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).
Fig. 7. Distribution and numbers of birds observed in Bristol Bay along seaward migration route of sockeye salmon (from Bartonek and Gibson 1972).
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.