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



461 



otoliths are routinely collected by most fisheries agencies 

 to determine the age of their respective principal fish spe- 

 cies for assessment and management purposes. However, 

 one drawback in using otolith morphometries for stock dis- 

 crimination is that otoliths frequently are broken or lost 

 during routine collection and processing, effectively limit- 

 ing the number of samples available for analysis. 



In these types of studies, it is also essential to consider 

 any confounding variation that may be present owing to 

 differences between samples in age group, year class, or 

 sex, so as to not mistake stock differences for sample dif- 

 ferences. Failure to account for such extraneous influences 

 may result in falsely attributing morphological differences 

 between separate stocks to a "stock effect," whereas the dif- 

 ferences may in fact simply be reflecting variation between 

 samples in age structure, sex ratio, or sampling year (Cas- 

 tonguay et al.. 1991). Significant variation in otolith mor- 

 phometries of silver hake observed in our study between 

 different age gi'oups, year classes, and to a lesser extent sex, 

 emphasized the need to examine these factors before eval- 

 uating the stock status of the species with this technique. 

 Likewise, significant differences in these effects have been 

 detected in other morphometric-based stock identification 

 studies for marine fish species, such as Atlantic salmon 

 (L'Abee-Lund, 1988), Atlantic mackerel (Castonguay et al, 

 1991 1, and Atlantic cod. Gad us morhua ( Campana and Cas- 

 selman, 1993 ). Thus, although it is clearly necessary to cor- 

 rect for these confounding variables, which can influence 

 any stock identification procedure, they do not limit the use 

 of otolith morphometries for stock discrimination provided 

 they are examined and accounted for before making any 

 conclusions about the stock structure of a species. Indeed, 

 calculating linear discriminant functions of otolith mor- 

 phometric characteristics for 1-year-old silver hake on a 

 yearly basis, differentiated by sex, should provide a level 

 of discrimination not yet seen for silver hake stocks in 

 U.S. waters of the northwest Atlantic and may assist in 

 estimating levels of stock mixing (an objective yet to be 

 realized for silver hake or for most of our commercially 

 exploited fish stocks). 



There was clearly a need for a more advanced and efficient 

 method in distinguishing between silver hake stocks or in 

 determining mixing levels between stocks. Assessment and 

 management of silver hake stocks are currently based on 

 stock structure results from the late 1970s and early 1980s, 

 on discriminant analysis of external morphometric charac- 

 ters, coupled with a qualitative analysis of research survey 

 and commercial catch data (Almeida, 1987). Since that time, 

 the status of silver hake has certainly changed — spawning 

 stock biomass and recruitment levels have declined (Helser 

 and Brodziak, 1998) — and stock discrimination technologies 

 through image processing have certainly improved ( Cadrin 

 and Friedland, 1999). We suggest that the use of whole oto- 

 lith morphometries in the analysis of silver hake stocks will 

 give fishery scientists and managers greater logistical ben- 

 efits (in terms of time and expense), not to mention greater 

 discrimination success, than the stock discrimination meth- 

 ods used previously. 



Our study indicated that successful identification of 

 silver hake stocks can be achieved by using whole otolith 



morphometries in combination with image processing and 

 discriminant function analysis. We are confident that this 

 is a relatively inexpensive, objective method which can 

 facilitate routine discrimination of silver hake stocks, 

 as well as other marine fish species, by efficiently obtain- 

 ing accurate measurements to produce valid, repeatable 

 results. This method may prove to be particularly useful 

 for rapid (in-season) assessments for determining levels 

 of stock mixing that are often a requirement of contempo- 

 rary management plans. Discrimination between stocks is 

 important for fisheries stock-rebuilding strategies, such as 

 those for silver hake, because genetic and stock biodiver- 

 sity of the species needs to be maintained to ensure all 

 divisions of a stock contribute to the replenishment of the 

 resource. 



Acknowledgments 



We thank Fred Nichy for his support and guidance; John 

 Boreman, Frank Almeida, Kathy Lang, and two anony- 

 mous reviewers for reviews of this manuscript: Nina Shep- 

 herd for aging silver hake; Jay Burnett for his helpful 

 suggestions; Donald Hoagland and Jim Phillips for their 

 support during the initial phase of the study; and the staff 

 of the Fishery Biology Program for their assistance with 

 the image analysis. This work was performed while G. 

 A. Begg held a National Research Council (NOAA/NMFS/ 

 NEFSC) Research Associateship. 



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