432 



Fishery Bulletin 97(3), 1999 



40. 



30 - 



.2 20 - 



a. 



10 - 



0.0 



nual total exploitation rates have 

 been less than lO'^ during most 

 of the years of the experiment. 

 However, there is a strong indi- 

 cation that exploitation of tagged 

 albacore by the troll fleet may not 

 be representative of exploitation 

 patterns generally by this fleet. 

 The total number of tag returns 

 from the troll fleet was 148 (Table 

 1). The U.S. baitboat fleet, which 

 fished in an area similar to that 

 fished by the troll fleet during the 

 1970s and 1980s, reported 403 

 tag recaptures. However, during 

 the period of the tagging experi- 

 ment, troll catches were some 

 eight times baitboat catches ( Liu 

 and Bartoo, 1995). There is no 

 ready explanation for this incon- 

 sistency. One possibility is that 

 differential reporting rates ex- 

 isted between the fleets. How- 

 ever, given the earlier assertion 

 that tag-reporting rates were 

 high in general, differential tag 

 reporting is unlikely to be the 

 cause of such a discrepancy. An- 

 other possibility is that albacore 

 become "hook shy" after they 

 have been captured by trolling 

 and are thereafter less likely to 

 respond to trolled lures. Because 

 the majority of albacore tag re- 

 leases were troll-caught, such be- 

 havior would render the tagged 

 population less available to the 

 troll fleet than the albacore popu- 

 lation in general. If this was in- 

 deed the reason for the paucity 

 of troll-caught returns, relative 

 availability of tagged albacore to 

 the troll fleet would need to be of 

 the order of 3.5% (compared with 100% for the U.S. 

 baitboat fleet) to reconcile the tag-return and catch 

 data for these fleets. Such reduced availability would 

 result in significant underestimation of exploitation 

 rates if not explicitly accounted for in the analysis. 

 If we set (/)^^=: 0.035 for 3</<16 for the troll fleet, esti- 

 mated exploitation rates that would apply to the 

 untagged population are considerably higher, declin- 

 ing from around 40% in the 1970s to less than 10%- 

 since the mid-1970s (Fig. 11). At this point, we can- 

 not be sure that the "hook shy" hypothesis is viable 

 or not; therefore, both of the time series shown in Fig- 



- Exploitation rate 



- Natural rrwrtality rate 



r 0.8 



0.6 



0.4 



0.2 B 



0.0 



0.2 0.4 0.6 0.8 



Assumed tag-reporting rate 



1.0 



1.2 



Figure TO 



Estimates of annual natural mortality rate and annual average exploitation rate 

 (total for all fleets) for model 8 (see Table 4 for description) as a function of the tag- 

 reporting rate (/J). The same value of /J is assumed to apply to each fleet. 



50 



40 



2 30 



c 

 o 



S 20 



D. 



W 10 







Model 8 



Reduced availability 



1970 1972 1974 1976 1978 



1980 1982 

 Year 



1984 1986 1988 1990 1992 



Figure 11 



Estimates of annual exploitation rates of North Pacific albacore under hypotheses 

 of full availability of tagged albacore to the troll fleet (model 8), and reduced avail- 

 ability due to "hook shyness." 



I 



ure 11 should be considered as equally valid possibili- 

 ties until additional information becomes available. 



Acknowledgments 



We are grateful to Babera Kaltongga for assistance 

 in compiling tagging and fishery data. Eric Prince 

 and Joseph Powers provided valuable comments on 

 an earlier version of the manuscript. The manuscript 

 was improved by the comments of two anonymous 

 reviewers. 



