REEVALUATION OF FISHING EFFORT AND APPARENT ABUNDANCE 



IN THE HAWAIIAN FISHERY FOR SKIPJACK TUNA, 



KATSUWONUS PELAMIS, 1948-70 



Richard N. Uchidai 



ABSTRACT 



Catch per effective trip, used in 1948-64 as an index of apparent abundance of skipjack tuna, Kat- 

 suwonus pelamis , in Hawaiian waters, is biased because effective trip, defined as one on which fish were 

 caught, underestimates effort. Catch per day fished, calculated from data collected in 1965-70, is a 

 refined index because effort includes days with or without catches. This paper describes the existence of 

 a linear relationship between catch per effective trip and catch per day fished in 1965-70, and a method 

 of estimating the latter from the former in 1948-64 based on this relationship. Fishing intensity, which 

 was measured by standard effective trips in past studies, is calculated in standard days fished. Changes 

 in catch per standard day fished are not associated with changes in relative fishing intensity. Skipjack 

 tuna abundance in Hawaiian waters, therefore, is fishery independent and is probably influenced by 

 availability and strength of year classes. 



In the study of the dynamics of any exploited fish 

 population, data on commercial catch and fishing 

 effort can be interpreted in a number of ways, 

 giving various estimates of apparent abundance. 

 The ultimate objective, however, is to obtain the 

 best possible estimate of apparent abundance. 



Prior to 1965, studies on catch and effort statis- 

 tics in the Hawaiian pole-and-line fishery for 

 skipjack tuna, Katsuwonus pelamis, defined 

 fishing effort as a "productive" or "effective" trip, 

 that is, one in which skipjack tuna were caught 

 (Yamashita 1958; Shippen 1961; Uchida 1966, 

 1967). Effective trip underestimated the actual 

 amount of fishing pressure, but it was used be- 

 cause catch report forms used by the fishermen in 

 1948-65 provided no spaces for recording zero- 

 catch trips. 



Zero-catch trips should be considered as effort 

 expended to catch fish because they include time 

 spent searching for schools of fish. But the rela- 

 tive importance of search and fishing time de- 

 pends on type of gear used. Gulland (1969) used 

 whaling as an example of a fishery where the im- 

 portant measure was time spent searching, the 

 gear being operational only for a few minutes. 

 The other extreme was bottom trawling, where 

 the important measure was time spent catching 

 fish with the gear on the bottom and searching 



^Southwest Fisheries Center Honolulu Laboratory, National 

 Marine Fisheries Service, NOAA, Honolulu, HI 96812. 



Manuscript accepted May 1975. 



FISHERY BULLETIN: VOL. 74, NO. 1, 1976. 



was minimal. Beverton and Parrish (1956) 

 suggested that where searching time is impor- 

 tant, the gear may have to be regarded as being 

 engaged in searching for fish but giving no catch 

 until a school is encountered. For pole-and-line 

 fishing, where much time is devoted to searching 

 for schools of fish, Shimada and Schaefer (1956) 

 used the day spent on the grounds as the basic 

 unit of fishing time. 



Catch reports of 1965-70 were used to obtain 

 two indices of skipjack tuna apparent abundance: 

 catch per effective trip (C/ET), calculated from 

 data on trips with catches, and catch per day 

 fished (C/DF), calculated from total days fished 

 including zero-catch fishing days. The purpose of 

 this study is to determine whether a relationship 

 exists between C/ET and C/DF. The importance 

 of the relationship is that it affords a means of 

 converting C/ET to C/DF for 1948-64, those years 

 for which no data on C/DF exist but for which 

 good C/ET information is available. A corrected 

 measure of apparent abundance, derived from 

 standard days fished instead of standard effective 

 trip, is used to estimate the relative fishing inten- 

 sity in 1948-70. 



COLLECTION OF DATA 



Data on skipjack tuna catch and fishing effort 

 were obtained from the Hawaii State Division of 

 Fish and Game, which collects fish catch statis- 

 tics in the Hawaiian Islands. In addition, catch 



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