460 



Fishery Bulletin 98(3) 



16 



12 



8 



4 



Age group 1 



I 



-P- 



15 



9 



-1.5 -0,5 0.5 1.5 2.5 3.5 



20 1 Age group 2 



111 



-^-A- 



-2.5 -1.5 -0.5 0.5 15 2 5 



48 



36- 



24 



12 



Age group 3 



\ 



JA 



-4 5 -2 5 -0.5 15 3 5 



Figure 5 



Discrimination of silver hake sam- 

 ples from the northern (solid bars) 

 and southern regions (open bai-s) 

 based on frequency of canonical vari- 

 ate scores (CV Ii for 1-, 2-, and 

 3-year-old fish. 



that a certain degree of reproductive isolation has been con- 

 firmed in spawning and postspawning silver hake ( Almeida, 

 1987). Such isolation could restrict gene flow to a level 

 that effectively isolates population units (lies and Sin- 

 clair, 1982). However, some degree of intermixture prob- 

 ably occurs owing to the wide migratory patterns of silver 

 hake (Almeida, 1987). Therefore, further investigations 

 are required to interpret the degree to which morphologi- 

 cal differences between silver hake stocks are caused by 

 environmental or genetic influences, but as long as these 

 differences persist between stocks, the use of otolith mor- 

 phometries remains a viable tool for stock discrimination. 

 Otolith morphometries in combination with an image 

 analysis system has been found to be a relatively inexpen- 

 sive, objective, and efficient tool by which to distinguish fish 

 stocks ( Messieh et al., 1989; Jearld, 1995 ). Rapid and precise 

 measurements can be obtained in less time with this tech- 

 nique, compared with alternative manual-based procedures, 

 because all the measurements are automatically deter- 

 mined from an outline of the otolith perimeter traced with 

 an edge-detection algorithm (OPTIMAS, 1996). In addition 

 otolith morphometric data can easily be obtained because 



