Seasonal and Spatial Variations in Densities of Marbled Murrelets <i>Brachyramphus marmoratus</i> off Southwestern Vancouver Island

The ocean off Vancouver Island, British Columbia, is a productive area supporting diverse marine life, including a globally important breeding population of Marbled Murrelets Brachyramphus marmoratus. We report on the year-round at-sea distribution and densities of this murrelet off southwestern Vancouver Island based on monthly vessel surveys in 1993–2000. Surveys covered three ocean zones: Nearshore (sheltered waters, usually < 20 m deep, within 1–2 km of shore; 69 surveys), Inshore (exposed coastal waters, < 50 m deep, within 6 km of shore; 38 surveys), and Offshore (exposed open seas over the continental shelf, > 50 m deep, generally > 6 km from shore; 29 surveys). Year-round mean densities (± standard deviation, in birds/km²) for Nearshore, Inshore, and Offshore waters were 4.5 ± 5.8, 1.5 ± 1.2, and 0.2 ± 0.2, respectively. During peak breeding season (May–July), densities were 13.7 ± 2.3, 3.3 ± 0.2, and 0.2 ± 0.1 birds/km², respectively. Our data confirm that most murrelets leave this area outside of the breeding season, and very few move into the open ocean at this time. Post-breeding movements, molt locations, and winter distribution of these birds remain poorly known, highlighting the need for more surveys outside the breeding season and across the murrelet’s range. Due to high vessel traffic, including many oil tankers, and ongoing chronic oil spills, murrelets off southwestern Vancouver Island are exposed to high risk of oil pollution, particularly during the summer and close to shore. Gill nets and disturbance from boats are additional risks.