Distribution of Marbled Murrelets along the Oregon Coast in 1992

Abstract

Strip transects from boat and light aircraft were used to quantify the distribution and abundance of marbled murrelets (Brachyramphus marmoratus) along the length of the Oregon coast. Murrelets were abundant in central Oregon between Newport and Coos Bay, with observed densities averaging 78 birds/10-km by 100-m strip (N = 90, SE = 5.9). Murrelets were relatively scarce north of Lincoln City (x = 6.8 birds/10-km by 100-m strip, N = 32, SE = 1.2), with small concentrations near coastal State Parks that contained old-growth forest stands. Murrelet abundance was variable from Coos Bay to the California border (x = 28.4, N = 22, SE = 5.9). Highest densities occurred in a narrow band and decreased sharply at > 1 km from shore. There was evidence of a northward and offshore shift of the population late in July, which may have been related to a prey shift from surf smelt (Hypomesus sp.) to Pacific sandlance (Ammodytes hexapterus) or to post-breeding dispersal. The abundance of marbled murrelets seen during this study suggests that the population in Oregon is higher than previously estimated. Marbled murrelets (Brachyramphus marmoratus) are small diving seabirds that have the unique adaptation of flying inland to nest in large trees along the west coast of North America (Marshall 1988, Hamer and Nelson 1995a). At sea, marbled murrelets occupy inland passages, bays, and near-shore waters of the open coast; they rarely are seen over 3 km from shore (Sealy 1975b, Carter 1984, Carter and Sealy 1990, Rodway et al. 1992, Ralph and Miller 1995). Because detection and quantification of marbled murrelets is very difficult in their forest nesting habitat, surveys of the birds at sea is presently the most effective and accurate means of assessing population size and distribution. The need for information on abundance and distribution of marbled murrelets was accentuated by the species' listing as federally threatened in October 1992 (U.S. Fish and Wildlife Service 1992). All of the 3 prior at-sea studies of marbled murrelets in Oregon (summarized in Nelson et al. 1992) were made along short sections of the coastline, and each study used different methods and observation vessels, making comparison difficult. Besides those at-sea counts, numerous observations from shore of marbled murrelets in Oregon have been made over the years (Nelson et al. 1992, Nelson and Hardin 1993). To this point, information on population size and distribution in Oregon has been derived from inland and shore-based observations and the work of Varoujean and Williams (1995). Based on vessel and aerial transects, preliminary results of the first state-wide surveys of marbled murrelets along the Oregon coast are presented here. This ongoing research is directed towards establishing a population estimate and monitoring protocol for the species in Oregon.