We do not know the importance of competition between marine birds and commercial fish in the eastern Bering Sea. Only a few investigators have even alluded to competition between marine birds and fish for food. Ogi and Tsujita (1973) mentioned that competition seemed to exist between murres and juvenile 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; comparison of these two tables clearly indicates that competition could occur.

The principal factors determining the extent 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 depends 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 barnacles. 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 significant 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 pollock as these fish increase in size and seek deeper waters. The diet of cod changes from small crustaceans in shallow water to progressively 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.

Table 4. Food items eaten by the adult stage of seven commercially important species of fish in the eastern Bering Sea.
Food itemHerringSalmonWalleye pollockPacific codPacific ocean perchYellowfin solePacific halibut
Invertebrates
PteropodsXX——X——
Squid—X—XX—X
PolychaetesXXXX—XX
CopepodsXXX————
AmphipodsXXXXXX—
EuphausiidsXXX—XX—
DecapodsXXXXXXX
Fish
CapelinXXXX—X—
Sand lance—XXX——X
Table 5. Forage fish and invertebrate foods eaten by seven species of marine birds in the eastern Bering Sea.
Food itemShearwatersMurresPuffinsMurreletsFulmarsKittiwakesGulls
Forage fish
Sand lanceXXX——XX
Capelin——X————
Invertebrates
Copepods—————X—
EuphausiidsXX———X—
AmphipodsXX———X—
DecapodsXX———X—
Pteropods—X—————
Chaetognaths———————
Polychaetes—XX——X—
SquidXX——X——

As pollock increase in size, they continue to feed mainly on zooplankton, but they change from copepods near the surface to euphausiids at mid-depths and near the bottom. Euphausiids are large and abundant zooplankters which, for the most part, are available only to deep-diving birds. Adult pollock also consume herring, sand lance, capelin, and other small fish.

Both marine birds and fish are capable of exploiting a wide variety of food, and often their stomach contents reflect the relative abundance of food items in the area. Ogi and Tsujita (1973) illustrated the differences in the food taken by murres captured at different locations in the eastern Bering Sea. Carlson (1977) and Ogi and Tsujita (1973) reported on differences in the diet of juvenile sockeye salmon captured at various locations in Bristol Bay and the eastern Bering Sea. The diets of many species of birds and fish, however, seem to be largely determined by their physiological and morphological adaptations and resultant feeding behavior. For instance, adult sockeye and pink salmon have well-developed gill rakers and feed largely on zooplankton, whereas chinook and coho salmon have poorly developed gill rakers and feed almost entirely on fish. In the eastern Bering Sea, murres appear to prefer the Pacific sand lance, whereas the slender-billed shearwater consumes mainly euphausiids (Ogi and Tsujita 1973). Thus, murres may be greater competitors with piscivorous fish than are shearwaters. Shearwaters are probably more important as competitors with zooplankton-eating fish that inhabit shallow water in juvenile stages and with pelagic fish species (such as pollock, herring, salmon, and capelin) that are heavily dependent on euphausiids.

Some species of marine birds may interact with fish as predators and competitors. As an example, pursuit diving birds, such as murres and puffins, may be important predators on juvenile salmon in the eastern Bering Sea, but these same birds may compete for food with adult salmon. Surface-feeding birds, such as fulmars, shearwaters, kittiwakes, and gulls, may be important as both predators and competitors with herring and capelin and some demersal fish.