Graduate Thesis Or Dissertation
 

Foraging strategies of Glaucous-winged Gulls : influences of sea otter predation

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  • Diets and foraging strategies of Glaucous-winged Gulls were studied in areas with and without sea otters in the western Aleutian Islands, Alaska. Gulls foraged on invertebrates (e.g., sea urchins, limpets, chitons, mussels, and others) in the rocky intertidal community and on fish at sea; this study was conducted on gulls foraging intertidally. Sea otters affected foraging strategies and diets of gulls by reducing the size and density of intertidal prey available to them. In the presence of low densities of sea otters (which had depredated large sea urchins) gulls adjusted their foraging strategies by being more selective while feeding on urchins. In the presence of high densities of sea otters (which had depredated most intertidal prey) gulls shifted their diets from intertidal invertebrates to fish and the diversity of their diets was reduced. Observations demonstrated that gulls foraged intertidally during low tides and that most foraging occurred in the lowest intertidal zones that were exposed. Consequently, gulls foraged in different zones during spring and neap tides. When all zones were exposed gulls selected the Alaria and Laminaria zones, which offered the highest net rate of energy gain (En) . Gulls also selected particular prey species and prey sizes. Selective foraging of gulls increased their En 126% in areas without sea otters and 181% in areas with low densities of sea otters. Prey preference experiments demonstrated that preferences of gulls for chitons and urchins were significantly correlated to En, but assimilation rate, experience and search images were also influential. Highly preferred prey species (chitons) were not strongly selected for in the field because of their ability to adhere to the substrata. Foraging behavior of gulls indicated that they hunted by En expectation and left prey patches when a threshold En was reached. Foraging behavior of gulls in the rocky intertidal community supported optimal foraging theory for optimal diets, patch choice, and time allocation to patches.
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