IMULATING THE EFFECTS OF PREDATION AND EGG-HARVEST AT A GULL COLONY Final Report to: Glacier Bay National Park Gustavus, Alaska

We developed an individual-based simulation model to explore the effects of harvesting eggs from a glaucous-winged gull (Larus glaucescens) colony that also experiences egg loss from avian predators. The model has direct application to Glacier Bay National Park, where resource managers are concerned about potential negative effects of traditional Native Alaskan harvesting of gull eggs at colonies within the park. The model simulates the sequence of egg laying, relaying, and incubation to hatching for individual nests and calculates hatching success, incubation length, total egg harvest, and total eggs laid in all nests during the simulation. Stochasticity is incorporated in the 1 distribution of nest lay dates and in the number of nests attacked during predation and harvest events. Model outputs suggest that (1) harvest strategies are largely ineffectual when background predation rates are high, and, (2) harvesting early, at one time, and from no more than 20% of the colony provides a constant harvest with the least impact to gulls when background predation rates are low to moderate (and within the natural range observed in the field). Glaucous-winged gulls (Larus glaucescens) are common along the west coast of North America from Washington to the Alaska Peninsula (Verbeek 1993). Their average clutch size is 3 eggs, and females lay at 2-day intervals until clutches are complete and incubation begins. The loss of all eggs in a nest prior to clutch completion may result in protracted laying, in which case females continue to lay until their clutch is complete. Replacing a clutch lost after the onset of incubation requires 12-13 days to resume follicle growth and lay the first egg of the replacement clutch. Replacement-laying is common in ground-nesting gulls, which have evolved to replace eggs lost to factors such as floods and predators (Brown and Morris 1996). Common predators of glaucous-winged gull eggs include conspecifics (Verbeek 1988) (Good et al 2000), common ravens (Corvus corax) (Patten Jr 1974) American crows (Corvus brachyrhynchos) (Verbeek 1988) bald eagles (Haliaeetus leucocephalus) (Thompson 1989, Good et al 2000) and humans (Vermeer et al 1991). Egg predation by one predator species, such as humans or bald eagles, can also facilitate predation by conspecifics (Hand 1980, Good et al 2000). On South Marble Island in Glacier Bay, Alaska, glaucous-winged gull eggs are commonly preyed upon by bald eagles and harvested by native peoples. The harvest of seabird eggs is a traditional subsistence activity of the Tlingit peoples …