134 



Fishery Bulletin 92(1), 1994 



1 -10 Boat-Days 

 CD 11 -50 Boat-Days 

 J\ 51 - 100 Boat-Days 



101 -200 Boat-Days 



201 - 500 Boat-Days 



Greater than 500 Boat-Days 



Figure 2 



(A) Geographic distribution of total purse-seiner fishing effort during 

 1980-88 which lead to dolphin (Delphinidae) sets. Effort levels are ex- 

 pressed in boat-days of fishing by 2.5-degree quadrangles. (B) Geo- 

 graphic distribution of total purse-seiner fishing effort during 1980-88 

 which lead to non-dolphin sets. Effort levels are expressed in boat-days 

 of fishing by 2.5-degree quadrangles. 



native catchability coefficients 

 include Holt ( 1958), Jones ( 196 1 ), 

 and Bartoo and Coan (1978). 



Materials and methods 

 Data 



The Inter-American Tropical Tuna 

 Commission's (IATTC) logbook 

 and length-frequency data bases 

 were used in this study. The log- 

 book data base, described in Or- 

 ange and Calkins (1981), Punsly 

 (1983; with emphasis on set 

 types), and Punsly (1987; with 

 emphasis on yellowfin catch 

 rates), contains information on 

 the fishing activities of about 90^ 

 of the purse seiners in the EPO. 

 Total catches were estimated by 

 multiplying the logbook catches 

 by the ratio of the sum of the un- 

 loading weights to the sum of the 

 logbook catches. Geographic dis- 

 tributions of the logbook data on 

 catch and effort, during 1980-88, 

 for both dolphin-associated and 

 unassociated schools are shown in 

 Figures 1 and 2. The length-fre- 

 quency data base, described by 

 Hennemuth (1957). Punsly and 

 Deriso (1991), and Tomlinson et 

 al. (1992), has information from 

 samples of about 12-15^ of the 

 catch. Age-specific yellowfin abun- 

 dances from cohort analysis 

 (Pope, 1972; also called sequential 

 computation of stock size in 

 Ricker, 1975; and virtual popula- 

 tion analysis in Gulland. 1965) 

 were taken from Bayliff ( 1990). 



Data from 1980 to 19S8 were 

 used in this study. Data before 

 1980 were not used because of the 

 difficulty in modeling the closed 

 seasons for yellowfin (Cole, 1980). 

 Data after 1988 were not used be- 

 cause cohort analysis cannot pro- 

 duce accurate abundance esti- 

 mates for cohorts which have not 

 been in the fishery for a sufficient 

 period of time. 



Semi-annual age groups used in 

 this study were described in detail 



