INTERACTIONS AMONG MARINE BIRDS AND COMMERCIAL FISH 



213 



plankton-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 com- 

 petitors 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 deter- 

 mined only if we know their distribution, 

 abundance, and food habits, especially while 

 they are associated with one another. Infor- 

 mation is particularly lacking for all life his- 

 tory 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 diffi- 

 cult because their rapid digestion soon de- 

 stroys 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 coin- 

 cides with the distribution of the eggs and lar- 

 vae of pollock, certain flatfish, rockfish, sable- 

 fish, 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, tem- 

 perature, through its influence on the rate of 

 metabolism, is a major variable in determin- 

 ing the amount of energy needed for mainte- 

 nance and for performing such essential ac- 

 tivities 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 4C (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 shell- 

 fish. 



Variations in sea temperature should, there- 

 fore, influence the extent to which fish are vul- 

 nerable 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 preda- 

 tion would be extended. At lower tempera- 

 tures 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 



