Punsly et al.: Potential non-dolphm-associated tuna catches in the eastern Pacific Ocean 



135 



in Bayliff (1992, p. 52). Monthly age compositions 

 were estimated by combining 1-cm length-interval 

 data into semi-annual age groups by fitting 

 multinormal distributions to the data with the aid 

 of the computer program NORMSEP, (Abramson, 

 1971), and constraining the fit to the growth param- 

 eters of Wild (1986). "X" and "Y" cohorts were de- 

 fined as those fish reaching 30 cm, which correspond 

 to the approximate age of first recruitment, during 

 the fourth and second quarters of the year, respec- 

 tively. Age groups in our study, 0.5 to 5.5 in 0.5 year 

 increments, correspond to the Y0, XI, Yl ... Y5 co- 

 horts, respectively, in Table 21 of Bayliff (1992). 



Estimates of fishing effort 



The total monthly effort by purse seiners was esti- 

 mated as 



E = f Y l\ 



om J om om I .' om ' 



where o, refers to the observed mixture of set types, 

 Y is the yellowfin catch unloaded by purse sein- 

 ers in month (m),y om is the yellowfin catch reported 

 in the IATTC logbooks and f is the effort, in boat- 

 days of fishing, reported in the logbooks. Effort on non- 

 dolphin sets for all purse seiners was estimated by 



^nm / , / .lnmcs^omcs/y 





v„ 



where f nmcs is the fishing effort which lead to non- 

 dolphin (n) sets by monitored vessels of size (s) from 

 country (c), Y omcs is the total catch of yellowfin from 

 unloadings by size (s) vessels from country (c), and 

 y is the total yellowfin catch by monitored ves- 



J omcs J "* 



sels. These estimates were stratified by country and 

 size of vessel because the proportion of dolphin sets 

 is affected by these two factors. 



Estimation of yellowfin catches if all effort 

 were non-dolphin 



This method used age-specific, monthly catchability 

 coefficients by fishing mode and allowed the future 

 population structure to be affected by previous 

 catches. First, age-specific catchability coefficients 

 for non- dolphin sets in) in each month (m) were 

 estimated for each semi-annual age group (/'): 



Qnmj ~ ^nmj y^nm^ mj J i 



where C are the monthly, total, non-dolphin 

 purse-seine catches (in numbers of fish) of semi- 

 annual age group (j) and N mj are the age-specific, 

 monthly, average abundances estimated by the co- 

 hort analysis (Bayliff, 1990). Beginning with the 

 population structure in January 1980, obtained from 

 cohort analysis, we estimated what the catch in each 

 month of each semi-annual age group would have 

 been without dolphin sets; i.e., 



pmj 



(N mj q nmj E om )/(q nm] E om + Mj ) 



where Mj is the age-specific, instantaneous, monthly 

 natural mortality (Bayliff, 1992, p. 52). Yield in 

 weight was estimated by 



Y =W (i)C , 



1 pmj m y J ' pmj ' 



where W(j) is the estimated mean weight of age (j) 

 yellowfin in month m caught during 1980-88. The 

 subsequent month's abundance of semi-annual age 

 group (j) was estimated to be 



Estimates of skipjack catches if all effort 

 were non-dolphin 



Skipjack are suspected to be mostly transient in the 

 EPO (Joseph and Calkins, 1969), so we assumed 

 that depletion is probably unimportant. Thus, the 

 ratio of the total effort to the non-dolphin effort was 

 used to estimate skipjack catches: 



Y pm (SJ): 



Y nm (SJ)E om /E nn 



where Y m (SJ) is the potential (p) non-dolphin, skip- 

 jack catch and Y nm (SJ) is the actual non-dolphin-set, 

 skipjack catch. In essence, skipjack catches were 

 estimated to be linear extrapolations of catch rates 

 to higher levels of effort. 



£*m+Xj mj 



-iq nm .E om+ M,) 



except for the months of recruitment (May and 

 January), when N JAN2 and N MA y, 3 were set e Q ual to 

 the historical recruitment previously estimated for 

 that time period by cohort analysis. Yellowfin form 

 the first semi-annual age group (those fish hatched 

 in the middle of the current year) were not included 

 in the analysis because they were not recruited until 

 the next year, when they became semi-annual age 

 group 3. Each January, the semi-annual age groups 

 were graduated as follows: 



N JANJ+2 =N 



DEC. J 



iaDEi ' 



 +M, 



