Ruggerone et al.: Seasonal growth of Oncorhynchus nerka in relation to competition with O. gorbuscha 



367 



their third year at sea, but spring growth was relatively 

 low during the second year. Growth during the second 

 year was relatively high during summer, a pattern that 

 was different from SW1 and SW3 growth. Seasonal 

 scale growth patterns of sockeye salmon indicate that 

 the response of salmon to the 1977 ocean regime shift 

 varied with age and season but that the greater growth 

 during early marine life was associated with greater 

 adult returns. The shift in seasonal growth patterns 

 of sockeye salmon likely reflected their opportunis- 

 tic forging behavior and the changes in prey species 

 abundances caused by climate change (Kaeriyama et 

 al, 2004). 



Greater growth of sockeye salmon when they initially 

 entered the Bering Sea after the 1977 ocean regime 

 shift may reflect differences in seaward migration pat- 

 terns. Prior to the 1977 regime shift, juvenile sockeye 

 salmon were observed in a narrow band that extended 

 from the shore along the Alaska Peninsula to as far as 

 50 km offshore (Straty, 1981; Hartt and Dell, 1986). 

 However, recent survey results indicate that juvenile 

 sockeye salmon are broadly distributed in the eastern 

 Bering Sea from the Alaska Peninsula to north of 58°N 

 and that the highest catch rates occur beyond 50 km 

 offshore (Farley et al. 7 ). Zooplankton are more abundant 

 in offshore, deeper waters of Bristol Bay than within 

 near shore waters (Straty, 1981; Napp et al., 2002), 

 indicating that the recent northerly seaward migration 

 patterns of juvenile sockeye salmon may place them in 

 areas of higher prey densities and lead to higher early 

 marine growth rates. 



Sockeye salmon scale growth during the third year 

 of growth (SW3) was relatively low after 1977, indicat- 

 ing that density-dependent growth was most apparent 

 during this late life stage when mortality is likely rela- 

 tively low (Ruggerone et al., 2002). Our study indicated 

 the reduced SW3 growth after the 1977 regime shift 

 occurred after peak spring growth, indicating that in- 

 terspecific competition was most apparent during sum- 

 mer and fall. During the spring homeward migration 

 (SWPL) period, scale growth was above average after 

 1977. Age-specific size of adult sockeye salmon return- 

 ing to Bristol Bay was density dependent, but size at a 

 given density was greater after 1977 (Rogers and Rug- 

 gerone, 1993; Ruggerone et al., 2003). 



7 Farley, E. V., Jr., R. E. Haight, C. M. Guthrie, and J. E. 

 Pohl. 2000. Eastern Bering Sea (Bristol Bay) coastal 

 research on juvenile salmon, August 2000. Doc. 499, 18 p. 

 North Pacific Anadromous Fish Commission, 889 Pender 

 Street, Vancouver, Canada. 



Farley, E.V., Jr., CM. Guthrie, S. Katakura, and M. 

 Koval. 2001. Eastern Bering Sea (Bristol Bay) coastal 

 research on juvenile salmon, August 2001. Doc. 560, 19 p. 

 NPAFC, 889 Pender Street, Vancouver, Canada. 

 Farley, E.V, Jr., B.W. Wing, A. Middleton, J. Pohl, L. Hulbert, 

 M. Trudel, J. Moss, T. Hamilton, E. Parks, C. Lagoudakis, and 

 D. McCallum. 2002. Eastern Bering Sea (BASIS) Coastal 

 Research (August-2002) on Juvenile Salmon. Doc. 678, 27 

 p. NPAFC, 889 Pender Street, Vancouver, Canada. 



Salmon survival and scale growth 



Biologists have suggested that rapid growth early in 

 life can lead to greater growth in subsequent periods 

 because larger animals have a greater variety of prey 

 and prey size available to them (Pearcy et al., 1999). 

 Aydin (2000) hypothesized that rapidly growing salmon 

 in their first year at sea would more quickly reach a 

 threshold size for feeding on abundant, energy-rich 

 micronekton squid, leading to even greater growth in 

 their second year. However, comparison of early marine 

 scale growth (SW1 through SW2, circuli 8) with late 

 season SW2 growth of individual Kvichak and Egegik 

 sockeye salmon indicated a negative rather than positive 

 relationship. Individual salmon having relatively great 

 early marine scale growth tended to experience reduced 

 scale growth during the later portion of their second year 

 when sockeye salmon reach the size needed to readily 

 consume larger prey such as squid. This finding reflects 

 the growth of sockeye salmon survivors and not those 

 that died at sea. Thus, we interpret this counterintui- 

 tive finding as an indication that slow growing sockeye 

 salmon during late SW2 survived primarily when their 

 early marine growth was relatively high. Salmon that 

 experienced both low early marine growth and low SW2 

 growth apparently did not survive and were not repre- 

 sented in the scale collection. These observations do not 

 necessarily reject the hypothesis that high early marine 

 growth leads to high subsequent growth. In fact, other 

 analyses of sockeye scales indicate spring growth is 

 positively correlated with fall growth within a given 

 year (G. Ruggerone, unpubl. data). 



Effect of freshwater age on seasonal scale growth 



Scale growth during the first year at sea was differ- 

 ent among salmon spending one versus two winters in 

 freshwater. Early SW1 scale growth of sockeye salmon 

 spending two winters in freshwater (age-2.) was sig- 

 nificantly greater than that of salmon spending only 

 one winter in freshwater. This trend might reflect dif- 

 ferences in migration timing or size (or both) of age-2 

 versus age-1 smolts. Age-2 smolts are approximately 

 17 mm longer than age-1 smolts and most age-2 smolts 

 enter marine waters before age-1 smolts (Crawford and 

 West 8 ). After peak growth in spring, scale growth of age- 

 1. smolts exceeded that of age-2. smolts. The different 

 early marine growth patterns of age-1. and age-2. smolts 

 did not appear to significantly affect the size of the fish 

 at the end of the growing season. For example, during 

 1958-72, age-2. 1 sockeye salmon sampled immediately 

 south of the Aleutian Islands were 25 mm longer than 

 age-1. 1 sockeye salmon (French et al., 1976). The size 

 difference between age-2. and age-1. smolts declined to 

 8 mm during the second growing season. 



; Crawford, D. L., and F. W. West. 2001. Bristol Bay sock- 

 eye salmon smolt studies for 2000. Reg. Info. Rept. 2A01- 

 12, 164 p. Alaska Dept. Fish Game, 333 Raspberry Road, 

 Anchorage, AK. 



