Environmental Controls of Marine Food Webs: Food Habits of Seabirds in the Eastern Chukchi Sea

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Abstract
Food habits of thick-billed murres (Uria lomvia), common murres (U. aalge) and black-legged kittiwakes (Rissa tridactyla) were examined at 2 breeding colonies in the eastern Chukchi Sea between 1976 and 1980. Cods (Gadidae), sculpins (Cottidae), sand lance (Ammodytes hexapterus) and capelin (Mallotus villosus) were the dominant (biomass) prey of thick-billed murres. Cods, sand lance and capelin were the dominant prey of common murres and kittiwakes. Annual changes in the consumption of fishes by murres and kittiwakes suggest that available fish biomass of the eastern Chukchi Sea was low in 1976 and 1977, but increased each year through 1979 and 1980. The average size of several taxa of fishes eaten by the birds increased also between most years. Invertebrates were less important than fishes in all years, were eaten more frequently by thick-billed murres, and were eaten more frequently in 1976-1977 than in 1978-1979. A pattern of climatic cooling in the early 1970''s followed by warming in the second half of the decade caused annual differences in the extent and duration of sea ice, and apparently in the spatial and temporal development of Alaska coastal water, a major oceanographic feature of the Bering-Chukchi shelf. Fluctuations in the physical environment could have led to changes in fish populations through direct physiological and behavioral effects, or indirectly by altering the abundance of important zooplankton prey populations. Variability in the reproductive success of murres and Kittiwakes in the northern Bering Sea and eastern Chukchi Sea corresponded with the apparent changes in fish stocks: reproductive success was very low in 1976 but improved in successive years. Short-term responses of sea birds to environmental fluctuation and its proposed effect on supporting food webs suggest that the stability of regional seabird populations could also be affected by long-term changes in weather and climate.

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