Bolles and Begg: Distinction between silver hake stocks in U.S. waters 



459 



aries or determine the mixing components 

 between the silver hake stocks, further 

 investigation into the appHcation of this 

 technique may show that otolith morpho- 

 metries, when coupled with image process- 

 ing and discriminant analysis, is capable 

 of achieving these objectives. 



Subsequently, to understand better the 

 fishery dynamics of silver hake stocks 

 and to manage them accordingly, we must 

 question what caused these differences — 

 whether they are differences in fish growth, 

 or internal or external morphometric char- 

 acters, both between and within stocks. 

 Otolith morphometric differences between 

 silver hake stocks are probably the result 

 of both environmental and genetic influ- 

 ences. For example, the hydrodynamics of 

 northwest Atlantic waters are considered 

 to be heterogeneous; environmental condi- 

 tions differ between the Gulf of Maine and 

 the Middle Atlantic (Brooks, 1996). Cer- 

 tainly, we observed significant and consis- 

 tent differences in the bottom temperatures 

 between the two stock regions inhabited by 

 silver hake, where those fish in the north- 

 ern region experience lower temperatures 

 than those in the southern region. Because 

 fish growth and external body morphomet- 

 ries are known to be influenced by such 

 environmental factors (Ihssen et al., 1981; 

 Pawson and Jennings, 1996), one would 

 also expect that internal morphometries, 

 such as otolith length and width, would 

 also differ in response to these variables 

 because fish growth and otolith growth 

 tend to be related. In accord with these 

 assumptions, otolith shape has been found 

 to vary in response to environmental condi- 

 tions and is also highly correlated with fish 

 growth (Campana and Casselman, 1993). 

 Similarly, the results of our study showed 

 significant correlations between fish and 

 otolith growth for all the otolith morpho- 

 metric characteristics measured. There- 

 fore, it is likely that the differences found 

 between silver hake stocks in growth rates 

 ( Helser, 1996; our study ), external morphometries ( Conover 

 et al., 1961: Almeida, 1987), and internal morphometries 

 (our study) are to some extent reflective of localized envi- 

 ronmental conditions. 



Assuming that environmental variables can effect otolith 

 growth, we analyzed bottom ocean temperature between 

 the two stock regions across the life history range of our 

 samples to investigate whether the differences in otolith 

 morphometries found in this study were due to a weather 

 phenomenon during any given sampling year (Fig. 2). Tem- 

 perature was found to differ consistently on a seasonal 

 basis during all years, as would be assumed, and there 

 was no marked change in temperature between any given 



years, within any given season. As a result, the possibil- 

 ity of a severe temperature shift that could influence oto- 

 lith morphometries and make this method unsuitable for 

 stock discrimination was not apparent in our study. How- 

 ever, because temperature was shown to differ between 

 the proposed stock regions and because silver hake dis- 

 tribute themselves in response to temperature (Almeida, 

 1987; Helser, 1996), it can be assumed that this variable 

 could effect otolith growth. 



Alternatively, growth and otolith morphometric differ- 

 ences found between silver hake stocks could also be appar- 

 ent, regardless of environmental variation, and reflect a 

 genetic influence. This could be a valid assumption given 



