Friedland et al.: Post-smolt growth, maturation, and survival of two stocks of Atlantic salmon 



655 



plasticity (Via and Lande, 1985; Saunders, 1986; 

 Stearns, 1992; Scheiner 1993; Trippel, 1995). Growth 

 and environment during the post-smolt period play 

 an important role in influencing the sea-age at matu- 

 ration of salmon (Scarnecchia, 1983; Martin and 

 Mitchell, 1985; Neilson and Geen, 1986; L'Abee-Lund, 

 1989; Scarnecchia et al., 1989; Skilbrei, 1989). 

 Friedland and Haas ( 1996) found that summer post- 

 smolt growth was positively correlated with matu- 

 ration, suggesting that physical readiness to mature 

 may be achieved during the summer period. Exam- 

 ining post-smolt growth in stocks with differing 

 maturation rates should provide new insights into 

 the mechanisms controlling salmon maturation. 



In this study, we compare the marine survival and 

 age at maturity of two hatchery dependent stocks of 

 Atlantic salmon. We also describe and analyze cir- 

 culi spacing and other length measurements from 

 the scales of two seawinter returns to characterize 

 growth during the post-smolt phase and to examine 

 the role of post-smolt growth in affecting survival 

 and maturation. 



Materials and methods 



Figure 1 



Map of the northwest Atlantic Ocean with detail showing loca- 

 tion of the Penobscot and Connecticut rivers. 



Return rate and cohort age at maturity 



Return rate by individual sea-age and cohort sea- 

 age at maturity were calculated for two Atlantic 

 salmon stocks, the hatchery components of the 

 salmon runs in the Penobscot and Connecticut riv- 

 ers in the United States. These rivers are situated 

 at the southern end of the range of Atlantic salmon 

 in North America (Fig. 1 ). Salmon from these rivers 

 migrate to feeding areas as distant as the Labrador 

 Sea. We analyzed data from 1977 to 1990, a period 

 during which smolt releases in the Penobscot River 

 ranged from 200,000 to 687,000 fish per year and in 

 the Connecticut River from 32,000 to 476,000 fish 

 per year (Table 1 ). Early in the time series, releases 

 in both rivers were mostly two-year-old smolts, 

 whereas in recent years, most of the releases have 

 been yearlings. Freshwater returns of both 1SW (one 

 seawinter) and 2SW salmon were ascertained from 

 fishway trap counts and from recreational catches 

 in the Penobscot River, as well as from trap counts 

 in the Connecticut River (no legal Atlantic salmon 

 fishery exists for this river). Return rates were cal- 

 culated as simple ratios, expressed as returns per 

 1,000 smolts released. 



The fraction of a cohort or smolt year class that 

 matured after only a single winter at sea was de- 

 rived by using both counts of adult returns by sea- 

 age and fishing mortality that impacted the cohort. 



Fish from the Penobscot and Connecticut rivers were 

 exploited in marine fisheries in Canada and 

 Greenland during the study period. To account for 

 this, we applied annual instantaneous fishing mor- 

 tality rates (F in Table 1) for North American fish 

 that were maturing as 2SW salmon (Friedland and 

 Haas, 1996). 



The observed return of 1SW salmon to freshwater 

 (fl ; )is the product of the maturation fraction^) and 

 the size of the cohort immediately before the 1SW 

 fish return to homewaters and the 1SW fisheries have 

 commenced (A/ Q ). Because we were modeling the co- 

 hort split immediately before the 1SW runs occur, 

 natural mortality for this fraction of the cohort (M ; ) 

 was zero. 



i?, = N *¥*exp 



1) 



The observed return of 2SW salmon (i? 9 ) is assumed 

 to be the remainder of the cohort that did not ma- 

 ture as 1SW fish and that survived for an additional 

 10 months at sea, plus the mortality associated with 

 fishing (M 2 =0.1 is from Friedland and Haas, 1996, 

 and F is given in Table 1 ). The additional time 2SW 

 salmon stay at sea is assumed to be 10 months, as 

 opposed to one year, because 2SW salmon usually 

 return to the river earlier in the year than 1SW fish. 



