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



365 



and Ludwig (1966) reported that sockeye salmon in the 

 Gulf of Alaska tended to form annuli during December 

 and January, whereas salmon sampled farther west in 

 the relatively cold waters below the Aleutian Islands 

 appeared to form annuli during March (Birman, 1960). 

 For example, sockeye salmon collected from the east- 

 ern range of Bristol Bay sockeye salmon in the Gulf of 

 Alaska (e.g., 152-160°W) averaged 1.2 circuli beyond 

 the winter annulus during January and 3.6 circuli in 

 April. We observed peak circuli growth of Kvichak and 

 Egegik sockeye salmon to occur near circuli 5 to 6 (all 

 ages), indicating that peak scale growth occurred from 

 approximately early May to mid-June. This finding is 

 consistent with scale growth in the year of homeward 

 migration when Bristol Bay sockeye salmon averaged 

 approximately 1 to 2 circuli after peak circuli growth 

 before reaching Bristol Bay. on average, during the first 

 week in July. The estimated date of peak scale growth 

 is also consistent with observations of peak biomass 

 of zooplankton in the Gulf of Alaska and Bering Sea, 

 which typically occurs during May or June (Brodeur et 

 al., 1996; Coyle et al., 1996; Mackas et al., 1998; Mackas 

 and Tsuda, 1999). However, Ishida et al. (1998) reported 

 that salmon growth was greatest between June and July, 

 a period apparently later than peak scale growth and 

 peak zooplankton biomass. Furthermore, scale growth 

 may lag behind body growth (Bilton, 1975). Based on 

 these observations, the observed divergence in scale 

 growth between odd- and even-numbered years likely 

 began after zooplankton biomass declined and during a 

 period of high potential body growth of salmon. 



Differences in SW2 scale growth between odd- and 

 even-numbered years at sea began three to five circuli 

 after peak growth, rather than immediately after the 

 peak as shown among fish during their third year at 

 sea (SW3). Because younger salmon begin circuli for- 

 mation earlier in winter than do older salmon (Bilton 

 and Ludwig, 1966; Martinson and Helle, 2000), it is 

 likely that the differences in time of SW2 scale growth 

 was only slightly later than that scale growth during 

 SW3. The reason for the somewhat later differences 

 between odd and even years of younger sockeye salmon 

 might relate to the degree of diet overlap with pink 

 salmon. In the central North Pacific Ocean and Ber- 

 ing Sea, pink salmon in their second growing season 

 have greater diet overlap with larger sockeye salmon 

 (Davis, 2003), such as sockeye salmon in their third 

 season at sea. Thus, competition for prey may be great- 

 est between pink salmon and the larger, older sockeye 

 salmon, leading to earlier growth differences between 

 the SW3 than the SW2 growth period. Alternatively, 

 this pattern may reflect differences in the distribution 

 of age-2 and age-3 sockeye salmon: age-3 salmon maybe 

 distributed farther west where overlap with Asian pink 

 salmon is greater. 



that affect the degree of competition. Little or no overlap 

 occurs between these stocks during the first growing 

 season (SW1) and there are typically small numbers 

 of pink salmon originating from Bristol Bay (Rogers 1 ). 

 Little sampling has occurred during winter (Myers 6 ), 

 but data collected during fall and spring indicate that 

 some overlap between Asian pink salmon and Bristol 

 Bay sockeye begins in the central North Pacific Ocean 

 during winter (French et al., 1976; Takagi et al., 1981; 

 Myers et al. 3 ). The degree of overlap likely increases 

 into spring when both species reach their southernmost 

 distribution, which is somewhat farther south for pink 

 salmon. As the temperature begins to increase, both 

 species migrate northwest — pink salmon leading the 

 migration. Both species enter the Bering Sea but many 

 Bristol Bay salmon and some Asian pink salmon remain 

 in the North Pacific Ocean. In June, some Asian pink 

 salmon leave the high seas for coastal areas, whereas 

 others remain offshore through July (Myers et al. 3 ; Azu- 

 maya and Ishida, 2000). During odd-numbered years, 

 pink salmon are more broadly distributed on the high 

 seas and catch per effort in the Bering Sea remains high 

 through at least mid-July (up to 400 fish per 30 tans 

 (1.5 km) of gill net) compared with that during even- 

 numbered years (Azumaya and Ishida, 2000). Catch 

 per effort of pink salmon during July is somewhat lower 

 in the central North Pacific Ocean. Most pink salmon 

 in the Bering Sea likely originate from the eastern 

 Kamchatka Peninsula, which supports a major Asian 

 population that is dominated by odd-year pink salmon. 

 Thus, the period of overlap between Asian pink salmon 

 and Bristol Bay sockeye salmon is from approximately 

 winter through July and greatest overlap likely occurs 

 during late spring through at least mid-July. 



The relatively slow growth of sockeye salmon scales 

 during odd-numbered years at sea began in the period 

 of overlap with pink salmon and continued for months 

 after pink salmon left the high seas. This finding in- 

 dicates that prey availability was reduced for months 

 after most pink salmon left the high seas. Sugimoto and 

 Tadokoro (1997) examined zooplankton biomass dur- 

 ing June and July, 1950-81 and concluded that Asian 

 pink salmon caused the observed alternating pattern of 

 zooplankton biomass in the central North Pacific Ocean 

 and the eastern Bering Sea. Shiomoto et al. (1997) ex- 

 amined macrozooplankton biomass in the central North 

 Pacific Ocean during 1985-94 and also concluded that 

 Asian pink salmon, especially those from the eastern 

 Kamchatka Peninsula, reduced the biomass of macro- 

 zooplankton. Shiomoto et al. (1997) noted that lower 

 zooplankton biomass was still apparent in the central 

 North Pacific Ocean after many pink salmon had mi- 

 grated into the Bering Sea. These findings support the 

 hypothesis that predation by pink salmon altered zoo- 

 plankton biomass from spring through at least July. 



Interactions with pink salmon and prey 



Spatial and temporal overlap between Asian pink salmon 

 and Bristol Bay sockeye salmon are important factors 



6 Myers, K. 1996. Survey on overwintering salmonids in 

 the North Pacific Ocean: Kaiyo Maru, 5 January-29 Janu- 

 ary 1996. Report FRI-U W-9607, 54 p. Univ. Washington, 

 Seattle, WA. 



