708 



Fishery Bulletin 93(4). 1995 



Bayliff and Rothschild (1974) developed an ap- 

 proximate procedure for adjusting Jones's estimators 

 by the level of fishing effort at the time and place 

 each tag was recovered. A similar procedure could 

 be applied to the estimators developed here by ap- 

 propriately weighting the objective function. Equa- 

 tion 19, for example, could be modified to read 



m 



where f i is the observed fishing effort in the area 

 where tbe fth tag was recovered. Variations in other 

 factors, such as natural mortality and nonreporting, 

 could be treated analogously Bayliff and Rothschild 

 pointed out that this method of adjustment does not 

 account for the effort in the time-area strata encoun- 

 tered by the tag before its recovery. Even so, such ap- 

 proximate adjustments may be sufficient to reduce the 

 degree of bias to negligible levels. If true, the estima- 

 tion problem associated with Equation 20 would be sim- 

 plified considerably. This is an area that deserves more 

 attention, perhaps on an ad hoc basis via simulations. 

 In some situations it may be possible to circum- 

 vent the problem of inhomogeneous recovery rates 

 by dividing the feasible domain into regions where 

 the recovery rates are approximately constant and 

 by estimating the advection field in each region sepa- 

 rately. Tags of fish at liberty long enough to have 

 strayed into any of the other regions would be ex- 

 cluded from the analysis. 



Acknowledgments 



I thank J. Cramer, V. Restrepo, and S. Turner for 

 their comments on an early draft prepared as a work- 

 ing document for the ICES working group on long- 

 term management measures (Miami, 1994). The 

 present manuscript benefitted immeasurably from 

 the comments of M. Prager, W. Richards, and two 

 anonymous reviewers. 



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