NOTE O'Neill et al : Accuracy of at-sea commercial size grading of Penaeus spp. 



401 



range corresponding to the small commercial grades 

 is narrow (Table 1 ), it is perhaps not surprising that 

 small prawns tend to be misgraded more frequently. 

 Alternatively the grading of small prawns may be 

 less accurate because they are less valuable than 

 large prawns and therefore less time is spent on grad- 

 ing each individual. 



In small packs, misgraded prawns were generally 

 gi'aded into larger categories, whereas in large packs, 

 those misgraded were generally placed into smaller 

 categories. 



Incorrectly graded prawns from both pack sizes, 

 however, tend to be incorrectly graded by only one 

 size category, so that all prawns were graded to 

 within three and six millimeters carapace length of 

 their corresponding size grade. 



The high proportion of landings graded and the 

 accuracy of some of this gi'ading suggest that size 

 information contained in the NPF logbooks could be 

 valuable for stock assessment. Most prawns sold in 

 small (3-kg) packs have been accurately graded by 

 fishermen, and these gradings could be used as a 

 reasonable measure of the length-frequency distri- 

 bution of the prawns. However, prawns sold in larger 

 (12-15 kg) packs are graded less accurately, espe- 

 cially for the smaller grade sizes, and it is recom- 

 mended that data from quality inspections be used 

 to correct fishermen's grade-size distribution. 



Although the work outlined in this paper was done 

 for the Australian northern prawn fishery, similar 

 analyses using similar methods could be carried out 



for other fisheries if comparable data on size gradings 

 were available. 



Acknowledgments 



We thank A. Raptis and Sons and, in particular, 

 David Crighton and Paul Hand for providing the 

 quality-control data set and describing the quality- 

 control procedures. Ted Wassenberg, Chris Jackson, 

 and Carolyn Robins made constructive comments on 

 an earlier version of this manuscript. 



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