436 



Fishery Bulletin 103(2) 



at which 59c or less of the initial recruits survived. The 

 use of Anthony's standard to approximate M makes the 

 assumption that the fishable life span of an exploited 

 stock is the same as the longevity of the members of 

 the stock in an unexploited condition. It is unlikely 

 that this assumption will be met unless the fishery is 

 at an early stage in its development because fishing 

 may alter the age structure of the stock (Hilborn and 

 Walters, 1992). We note that although a limited num- 

 ber of scientists involved with ICES have used 3/t max 

 in a general way, the method has not been adopted as 

 a convention within ICES (O'Brien 6 ). Furthermore, we 

 did not find evidence that the approach is currently in 

 common use in stock assessments on the east coast of 

 the United States, with the exception of those for blue 

 crab. Nonetheless, the rule-of-thumb approach certainly 

 has the potential to be used widely, given its repeated 

 presentation in fishery literature and its accumulated 

 momentum in blue crab work. 



Recommendations 



The power of empirical relationships for predicting natu- 

 ral mortality can be rather limited (Vetter, 1988; Pas- 

 cual and Iribarne, 1993), and the uncertainty associated 

 with parameter estimates should be taken into account 

 whenever possible (Patterson et al., 2001). Further- 

 more, methods for directly estimating M are likely to be 

 preferable to making predictions based on life history 

 features. Nonetheless, such estimates may be needed 

 when available data are inadequate for making a direct 

 estimate. Given the results of our comparison, we recom- 

 mend that the regression estimator be used instead of 

 the rule-of-thumb approach when longevity is used to 

 predict M. The regression estimator is based on a least 

 squares fit to an extensive data set and thus matches 

 experience better than a rule-of-thumb approach based 

 on an arbitrary constant. 



We recommend that use of the 3/t max rule of thumb 

 be abandoned, despite it being entrenched in blue crab 

 literature. For a species like blue crab, for which t max is 

 less than 10 years, the differences in the estimates of M 

 from the regression estimator and 3/t max are not trivial 

 (-45%). Although the regression estimator was based 

 on data for fish, mollusks, and cetaceans (Hoenig, 1983) 

 and may not be applicable to other exploited taxa, such 

 as crustaceans, the model had a good fit to the data 

 across widely disparate taxa. Finally, estimates of M for 

 blue crab based on longevity are controversial because 

 of continued difficulty in determining an appropriate 

 'max- I n *" ne aDsence of data to directly estimate M for 

 this species, we suggest that the most prudent course 



O'Brien, C. M. 2004. Personal commun. Chair of ICES 

 Working Group on Methods of Fish Stock Assessments and 

 ICES Resource Management Committee. CEFAS (Centre for 

 Environment, Fisheries and Aquaculture Science) Lowestoft 

 Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, 

 England. 



of action is a review and comparison of other methods 

 for predicting M. 



Acknowledgments 



We thank Doug Vaughan for helping investigate the 

 use of the rule-of-thumb approach, and Russell Burke, 

 Romuald Lipcius, Jacques van Montfrans, and three 

 anonymous reviewers for helpful comments on the manu- 

 script. D.A.H. gratefully acknowledges the support of 

 the Willard A. Van Engel (WAVE) Fellowship for Crus- 

 tacean Research. This work was supported by fund- 

 ing from the NOAA Chesapeake Bay Office, award no. 

 NA03NMF4570376. 



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