds. 

 There are several possible answers. Fishermen 

 reduce costs by remaining close to port as long 

 as they can make profitable catches. The con- 

 centration of effort inshore also may be dictated 

 by the quality and quantity of live bait. Even 

 though it may occasionally survive as long as 

 a week, the delicate nehu may die w^ithin a 

 few hours. The fishermen logically would fish 

 inshore to use the bait quickly before mortality 

 becomes heavy. Furthermore, the need to re- 

 plenish live-bait supplies to some extent re- 

 stricts trips to the distant offshore grounds, 

 where live bait is unavailable. 



EFFECTIVE TRIPS BY SIZE CLASSES OF VESSELS 



The average number of effective trips per 



vessel per year fluctuated widely in 1952-62 



(table 8). The average number of effective 



trips per Class 1 vessel per year (fig. 6) 



Table S. — The total nvmber nf effective trips and the average 

 niimlier of effective trips per vessel by Class 1 and Class 2 

 Hawaiian skipjack tuna vessels, V>r>'2-62 



1952 1953 1954 1955 1956 1957 1958 1959 19G0 19G1 1962 



Figure 6. — Average number of effective trips per vessel 

 per year by Class 1 and Class 2 Hawaiian skipjack 

 tuna vessels, 1952-62. 



fluctuated between 51 and 72 in 1952-58, rose 

 sharply to 97 in 1959, and ranged between 77 

 and 91 in 1960-62. For Class 2 vessels, the 

 average rose very sharply from 58 to 88 in 

 1952-53, then declined to 75 in 1954 and to 62 

 in 1955. After 1955 the average appeared to 

 increase gradually. The reason for the increase 

 in the number of effective trips, particularly 



the sharp increase since 1959 among Class 1 

 vessels, is not known. 



APPARENT ABUNDANCE 



The catch per unit of effort does not provide 

 estimates of true abundance but of apparent 

 abundance, since it is affected by availability^ 

 and vulnerability- to the fishing gear. In the 

 section that follows, I discuss the catch per 

 effective trip, the factors affecting it, and the 

 method used to obtain a standard unit of effort. 



CATCH PER EFFECTIVE TRIP BY SIZE 

 CLASSES OF VESSELS AND AREAS 



Data on Y/g (catch per effective trip) by 

 size classes of vessels and areas are given in 

 table 9 and plotted in figure 7. The inshore 

 Y/g for Class 1 vessels fluctuated within a 

 relatively narrow range, whereas that for off- 

 shore fishing fluctuated more widely. The 

 curves for Class 1 vessels offshore and Class 2 

 vessels inshore were similar. Catches of Class 

 1 vessels that fished offshore fluctuated widely 

 and followed the curve for the total catch. The 

 inshore and offshore Y/g for Class 1 vessels and 

 total catch were significantly correlated 

 (r = 0.675; df ^ 9; p = 0.03 and r = 0.923; 

 df — 9; 7J<0.001, respectively). A similar com- 

 parison of data for Class 2 vessels showed that 

 both the inshore and offshore Y/g were sig- 



1 "Availabilil\' is the iiorlion (a percentage) of the recruited 

 population that is physirally within the geographic range of the 

 fishery durintr the fishing season." (Ahlstroni. 1960: p. 1361.) 



- "Vtilnerabijity is tlie accessibility of the fish within the geographic 

 range of tlie fishery to the efforts of a fishery." (Ahlstroni, 1960: 

 p. 1361.) 



U.S. FISH AND WILDLIFE SERVICE 



189 



