Juvenile Sockeye Salmon Distribution, Size, Condition, and Diet During Years with Warm and Cool Spring Sea Temperatures Along the Eastern Bering Sea Shelf
Interannual variations in distribution, size, indices of feeding, and condition of juvenile Bristol Bay sockeye salmon (Oncorhynchus nerka) collected in August - September (2000-03) during Bering-Aleutian Salmon International Surveys were examined to test possible mechanisms influencing their early marine growth and survival. Juvenile sockeye salmon were mainly distributed within the southern region of the eastern Bering Sea, south of 57°0'N during 2000 and 2001 and farther offshore, south of 58°0'N during 2002 and 2003. In general, juvenile sockeye salmon were significantly larger (P < 0.05) and had significantly higher indices of condition (P < 0.05) during 2002 and 2003 than during 2000 and 2001. The feeding index was generally higher for age-1.0 sockeye salmon than age-2.0 during all years.
Among-year comparisons suggested that Pacific sand lance (Ammodytes hexapterus) were important components of the juvenile sockeye salmon diet during 2000 and 2001 (20% to 50% of the mean wet mass) and age-0 walleye pollock (Theragra chalcogramma) were important components during 2002 and 2003 (50%-60% of the mean wet mass). Warmer sea temperatures during spring and summer 2002 and 2003 likely increased productivity on the eastern Bering Sea shelf, enhancing juvenile sockeye salmon growth.
By Ed Farley
A Review of the Critical Size, Critical Period Hypothesis for Juvenile Pacific Salmon
Pacific salmon experience relatively high mortality rates during the first few months at sea, and it is believed that the high mortality rates may be partly related to size. Size-dependent marine mortality of juvenile salmon may be concentrated during two specific early marine life-history stages. The first stage may occur just after juvenile salmon enter the marine environment, where smaller individuals are believed to experience higher size-selective predation. The second stage is thought to occur following the first summer at sea, when smaller individuals may not have sufficient energy reserves to survive late fall and winter. Thus, larger individuals within a cohort likely have higher probability of survival, emphasizing the importance of size during the first summer at sea.
We consider size of juvenile Pacific salmon after the first summer at sea to be the trait on which size-selective mortality operates. The idea is based on the critical size, critical period hypothesis, where those individuals within a cohort that do not reach a critical size during their first summer at sea have higher rates of late fall and overwinter mortality. The results suggest that early marine growth of juvenile Bristol Bay sockeye (O. nerka), Prince William Sound hatchery pink (O. gorbuscha), and British Columbia coho (O. kisutch) salmon from geographically distinct regions (Bering Sea, northern Gulf of Alaska, coastal British Columbia, respectively) is important and that these salmon must attain sufficient growth and size during their first summer at sea to survive the first winter and subsequent years at sea.
A critical size, critical period relationship to marine survival is a reflection of the carrying capacity of an ecosystem. A recognition that insufficient growth in the first marine spring and summer probably will result in death during the winter is also recognition that there is a matching of numbers of juveniles entering the ocean with the prey that is immediately available to juvenile salmon. Natural regulation in the absence of fishing or hatcheries would result in reduced adult returns in periods of reduced prey production and large returns in periods of favorable ocean environmental conditions. In managed populations, it should be possible to use early marine growth to optimize the production of smolts entering the ocean and to forecast marine survival. Producing too many smolts during years with low ocean productivity simply results in salmon dying in the first marine winter either directly from starvation or indirectly by being easy prey.
