INTERACTIONS AMONG MARINE BIRDS AND COMMERCIAL FISH 



215 



25 



i 20 



-J 15 



10 



5 10 



TEMPERATURE (C.) 



15 



Fig. 8. The relation of temperature to the rate of de- 

 velopment to hatching of lemon sole, as compared 

 with two European flatfishes (Ketchen 1956). 



pendent activities of fish and, in turn, in mag- 

 nitude of marine bird predation and com- 

 petition. 



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 his- 

 tory stages, including those on which marine 

 birds and other fishes depend for food. What 

 effect this reduction has had on the abun- 

 dance 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 quan- 



tified. 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 south- 

 ern 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 fish- 

 ing 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 knowl- 

 edge of the abundance, distribution, and sea- 

 sonal 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, 



