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Fishery Bulletin 98(3) 



scheme; growth rates differed between silver hake from 

 northern Georges Bank and the Gulf of Maine and between 

 fish from southern Georges Bank and the Middle Atlan- 

 tic. Such within-stock differences further emphasizes the 

 uncertainty that currently exists over both stock boundar- 

 ies and the degree of mixing between silver hake stocks in 

 U.S. waters of the northwest Atlantic. 



Similarly, the uncertainty in stock boundaries and the 

 degree of stock mixing was evident in most of the past 

 silver hake stock discrimination studies (Conover et al.. 

 1961; Konstantinov and Noskov, 1969; Nichy, 1969: Ander- 

 son, 1974), although we believe that our approach has 

 the potential to resolve some of these questions. A major 

 reason for this uncertainty has been due to a lack of inten- 

 sive genetic studies to investigate the population struc- 



ture of the species, and therefore a subsequent lack of 

 knowledge concerning the actual reproductive isolation 

 of the species. Schenk (1981) demonstrated two geneti- 

 cally distinct groups but, owing to a limited geographic 

 sample area, was not able to define boundaries between 

 the groups. However, it must be remembered that a "stock" 

 in its working definition for practical fisheries manage- 

 ment is a management unit and not a discrete biological 

 population unit. Consequently, stocks of fish with dif- 

 ferent phenotypic (i.e. morphometric) characteristics or 

 life history (i.e. gi-owth) dynamics should be considered 

 as separate management units and modeled separately 

 regardless of genetic characteristics for stock assessment 

 and management purposes (Cadrin and Friedland, 1999). 

 Although our study did not delineate actual stock bound- 



