368 



Fishery Bulletin 103(2) 



Difference in growth by ocean age 



Barber and Walker (1988) reported that peak SW2 scale 

 growth for Bristol Bay sockeye salmon (Ugashik stock) 

 was less than peak growth during SW1 and SW3. They 

 suggested that this trend reflected lower prey availability 

 for sockeye salmon in the North Pacific Ocean than in the 

 Bering Sea (Mackas and Tsuda, 1999). But Bristol Bay 

 sockeye salmon also develop in the Bering Sea during 

 their second growing season (French et al., 1976; Myers 

 et al. 3 ). Kvichak and Egegik sockeye salmon scales, 

 1955-2000, exhibited relatively low growth throughout 

 SW2 year compared to SW1 and SW3 years. We suggest 

 that low SW2 growth may also be related to the inabil- 

 ity of sockeye salmon to efficiently capture large prey 

 (Aydin, 2000) and to a lower bioenergetic efficiency when 

 consuming smaller prey. Salmon in their third year at 

 sea may experience greater prey availability and capture 

 efficiency because they are larger. 



Late season growth of ocean age-2 sockeye salmon 

 during SW2 was significantly greater than that of ocean 

 age-3 sockeye salmon. This finding indicates that the 

 greater size-at-age of ocean age-2 sockeye salmon com- 

 pared to ocean age-3 sockeye salmon at the end of the 

 second growing season (French et al., 1976) may be 

 largely related to increased growth during the later 

 portion of the second growing season at sea. 



Conclusions 



Seasonal scale growth patterns of Kvichak and Egegik 

 sockeye salmon exhibited significant differences in SW2 

 and SW3 scale growth during odd- versus even-num- 

 bered years. Differences in scale growth did not begin 

 until after peak scale growth and difference began 

 somewhat later for younger SW2 sockeye salmon. The 

 persistence of this pattern over the past 45 years may 

 be caused by pink salmon, especially those from eastern 

 Kamchatka that are highly abundant during odd-num- 

 bered years. During odd-numbered years, pink salmon 

 reduced prey abundance prior to migrating to coastal 

 areas in June and July (Shiomoto et al., 1997; Sugimoto 

 and Tadokoro, 1997). This prey reduction, coupled with 

 declining abundance and ontogenetic vertical migra- 

 tions of some zooplankton (Mackas and Tsuda, 1999), 

 appears to have influenced (reduced) growth of sockeye 

 salmon from early summer through fall of odd-numbered 

 years. We hypothesize that the alternating odd- and 

 even-year growth pattern of sockeye salmon may be re- 

 enforced by the one- or two-year life cycle of prey, such 

 as squid, whose abundance may be out-of-phase with 

 the two-year cycle of pink salmon. These data, coupled 

 with previous findings of reduced smolt-to-adult sur- 

 vival of sockeye salmon that interacted with odd-year 

 pink salmon during the second year at sea (Ruggerone 

 et al., 2003), indicate that reduced growth of salmon 

 during the second year at sea can lead to measurable 

 salmon mortality. Sockeye mortality associated with 

 pink salmon likely occurs during winter when demand 



for prey by salmon exceeds the low availability of prey 

 (Aydin, 2000), but it may also occur in response to size- 

 selective predation. Our study indicates that salmon 

 growth and survival are influenced by complex food web 

 interactions, which are likely to significantly shift under 

 various scenarios of climate change that affect tempera- 

 ture, C0 2 , and phytoplankton community structure of 

 the Bering Sea (Hare et al. 9 ). 



Acknowledgments 



We appreciate the efforts of biologists and technicians 

 of the Alaska Department of Fish and Game who col- 

 lected salmon scales and associated data, and B. Agler 

 and D. Oxman who helped compile the data. S. Good- 

 man assisted with graphics. The manuscript benefited 

 from comments provided by N. Davis, G. Duker, and two 

 anonymous reviewers. This study was funded by the 

 Global Change Program, Biological Resources Division, 

 U.S. Geological Survey. 



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