Regional population monitoring of the marbled murrelet: Field and analytical methods

Abstract

The marbled murrelet (Brachyramphus marmoratus) ranges from Alaska to California and is listed under the Endangered Species Act as a threatened species in Washington, Oregon, and California. Marbled murrelet recovery depends, in large part, on conservation and restoration of breeding habitat on federally managed lands. A major objective of the Northwest Forest Plan (the Plan) is to conserve and restore nesting habitat that will sustain a viable marbled murrelet population. Under the Plan, monitoring is an essential component and is designed to help managers understand the degree to which the Plan is meeting this objective. This report describes methods used to assess the status and trend of marbled murrelet populations under the Plan. Our monitoring plan is specifically designed to estimate marbled murrelet density, population size, and population trend in each of five geographic areas (conservation zones) between the northern tip of Washington state and San Francisco, California. Within each zone, we defined an offshore boundary denoting the extent of the target population. We then divided the shoreline into 20-km segments (primary sample units, denoted as PSUs) and drew a stratified random sample of about 30 PSUs for each zone. For each PSU survey, observers in small boats followed a prescribed at-sea transect line and recorded perpendicular distances to all murrelets observed from mid-May to the end of July when murrelets detected on the water are most likely locally breeding birds. We use distance sampling methods to compute density and population estimates for the target population each year. This sampling design was implemented during the 2000 field season and sampling has continued each year thereafter. This report describes our sampling design, survey methods and analysis methods. To help illustrate our methods, we present results from the 2005 field season. The total population estimate that year was approximately 20,200 murrelets with a 95 percent confidence interval ranging from 16,000 to 24,500 murrelets. We emphasize that this is the first systematic sampling protocol aplied throughout the murrelet's listed range. Because these methods have not been used in the past, our current population estimates are not directly comparable to past estimates from other researchers.