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



39 



. 1988 



1987 

 1986 



1985 



1984 



1983 



1982 

 1981 

 1980 



— i — i — i — i — r- 



IZI B- 



I ! T I 



n^Tr-r^ 



o. 



Oil 



□£l 



TI 



- AGE 4 . 



r^ 



OZL 



TTT-rL 



J~ r— r 



~1 1 1 I~~I 1 1 ! T- 



AGE 5.5 



AGE 5.0 



AGE 3.5 



AGE 3 . 

 AGE 2.5 

 AGE 2 . 

 AGE 1 . 5 

 AGE 1.0 

 AGE . 5 



Figure 7 



Average annual differences between the observed and non-dolphin (Delphinidae) 

 catchability coefficients (boat-days -1 ). In the left panel, differences are summa- 

 rized by age within year. In the right panel, differences are summarized by year 

 within age group. Age refers to the age in years at the middle of the year. Nega- 

 tive values (those pointing down) indicate that those non-dolphin-set catchability 

 coefficients were greater than the observed coefficients. 



the observed levels when the non-dolphin-set 

 catchability coefficients were greater than or equal 

 to the observed overall catchability coefficients 

 (Fig. 7, negative values) for the age groups of the 

 greatest biomass (Figs. 3 and 4). Estimated total yel- 

 lowfin plus skipjack catches, if all effort were di- 

 rected at non-dolphin sets, ranged from 84% during 

 1985 to 104% in 1983. 



Estimates (Table 1, column QYF) of what the 

 catches would have been without dolphin sets, us- 

 ing the quarterly average (over years) non-dolphin- 

 set catchability coefficients for 1980-88, indicate 

 that yellowfin catchabilities on non-dolphin sets 

 increased in the late 1980's. Average quarterly 

 catchability coefficients produced noticeably higher 

 catches than the observed non-dolphin-set monthly 

 coefficients in 1983-85 when the observed coeffi- 

 cients on small fish were low. On the other hand, 

 average quarterly catchability coefficients produced 

 lower catches during 1986-88, when the observed 

 non-dolphin-set coefficients were high. 



Monte Carlo simulation 



The Monte Carlo simulations (Table 2) predicted 

 that, if total effort, recruitment, and non-dolphin-set 



catchability coefficients had varied randomly 

 throughout their 1980-88 distributions, and current 

 levels of effort and recruitment had been main- 

 tained, changing to a fishery with all effort directed 

 toward non-dolphin sets would have resulted in an 

 average reduction of 55,563 t (24.7%) of yellowfin 

 catch per year. The 95% confidence interval, based 

 on the 50th and 950th highest simulated differences 

 was 24,000 to 91,000 t (10%-42%). The entire fre- 

 quency distribution of the differences between the 

 two fishing-mode models in the 9th year is shown 

 in Figure 8. Simulated recruitment estimates were 

 selected from the observed values during 1980-88. 

 Thus, average recruitment used in the simulations 

 was higher than the mean actual recruitment to the 

 initial 1980 population structure, which was partly 

 a result of the poor recruitment during 1978 and 

 1979. Consequently, simulated catches were higher 

 for both the observed mixed mode fishery (229,000 

 t per year) and the non-dolphin-set only fishery 

 (175,000 t). 



Yield per recruit 



Estimated yellowfin catches from both the determin- 

 istic approach (Table 1) and the Monte Carlo simu- 



