Bertignac et al : Estimates of exploitation rates for Thunnus alalunga from tagging data 



429 



E 



3 

 u 



SO 



3 





U.S. baitboat 



-predicted 

 observed 



12 16 20 24 28 32 36 40 



Longline 



predicted 



...♦..-observed 



16 20 24 28 32 36 40 



Japan baitboat 



-predicted 

 observed 



80, 



16 20 24 28 32 36 40 



Other 



- predicted 

 observed 



12 16 20 24 28 32 36 40 



Troll 



-predicted 

 . observed 



12 16 20 24 28 32 36 40 



All fleets 



- predicted 

 observed 



12 16 20 24 28 32 36 40 



Time at liberty (quarters) 



70 



Z 



predicted 



 observed 



■ti^aMMttMlMMiiA 



1971 1973 1975 1977 1979 1981 1983 1985 1987 1989 1991 1993 



Year 



Figure 6 



Observed tag return numbers and tag returns predicted with model 2 (see Table 4 for description). 



higher than previous estimates obtained from fish- 

 ery data by Suda (1966) and Fournier et al. (1998). 

 Model 1-4 estimates of M (0. 496-0. 536/yr) are lower 

 than those of models 5-8 (0. 576-0. 632/yr) because 

 in the former, the models attempt to accommodate 

 the low numbers of initial tag returns by depressing 

 M, whereas in the latter, reduced availability for the 

 initial time periods rather than reduced M is found 

 to be a more likely solution. We noted that the corre- 

 lation coefficients between the estimates of M and 

 the 0, ^'s for the initial time periods were always 

 negative and were in the range of -0.09 to -0.45. 



The coefficients of variation of the (^,y estimates were 

 variable (0.16—1.0) and were particularly high for the 

 surface fleets at time periods at liberty beyond three 

 years when very few tagged albacore were recovered. 



For model 8, considerable differences in catch- 

 ability estimates among time periods for all fleets 

 were detected, although no consistent trends were 

 evident (Fig. 8). The confidence intervals on these 

 estimates were large in some cases. 



Estimates of the total exploitation rate (percent- 

 age of the population harvested) in year y, calculated 

 by 



