CATCH AND ESTIMATES OF FISHING EFFORT AND APPARENT ABUNDANCE IN 

 THE FISHERY FOR SKIPJACK TUNA {KATSUWONUS PELAMIS) IN HAWAIIAN 



WATERS, 1952-62 



By Richard N. Uchida. Fishery Biologist. Bureau of Commercial Fisheries, Biological Laboratory 



Honolulu, Hawaii SXj812 



ABSTRACT 



Detailed data on catch and effort are obtained each year 

 from all vessels that fish full time in the Hawaiian skipjack 

 tuna fleet. These data permit description of the fishery 

 and inferences about the abundance of skipjack. Our past 

 measures of abundance have been stated in terms of total 

 catch and catch per unit of effort calculated in terms of 

 productive trips of all sizes of vessel. This study offers 

 information on changes in the apparent abundance of 

 skipjack in Hawaiian waters calculated from standardized 

 units that will be unaffected by changes in the numbers of 

 small and large vessels in the fishing fleet. 



Effort was measured in terms of an "effective" trip, which 

 was defined as a trip in which skipjack were caught. Bias 

 introduced by the lack of data on zero-catch trips is dis- 

 cussed. 



The number of men hooking per trip declined in 1950- 

 60; however, those that remained in the fishery increased 

 their catch rate by shifting their emphasis in fishing tech- 

 nique from "grasping and unhooking" each fish to "flip- 

 ping," a method in which the fisherman swings the fish 

 aboard and by relaxing the tension on the pole, permits 



the hook to fall clear of the fish's mouth. The higher catch 

 rate from the "flipping ' method was one of the factors that 

 offset the effect of the decline in the number of men. 

 Another factor that appeared to increase the catch rate was 

 the reduction in the number of small vessels that did poorly. 



The vessels were separated into two size classes: Class 1, 

 with bait-carrying capacities of less than 800 gallons per 

 baitwell; Class 2, with bait capacities of more than 800 

 gallons per baitwell. 



The catch was standardized to Class 2 vessels. The catch 

 per standard effective trip (Y/f) and the total catch fluc- 

 tuated similarly in all years. The Y/f had no apparent 

 trend and averaged about 5,700 pounds. The Y/f and the 

 relative effective fishing intensity were not correlated 

 significantly over the 11-year period. Year-to-year changes 

 in apparent abundance seem to be independent of changes 

 in fishing effort. 



I concluded that variations in the availability and vul- 

 nerability of skipjack contribute to fluctuations in landings. 

 The variations in strength of year classes also may have 

 contributed importantly to fluctuations in the landings. 



In Hawaii, the skipjack tuna, Katsuivonus 

 pelamis (Linnaeus), or aku, as it is called 

 locally, supports the State's most important 

 commercial fishery, contributing about 66 per- 

 cent by weight to the total Hawaiian marine 

 catch, and accounting for about 40 percent of 

 the total annual ex-vessel value. The fish are 

 caught exclusively by pole and line from schools 

 which are concentrated at the stern of the 

 vessel by chumming with live bait. The fishery 

 is highly seasonal. Landings have ranged from 

 about 29,000 pounds in January, typically a 

 poor month, to about 3.7 million pounds in 

 July, when the catch usually is large. Four- 

 to 5-pound fish usually are caught throughout 

 the year, but between May and September, 



larger fish, ranging between 13 and 25 pounds, 

 are also taken. The latter contribute a large 

 percentage by weight to the total annual catch. 

 Not only does the catch fluctuate by month but 

 also by year. In 1952-62, the yearly landings 

 ranged between 6.1 and 14.0 million pounds, 

 apparently with changes in the numbers of 

 the larger fish at the islands. 



In the past, abundance of skipjack tuna in 

 Hawaiian waters has been measured in terms 

 of total catch and catch per unit of effort cal- 

 culated in productive trips of all sizes of vessels 

 — uncorrected fishing effort (Yamashita, 1958 ; 

 Shippen, 1961). In general, total catch is not 

 a dependable measure of abundance, because it 

 is affected seriously by changes in the amount 



FISHERY BULLETIN: VOLUME 66, NO. 2 



Published April 1967. 



181 



