Parrack: Estimating stock abundance from size data 



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abundance estimates were accurate, but those of pre- 

 ceding periods were not. Error variances of estimates 

 before the last period tended to be one to two orders 

 of magnitude higher than those of the last period. This 

 implies that abundance trends estimated in this man- 

 ner probably will be wrong. In this example, the prob- 

 lem is large enough to mask much of the 36% decrease 

 in abundance; the downward abundance trend would 

 not be clear in calculations of historical stock sizes. 



Discussion 



These Monte Carlo tests show that size-based methods 

 can be accurate and precise estimators of stock abun- 

 dance. Population characteristics need not conform 

 to the restrictive assumptions of traditional VPA 

 methods. Any sort of fish stock can be successfully ad- 

 dressed with size-based techniques, an important 

 aspect when assessing populations where ageing is im- 

 possible or where recruitment is not periodic. 



