Chapter XV: Part V: , Birds. U.S. Army, Signal Corps, Washington, D.C. Pages (9)

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.

Dependency of Marine Birds on Commercial Fish

The interactions of commercial fish and marine birds of the Bering Sea can be determined only if we know their distribution, abundance, and food habits, especially while they are associated with one another. Information is particularly lacking for all life history stages of commercial fish species and the seasonal movements of birds. We have some knowledge of the distribution and abundance of the various life history stages and the food habits of commercial fish in the Bering Sea. Little is known of the abundance, seasonal movements, and food habits of marine birds in this region, however, probably because marine birds have had little direct commercial value in the northern hemisphere. Food studies on marine birds are particularly difficult because their rapid digestion soon destroys the identity of the food.

We can make a reasonable guess as to some bird-fish associations for two regions of the Bering Sea where we have information on the distribution of marine birds and the various life history stages of commercial fish. For example, piscivorous birds, such as murres, puffins, black-legged kittiwakes, and slender-billed shearwaters, are extremely abundant in the summer along the seaward migration route of juvenile sockeye salmon (Fig. 7); the juvenile salmon, kittiwakes, and shearwaters all feed on plankton. Shuntov (1961) showed that kittiwakes are most abundant along the edge of the continental shelf in the Bering Sea in the summertime. This distribution coincides with the distribution of the eggs and larvae of pollock, certain flatfish, rockfish, sablefish, and several other species. These birds both exploit the fish directly (predation) and compete with them for plankton. Not enough information is available on the food habits of birds at the time fish eggs and larvae are present to evaluate this interaction.

Environmental Influence on Predation and Competition Between Marine Birds and Commercial Fish

Because fish are cold-blooded animals, temperature, through its influence on the rate of metabolism, is a major variable in determining the amount of energy needed for maintenance and for performing such essential activities as swimming and feeding--fish are less active, feed less, and grow more slowly in cold waters. For example, growth in young sockeye salmon is very slow at temperatures lower than 4°C (Donaldson and Foster 1941), and temperature profoundly affects their swimming speed (Brett et al. 1958). The rates of development of the eggs of some flatfish are closely correlated with water temperature (Ketchen 1956)--flatfish developed more rapidly at higher temperatures (Fig. 8). At lower temperatures, the rate of growth is also slower and, therefore, the duration of pelagic larval life is longer for demersal fish and shellfish.

Variations in sea temperature should, therefore, influence the extent to which fish are vulnerable to predation and competition. For example, eggs would take a longer time to hatch in colder than in warmer sea water. In both pelagic fish such as herring, whose eggs are laid in the intertidal zone, and in demersal fish with pelagic eggs such as the sole, the period of vulnerability of eggs to bird predation would be extended. At lower temperatures the length of the pelagic life of demersal fish and shellfish and their vulnerability to predation would also be greater than at higher temperatures. For example, the number of days between molts of the zoeal stages of snow crabs is temperature-dependent--the warmer the water, the less the time between molts (Kon 1970).

Temperature, through its effects on swimming speed, feeding activity, and growth of juvenile fish, might influence the magnitude of predation by birds on pelagic fish in the following ways: (1) lower sea temperatures would increase the vulnerability of juvenile fish to bird predation because swimming speed would decrease, and the time the fish are of a size that could be eaten by would-be predators would increase; (2) lower sea temperatures would reduce the feeding by fish and decrease the competition by fish for food exploited by birds; and (3) higher sea temperatures would have the opposite effect--the feeding by fish would increase consumption of the foods that birds feed on.

In the eastern Bering Sea, water temperatures may vary greatly between years for the same month (Fig. 9). Such variation should result in variation in the temperature-dependent activities of fish and, in turn, in magnitude of marine bird predation and competition.

Possible Influences of Man on the Interaction of Marine Birds with Commercial Fish

We have noted that the abundance and age and size composition of major stocks of fish in the Bering Sea have been drastically reduced by commercial fishing. This has resulted in the reduction in numbers of fish at all life history stages, including those on which marine birds and other fishes depend for food. What effect this reduction has had on the abundance and distribution of marine birds in the Bering Sea is unknown. It depends in part on the ability of birds to eat other fish or increase their use of zooplankton or nekton.

We can hypothesize on probable changes in bird and fish abundance that resulted from the heavy commercial harvest of fish but any such changes cannot be documented or quantified. A reduction in stocks of a fish species could result in a reduced supply of food for a species of bird and cause a shift in the diet of this bird to other species of fish or to more zooplankton. For a bird species with specific food preferences, this could mean a reduction in its abundance to a level supportable by the available food supply. For bird species with less specific food requirements, a reduction in a species of fish could mean a reduction in competition for food with that fish--which could increase survival of the birds.

Man's intentional harvest of marine birds, such as the shearwater in parts of the southern hemisphere, and his inadvertent harvest of other bird species which are entangled or caught in fishing gear reduce predation and competition by marine birds. This, in turn, may aid the survival of the fish stocks in the Bering Sea.

The status of most stocks of commercial fish and shellfish in the Bering Sea is such that reductions in harvest are warranted, have been proposed, or are in effect. If the 200-mile (61-km) limit of jurisdiction over the marine resources by adjacent coastal States is implemented, either as a result of the Law of the Sea Conferences or unilaterally by the United States, we can expect commercial fishing in the eastern Bering Sea to be more tightly regulated. Such action should result in a reduction in harvest of those fish species now in a depleted condition, which, in turn, could influence the abundance of marine birds. Now is an opportune time to implement the studies required to increase our knowledge of the abundance, distribution, and seasonal movements of marine birds and their relationship to commercial fish resources of the eastern Bering Sea.

Conclusions

• The eastern Bering Sea is a region of high biological productivity; it is one of the world's great producers of commercial fish and major congregating areas for marine birds.

• The vulnerability of fish to predation by marine birds depends on life history features, such as place of spawning, duration of larval stages, growth rate, sea temperature, and adult size of fish, and on the distribution, feeding behavior, and food habits of marine birds.

• The most apparent predation by marine birds on fish is on fish large or mature enough that some hard body parts persist and can be found in the stomach samples of birds.

• Little is known of the extent of bird predation on the pelagic eggs and larvae of demersal fish and shellfish in the Bering Sea because of lack of investigation and the rapid digestion of eggs and larvae by birds.

• Predation by marine birds on juvenile salmon is not well documented because of the lack of investigation in areas where both birds and fish are present.

• Marine birds and commercial fish eat similar zooplankton and fish in the eastern Bering Sea. The food exploited by both generally reflects the relative abundance of the types of food in the area, but food preference is displayed by some species of fish and birds.

• More is known about the food habits of the commercial fish than of the marine birds of the Bering Sea.

• Sea water temperature may be a major environmental factor in the Bering Sea since it influences both the extent to which fish are vulnerable to predation and the amount of competition with marine birds. Sea temperatures may vary greatly from year to year in the Bering Sea, and this may result in variations in the magnitude of predation and competition between birds and fish.

• The distribution of marine birds and the various stages in the life history of commercial fish are not well known for the eastern Bering Sea. Where these have been studied, they are intimately related. Such knowledge is required to gain some insight into even the potential for predation and competition in the dynamics of the marine bird and commercial fish populations of this region. In two instances, it is known that the occurrence of marine birds and the early life history stages of fish coincide so as to result in both potential predation on the fish by the birds and competition for food between the fish and the birds.

• The possibility exists that the commercial fish resources of the eastern Bering Sea will eventually come under the jurisdiction of the United States. This could mean reduced harvests of fish to restore depleted stocks. Such action could result in changes in the abundance of the marine birds of this region by creating an increased food supply for some and decreased supply for others.

Acknowledgments

We thank J. C. Bartonek and H. R. Carlson, H. Jaenicke, H. Larkins, and B. L. Wing for supplying various materials presented in this paper.

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