Friedland et al.: Growth patterns in postsmolts of Salmo salar 



477 



duced many early maturing fish; and the Penobscot 

 stock was intermediate between the other two. 



Discussion 



Our analysis suggests that the role of the Gulf of St. 

 Lawrence as salmon postsmolt nursery habitat var- 

 ies annually. In some years it appears that the growth 

 of wild postsmolts retained in the Gulf is as robust 

 as and patterned similarly to growth obsei^ved for 

 hatchery-origin postsmolts assumed to use open 

 ocean habitats (Reddin and Short, 1991). This corre- 

 lation suggests that either postsmolts from other 

 areas invade the Gulf and use it as a nursery area or 

 the Gulf region is continuous with a larger area of 

 similar growth conditions where the nursery is 

 formed. In other years, it appears that only smaller, 

 and presumbably less robust, postsmolts remain in 

 the Gulf area and that the nursery was formed else- 

 where. Variation in the suitability of nursery habi- 

 tat must interact with the ability of postsmolts to 

 migrate successfully to more favorable areas. In what 

 was first investigated as a mechanism controlling 

 the return migration of adults. Groot and Cooke 

 ( 1987 ) described an analogous situation with the dis- 



tribution of juvenile sockeye salmon in the Strait of 

 Georgia. It would appear that the dominant wind 

 patterns in the Strait can alter the annual migra- 

 tion of postsmolt sockeye salmon and place them in 

 different nursery areas each year (Peterman et al., 

 1994). Atlantic salmon postsmolt migration trajec- 

 tories may be similarly affected. In some years, 

 postsmolts are deposited in areas like the Gulf of St. 

 Lawrence and growth conditions are favorable 

 enough to retain them in the region for the entire 

 season. In other years they are not deposited or re- 

 tained in the same area. Regardless, our focus shifts 

 to the factors controlling migration and whether they 

 covary with the factors controlling sui-vival. 



We view nursery habitat for postsmolt salmon as 

 being dynamically defined because it shifts spatial 

 location on an annual basis to regions where the pro- 

 duction will support growth. Many marine fishes use 

 staged distribution separations between estuarine, 

 coastal, and offshore habitats (Blabber et al., 1995). 

 North American origin salmon are generally concen- 

 trated in the Labrador Sea as feeding adults or on 

 various migration routes back to their natal rivers 

 as maturing fish (Reddin and Shearer, 1987). How- 

 ever, postsmolt distributions are regulated in part 

 by passive displacement mechanisms and the swim- 



