FISHERY BULLETIN: VOL. 72, NO. 4 



(F +Xf =F +X -L 



where (F + X)* = estimate of total instantaneous 

 apparent mortality corrected 

 for tag shedding rate, 



Table 6. — Estimates of rates of exploitation and mortality of 

 bluefin tuna corrected for tag shedding. 



where s* 



where u^ 



where Uj 



where F* 



where X* 



s* =e-'^ + ^'' 



= estimate of annual survival 

 corrected for shedding rate, 



= estimate of total exploitation 

 of tagged bluefin tuna cor- 

 rected for immediate tag 

 shedding, 



«1 = Uilp 



= estimate of seasonal exploi- 

 tation corrected for immediate 

 tag shedding, 



F* =ut (-InsVd -s) 



= estimate of F corrected for 

 immediate tag shedding, 



X* = -ln(s*) -F* 



= estimate of X corrected for 

 immediate tag shedding. 



"1 



F* 



F=^ + X'' 



(1 - s*) 



where u ^ 



estimate of single season 

 exploitation of tagged and 

 untagged bluefin corrected 

 for all tag shedding, and 



U T 



Z 



where u 



** 



estimate of total exploitation 

 of tagged and untagged bluefin 

 corrected for all tag shedding. 



The estimates are shown in Table 6. Estimates 

 ofX* (other losses) range from 0.366 to 1.234 (av- 

 erage = 0.792). The estimates ofX* are considera- 

 bly higher than expected values of M (natural 



mortality) for bluefin. Bluefin are very long-lived 

 fish and values of M of 0.1 to 0.2, if M is constant, 

 would seem reasonable. Thus, there may be 

 significant amounts of Type-I (immediate) and 

 Type-II (long-term) tagging mortality, nonreport- 

 ing, and/or apparent mortality caused by emigra- 

 tion. 



It seems plausible that some Type-I tagging 

 mortality exists. In an earlier section it was noted 

 that, more returns were obtained from sport gear 

 releases than from commercial gear releases. 

 Statistically significant, more returns were ob- 

 tained from sport releases than commercial re- 

 leases in 1965, 1966, 1969, and 1970. A possible 

 cause of the difference in return rates could be that 

 sport tagged bluefin are of different ages than 

 commercially tagged bluefin. Bluefin of different 

 ages could suffer different rates of tagging mortal- 

 ity and shedding, and could behave differently. 

 Data have not been compiled in a fashion that 

 allows examination of the age at release by the 

 two gear t3^es. We recommend that it be and 

 assume for the present that age-dependent effects 

 are negligible. The differences in return rates 

 could also be caused by Type-I tagging mortality. 

 Experienced taggers report that sport-gear- 

 caught bluefin appear to be in better condition 

 than those caught by commercial gear. Also, 

 commercial gear tends to capture entire schools of 

 bluefin while sport gear captures one bluefin at a 

 time. Thus, bluefin released from sport gear tend 

 to be released into the immediate area of a school 

 of bluefin while bluefin released from commercial 

 gear are not. There may be an advantage for 

 bluefin to be in a school. Bayliff ( 1973) found that 

 return rates of purse-seine-caught yellowfin tuna 

 tagged immediately after a set is made can be 

 more than two times higher than yellowfin tagged 

 at a later time (Table 7). This evidence suggests 

 that Type-I tagging mortality is very important 



910 



