INTERACTIONS AMONG MARINE BIRDS AND COMMERCIAL FISH 



211 



mine how long they remain small enough for 

 birds to eat. Some of the smaller pelagic fish, 

 such as herring, capelin, and smelt, are vulner- 

 able to bird predation most of their lives; 

 larger pelagic species like salmon may be 

 preyed on for only a very short time. The 

 maximum size fish that can be eaten by 

 marine birds is, therefore, important in 

 evaluating predation on a given species of 

 fish. 



The literature on the food habits of marine 

 birds contains little on the sizes of fish con- 

 sumed. Tuck (1960) stated that murres prob- 

 ably will take fish up to 18 cm long. Ogi and 

 Tsujita (1973) estimated the lengths of Pacific 

 pollock in the stomachs of murres taken in the 

 eastern Bering Sea at 24 cm. 



Herring in the eastern Bering Sea reach an 

 age of 1 1 years and grow to about 33 cm. 

 Herring could, therefore, be taken during 

 most of their lives by murres but during only 

 the first few years by smaller birds such as 

 fulmars and shearwaters. Capelin and some 

 species of smelt would be vulnerable to birds 

 during all their lives. Although the size of 

 adult Pacific salmon varies with the species, 

 they are all so large that they are not preyed 

 upon by marine birds. Once in the ocean, juve- 

 nile salmon grow at such a rapid rate that 

 they are probably not very vulnerable to 

 marine birds after their first 4 to 6 months at 

 sea. Limited studies on the growth of juvenile 

 sockeye salmon in the eastern Bering Sea 

 (Straty 1974) indicate they may double their 

 size in their first 8 weeks at sea. A sockeye sal- 

 mon that entered the Bering Sea at 12 cm in 

 mid-June would be 24 cm long in August the 

 maximum size that a murre could eat; the fish 

 could be eaten by smaller marine birds for 

 much less time. Pink and chum salmon enter 

 the sea at a smaller size than sockeye salmon 

 and would be vulnerable to predation both by 

 a greater variety of marine birds and for a 

 longer period of time. 



Competition Between 



Commercial Fish and 



Marine Birds 



We do not know the importance of competi- 

 tion between marine birds and commercial 

 fish in the eastern Bering Sea. Only a few in- 



vestigators have even alluded to competition 

 between marine birds and fish for food. Ogi 

 and Tsujita (1973) mentioned that competi- 

 tion seemed to exist between murres and juve- 

 nile sockeye salmon for euphausiids in the 

 eastern Bering Sea. We have listed some of 

 the types of forage fish and invertebrates 

 eaten by commercial fish (Table 4) and marine 

 birds (Table 5) in the eastern Bering Sea; com- 

 parison of these two tables clearly indicates 

 that competition could occur. 



The principal factors determining the ex- 

 tent of competition between marine birds and 

 fish are the numbers of birds and fish, the 

 length of time that various life history stages 

 of the fish are in association with the birds, 

 and the abundance of the preferred foods at 

 these times. The impact of competition de- 

 pends on the adaptability of the birds and fish 

 to alternative types of food. 



The types and sizes of food eaten by fish 

 vary with the life history stage especially 

 with size at each stage. For instance, very 

 young herring eat the eggs and nauplii of 

 copepods or small copepodite stages and bar- 

 nacles. As herring grow, their diet includes 

 small fish and larger zooplankton, such as 

 mature copepods, amphipods, euphausiids, 

 and pteropods. Pacific cod shorter than 9 cm 

 feed on small crustaceans (Moiseev 1953), 

 whereas larger cod eat young crabs, shrimp, 

 and fish. Small juvenile sockeye salmon feed 

 mainly on larval stages of euphausiids (Straty 

 1974), but larger juveniles also eat the more 

 adult forms, which eventually make up a sig- 

 nificant part of their diet (Nishiyama 1974). 



The change in the diet of fishes with growth 

 results in competition with a changing variety 

 of marine birds. For example, deep-diving 

 birds may replace surface feeders as the major 

 bird competitors of the Pacific cod and pol- 

 lock as these fish increase in size and seek 

 deeper waters. The diet of cod changes from 

 small crustaceans in shallow water to progres- 

 sively larger food that eventually includes 

 herring, sand lance, shrimp, and crabs. The 

 change to herring and sand lance, and quite 

 possibly small crabs, places the adult cod in 

 competition with both the surface feeders and 

 pursuit diving birds, but adult cod do not 

 compete with birds for zooplankton. 



As pollock increase in size, they continue to 

 feed mainly on zooplankton, but they change 



