210 



R. R. STRATY AND R. E. HAIGHT 



this species in the Bering Sea, but because of 

 its size, it is an important forage fish for many 

 commercial fish species. 



The five species of Pacific salmon of the 

 eastern Bering Sea spawn in fresh water, un- 

 like herring, capelin, and sand lance. Their 

 eggs are not vulnerable to extensive predation 

 by marine birds; gulls take mainly salmon 

 eggs which have been dislodged from the 

 gravel and are drifting or being rolled along 

 the stream bottom by the current (Moyle 

 1966). After a few months to several years in 

 fresh water, the juvenile salmon (5-14 cm 

 long) enter the Bering Sea during late spring 

 or early summer and migrate through these 

 waters to feeding grounds, primarily in the 

 north Pacific Ocean. At maturity, the sur- 

 vivors return to their home streams and 

 rivers to spawn. It is during the seaward mi- 

 gratory phase of their life cycle that salmon 

 are most vulnerable to predation by marine 

 birds. 



The sockeye salmon (Oncorhynchus nerka) 

 is the most abundant and valuable species 

 harvested by American fishermen in the 

 waters adjacent to the Bering Sea and, as a re- 

 sult, the one that has been most extensively 

 studied during early marine life. Juvenile 

 sockeye salmon are between 8 and 14 cm long 

 when they enter the Bering Sea between late 

 May and early July. They are most abundant 

 in the upper 1 m of water at night and the 

 upper 2 m during the day (Straty 1974) well 

 within the regime that can be exploited by 

 many species of marine birds. 



The numbers of juvenile sockeye salmon mi- 

 grating seaward from the Bristol Bay region 

 of the Bering Sea in a single year has ranged 

 between 46.3 and 370.4 million (H. Jaenicke, 

 personal communication). This is equivalent 

 to between 409 and 3,267 metric tons (on the 

 basis of the mean weight of the juveniles when 

 they enter the Bering Sea). These large num- 

 bers of juvenile sockeye salmon, plus juvenile 

 chinook salmon (O. tshawytscha), coho sal- 

 mon (O. kisutch), chum salmon (O. keta), and 

 pink salmon (O. gorbuscha) from all other 

 rivers entering the Bering Sea, represent a 

 considerable input of energy from fresh water 

 in the form of prime forage fish for other 

 fishes, marine birds, and mammals. Young 

 salmon enter the Bering Sea each year over a 

 period of only 6 to 8 weeks and may follow 



Fig. 6. Distribution of juvenile sockeye salmon in 

 Bristol Bay and the eastern Bering Sea (adapted 

 from Straty 1974). 



rather discrete coastal migration routes 

 through the Bering Sea (Fig. 6), with the re- 

 sult that predators have access to an abun- 

 dant but transient food supply. 



The only published account of predation by 

 marine birds on juvenile salmon in the Bering 

 Sea is that of Ogi and Tsujita (1973). They 

 found juvenile sockeye salmon in the 

 stomachs of murres captured in gill nets in 

 the eastern Bering Sea. The predation did not 

 appear extensive, but most of the birds were 

 captured outside or on the fringes of the main 

 seaward migration route of the salmon. The 

 foods of marine birds should be studied in con- 

 junction with studies of the migrations of 

 juvenile salmon. 



Influence of Growth Rate 



and Adult Size of Fish on 



the Extent of Predation 



Incubation time for fish eggs, the length of 

 the pelagic larval period (Table 3), and the 

 growth rate of juvenile fish are species- 

 specific and temperature-dependent. The ex- 

 tent to which a fish species is subjected to pre- 

 dation by marine birds is directly related to 

 the rate at which development and growth 

 occur. For example, the less time it takes the 

 pelagic eggs of demersal fish and shellfish to 

 hatch and complete pelagic larval life, the less 

 is the time they will be preyed on by marine 

 birds. For fish species that are pelagic during 

 their entire life, the rate of growth will deter- 



