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Fishery Bulletin 98(3) 



elow and Schroeder, 1953; Almeida, 1987). In con- 

 trast, the southern stock is found predominantly in 

 waters from Cape Cod to Montauk Point, New York, 

 and during peak spawning, which occurs in June 

 and July, it is found on southern Georges Bank, in 

 the New York Bight, and as far south as Cape Hat- 

 teras (Almeida, 1987; Helser et al., 1995). 



Although silver hake in U.S. waters are cur- 

 rently assessed according to this two-stock scheme, 

 uncertainty exists in the stock definitions. A vari- 

 ety of other stock identification techniques have 

 also been used to identify these two stocks of silver 

 hake, including examining external morphometries 

 (Conover et al., 1961), growth rate comparisons 

 (Nichy, 1969), immunological analyses (Konstanti- 

 nov and Noskov, 1969), research survey distribu- 

 tion studies (Anderson, 1974), and genetic analyses 

 (Schenk. 1981 ). However, the stock boundaries sug- 

 gested in each of these studies have all tended to 

 vary from one another. More recently, Helser ( 1996) 

 found that silver hake growth rates differed both 

 between and within the two stocks — a finding that 

 is inconsistent with the stock boundaries currently 

 defined (Almeida, 1987) and that confounds the 

 delineation of the stocks and the degree to which 

 the two stocks mix. Given this finding, we believed 

 further investigation into the stock structure of 

 silver hake was needed to clarify the current man- 

 agement units defined for U.S. waters. In addi- 

 tion, we aimed to introduce a more efficient and 

 accurate method for distinguishing between silver 

 hake stocks than the previous methods that have 

 been used, one which would have the potential to 

 enable rapid and accurate in-season assessment of 

 the degree of stock mixing for silver hake. 



Otolith morphometric data have recently been 

 used to identify stocks of marine fish, including 

 Atlantic mackerel. Scomber scombrus (Dawson, 1991; Hop- 

 kinsM, Atlantic herring, Clupea harengus (Messieh et al., 

 1989), and Atlantic salmon, Salmo salar (Friedland and 

 Reddin, 1994). In contrast, otolith morphometric data have 

 not been used to differentiate between stocks of silver 

 hake, but because silver hake have been found to differ 

 phenotypically (Almeida, 1987), and possibly genetically 

 (Schenck, 1981 ), it was assumed that otolith morphometric 

 characteristics may also vary. Dery 1 1988) observed that 

 silver hake otoliths from the northern stock were narrower 

 and thicker in cross-section than those of the southern 

 stock, although no statistical analyses were used to test 

 her observations. Accordingly, it was thought that otolith 

 morphometries could differ between the two silver hake 

 stocks and could prove to be useful characters by which 

 to differentiate stocks in the future. The collection of such 

 data by means of image processing techniques has proven 

 to be useful in stock identification, providing accurate and 

 efficient measures that traditional morphometric methods 

 have not been able to provide (Jearld, 1995; Cadrin and 



1 Hopkins, P.J. 1986. 

 lith morphometries. 



Mackerel stock discrimination using oto- 

 K'ES Council Meeting 1986, 17 p. 



Figure 1 



NEFSC spring survey stations where silver hake samples were collected 

 for otolith morphometric-based stock discrimination analysis in 1992 

 (squares), 1994 (closed circles), and 1996 (open circles). GOM-NGB: Gulf 

 of Maine to northern Georges Bank; SGB-MID: southern Georges Bank to 

 the Middle Atlantic. 



Friedland, 1999). Given these benefits and Dery"s (1988) 

 observations, our study aimed to identify whole otolith 

 morphometric characteristics unique to silver hake stocks 

 in U.S. waters of the northwest Atlantic by using image 

 processing techniques. We address the advantages and 

 limitations encountered when using this method to distin- 

 guish fish stocks, and reevaluate the current silver hake 

 stock scheme given the recent within-stock growth differ- 

 ences observed by Helser ( 1996). 



Materials and methods 



Sample collection 



Silver hake were collected in 1992, 1994, and 1996 during 

 the spring Northeast Fisheries Science Center (NEFSC) 

 stratified random bottom-trawl surveys when the fish were 

 assumed to be on their spawning grounds. Silver hake 

 samples were obtained from the Gulf of Maine to northern 

 Georges Bank (GOM-NGB), and from southern Georges 

 Bank to the Middle Atlantic regions (SGB-MID) (Fig. 1, 

 Table 1 ). Silver hake were measured (fork length IFLj, cm). 



