FISHERY BULLETIN: VOL. 72, NO. 4 



Table 9. — Returns of tagged bluefin tuna per boat-day by years 



at liberty. 



analytical formulation of this mortality model has 

 two independent variates — cumulative time and 

 cumulative effort. Not only are these two vari- 

 ables so highly correlated that it is virtually im- 

 possible to obtain useful estimates of the two re- 

 gression coefficients, but also there are too few 



data points available for multiple regression 

 analysis. The regression coefficient of the simple 

 cumulative time regression is a crude measure 

 of the total mortality rate. The fitted regression 

 lines are shown in Figure 3 for July releases and 

 in Figure 4 for all releases for each year. The 

 values of the coefficients are given in Table 10. 



With the exception of 1966, when the returns 

 per boat-day increased during the year following 

 release (and the data were eliminated from the 

 regression), the logarithms of the recaptures per 

 unit effort decrease in a linear fashion and it is 

 clear the regressions provide reasonable fits to the 

 data points. Seven of the eight estimated survival 

 rates shown in Table 10 are lower than their coun- 

 terparts in Table 5. Both of these analyses provide 

 at best only crude approximations of the true sur- 

 vival rates; however, they do show fair general 

 agreement. The catch-per-effort estimates indi- 

 cate the loss rates are tending to increase with 

 time. 



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YEARS AT LIBERTY 



Figure 3. — Regression of recaptures per boat-day vs. time at 

 liberty for July releases of tagged bluefin tuna. 



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05 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 



YEARS AT LIBERTY 



5.0 



Figure 4. — Regression of recaptures per boat-day vs. time for all 

 releases, by year, of tagged bluefin tuna. 



912 



