Harley and Suter: Potential use of time-area closures to reduce the catches of Thunnus obesus in eastern Pacific Ocean 



57 



Quarter 1 



Quarter 3 



1997 1999 



Year 



Quarter 2 



~i 1 1 1 1 1 1 r^ 



1995 1997 1999 2001 



Year 



n 1 1 1 1 1 1 r 



1995 1997 1999 2001 



Year 



Quarter 4 



-I 1 \ 1 1 1 1 r 



1995 1997 1999 2001 



Year 



2 3 



Quarter 



2 3 



Quarter 



Figure 5 



Predicted changes in annual purse-seine catches of bigeye {Thunnus obesus) and skipjack tKatsuwonus pelamis) tuna 

 associated with a closure of the area between 5'N-IO^S and 90'-120^"W for each quarter of the year. See Figure 4 for 

 further description of the panels. 



quarters, the predicted reductions in bigeye tuna catches 

 associated with the practical closure ranged from 0.3% to 

 24.5% (777 metric tons [t] to 20,206 t), and the change in 

 skipjack tuna catches ranged from a 1.1% increase to a 

 17.0% reduction (1204 t to 32,773 t) (Fig. 5). The extreme 

 values of skipjack tuna catch were associated with first- 

 quarter closures. As with the hotspot closure, the great- 

 est reductions in bigeye tuna catch were associated with 

 second- and third-quarter closures (average reductions of 

 13.4% and 11.5%, respectively, across years) and again 

 the mean reduction in skipjack tuna catches did not 

 vary greatly across quarters (mean reductions ranging 

 from 3.8% to 4.9%). Based on the median of the ratios, 

 the greatest contrast between bigeye and skipjack tuna 

 reductions was associated with a second-quarter closure; 

 the average percentage reduction in bigeye tuna catches 

 was 13.4%, versus 4.9% for skipjack tuna catches. 



Overall, the hotspot closure predicted slightly greater 

 reductions in bigeye tuna catches and slightly lesser 

 reductions in skipjack tuna catches than did the prac- 



tical closure, but the difference in the median of the 

 ratios (5.0 times for the hotspot closure and 3.8 times 

 for the practical closure) is probably not significant. 

 Results for both closures may indicate that a closure 

 during the second or third quarters is optimal. Because 

 the predicted variability in performance was less for a 

 third-quarter closure than a second-quarter closure (in 

 both analyses), the former was preferred as a manage- 

 ment tool. 



Discussion 



Time-area closures are one of a number of fisheries man- 

 agement options (Hilborn et al., 2004). In our study we 

 investigated, using simulations that use historical catch 

 and effort data, whether time-area closures could be a 

 useful tool to reduce bigeye tuna catches in the purse- 

 seine fishery without leading to large reductions in the 

 catches of skipjack tuna. 



