654 



AbStr3Ct.— Marine survival and 

 sea-age at maturity of two hatchery-de- 

 pendent stocks of Atlantic salmon were 

 compared in respect to differences in 

 post-smolt growth as evidenced by the 

 circuli spacing patterns of their scales. 

 The two stocks, the Penobscot and Con- 

 necticut, are located at the southern 

 extent of the range of Atlantic salmon 

 in North America. Return rates for 1SW 

 (one seawinter) and 2SW salmon and 

 the fraction of the smolt year class or 

 cohort that matured as 1SW fish were 

 found to be significantly higher for the 

 Penobscot stock. Using image process- 

 ing techniques, we extracted intercir- 

 culi distances from scales of 2,302 2SW 

 fish. Circuli spacing data were ex- 

 pressed as growth indices for spring 

 (when post-smolts first enter the 

 ocean), summer (when growth appears 

 maximal), and winter (when growth 

 appears to be at a minimum). Circuli 

 spacings of the Penobscot fish were 

 wider during the summer season than 

 were those for conspecifics from the 

 Connecticut River of the same smolt 

 year class. The results suggest that 

 post-smolt growth may play a signifi- 

 cant role in deciding age at maturity 

 and survival patterns for Atlantic 

 salmon stocks. 



Post-smolt growth, maturation, 

 and survival of two stocks 

 of Atlantic salmon 



Kevin D. Friedland 

 Ruth E. Haas 

 Tim F. Sheehan 



National Marine Fisheries Service, NOAA 

 1 66 Water Street 



Woods Hole, Massachusetts, 02543 

 e-mail address kfnedla@whsun 1 wh.whoi.edu 



Manuscript accepted 14 May 1996 

 Fishery Bulletin 94:654-663 (1996) 



Variation in the marine survival 

 and sea-age of maturation of indi- 

 vidual Atlantic salmon stocks, 

 Salmo salar L., has ramifications 

 for the management of these stocks 

 in both freshwater and marine en- 

 vironments. Mixed-stock fisheries 

 for salmon indiscriminately harvest 

 stocks of varying productivity, often 

 leading to the over harvesting of 

 weaker stocks even when exploita- 

 tion is at a level considered safe for 

 the stock complex (Ricker, 1958). 

 Homewater or terminal fisheries 

 can also over harvest weak stocks 

 when shifts in age at maturity and 

 survival occur (Minard and Mea- 

 cham, 1987). Understanding these 

 periods of changing stock dynamics 

 will help to ensure that manage- 

 ment measures are taken to protect 

 the stocks and enhance the fishery. 

 Events during the post-smolt year 

 are critical to the survival and matu- 

 ration of Atlantic salmon (Mills, 

 1989; Salminen et al., 1995); how- 

 ever, the factors shaping salmon 

 population abundances during this 

 period are poorly known (Friedland 

 et al., 1993). Many sources of mor- 

 tality, such as predation, disease, 

 and parasitism, affect young salmon 

 during their first year at sea. The 

 effect of these sources of mortality 

 is to some extent influenced by the 

 size and condition of the post- 



smolts; thus, their effect would be 

 expected to vary between years and 

 among stocks (Mathews and Ishida, 

 1989; Holtby et al. 1990; Salminen 

 etal., 1995). For example, predation 

 during the post-smolt year may de- 

 termine overall survivorship in 

 salmon, but it is not known how 

 much of the predation on young 

 salmon is controlled by predator 

 abundance versus the size and 

 growth of the post-smolts them- 

 selves (Hislop and Shelton, 1993; 

 Hargreaves, 1994). Predation theory 

 suggests that the probability of sur- 

 vival for an individual increases 

 considerably as its body size exceeds 

 sizes vulnerable to potential preda- 

 tors (Werner and Hall, 1974; Mik- 

 heev, 1984; L'Abee-Lund et al., 

 1993). If juveniles remain in a de- 

 velopmental stage that makes them 

 vulnerable to mortality, cumulative 

 mortality will increase (Cushing, 

 1975). If a stock experiences de- 

 pressed post-smolt growth, it may 

 be susceptible to high mortality for 

 a longer time compared with other 

 stocks exploiting the same feeding 

 area. Likewise, similar growth-me- 

 diated mechanisms can be envi- 

 sioned for other sources of mortal- 

 ity such as diseases and parasitism. 

 The onset of sexual maturation in 

 fishes is an inherited trait that op- 

 erates within a range of phenotypic 



