Hannah; A new method for indexing spawning stock and recruitment for Pandalus /ordani 



493 



(2 of 3 observed) years of low spawner abundance 

 result mainly from recruitment failures at age 1. This 

 finding suggests that the classic management strat- 

 egies, aimed at maintaining age-1 escapement rates, 

 such as maximum count-per-pound and minimum 

 codend mesh size, are unlikely to be very successful 

 at reducing recruitment variability and increasing 

 average recruitment for ocean shrimp. The reason 

 for this is that by allowing increased escapement from 

 an extremely depressed year class, there will be likely 

 little impact on total spawner abundance. Rather, a 

 strategy that reduces or eliminates exploitation of 

 all age classes, once a year-class failure has been 

 detected, has more potential to increase spawner 

 abundance, simply because the remaining older 

 shrimp may be more abundant than the failed age-1 

 year class. Another alternative, of course, is to man- 

 age at much lower exploitation rates, such that more 

 shrimp survive to spawn at ages 2 and 3, although 

 such a strategy is unlikely to maximize yield. The 

 precise impact on yield of such an approach is diffi- 

 cult to predict without better knowledge of natural 

 mortality rates. Natural mortality rates of ocean 

 shrimp have been shown to vary widely, possibly in 

 response to changes in the abundance of predator 

 stocks (Hannah, 1995), such as Pacific whiting 

 (Merluccius productus). Improved estimates of natu- 

 ral mortality rates for shrimp in turn depend on bet- 

 ter knowledge of the elemental efficiency of shrimp 

 trawl gear (Hannah, 1995). In addition, changes in 

 management of this fishery should be geared towards 

 maximizing economic rather than biomass yield 

 (PFMC, 1981), suggesting that a comprehensive bio- 

 economic model is needed before the findings of this 

 study can begin to be used. 



Acknowledgment 



This paper was funded in part by a grant/coopera- 

 tive agreement from the National Oceanic and At- 

 mospheric Administration. This project was financed 

 in part with Federal Interjurisdictional Fisheries Act 

 funds (15'7c federal, 25'/f state of Oregon funds) 

 through the U.S. National Marine Fisheries Service 

 (contract NA76FI0139). Helpful reviews of early 

 drafts were provided by David Fox and David 



Sampson. 



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