The Marbled Murrelet (Brachyramphus marmoratus) nests in old-growth forests along the Pacific Coast of North America from central California to southern Alaska and the Aleutian

361 Abstract: We assessed the occurrence patterns of Marbled Murrelets (Brachyramphus marmoratus) offshore of Waddell Creek, in central California. Data were derived primarily from cruises during the height of the murrelet breeding season, in June, between 1986 and 1994, as well as some cruises during the prebreeding period, February to early April. The large majority of sightings occurred within 10 km of Point Año Nuevo, directly offshore the species’ breeding area. Only three sightings occurred farther offshore (12-24 km). The physical factors that explained a small but significant portion of variability in murrelet occurrence were: (1) inverse relationships related to distance to breeding area (including ocean depth); and (2) relationships to recently upwelled water. Murrelets were least abundant during periods of El Niño-Southern Oscillation. An analysis of the availability of potential prey species indicated that murrelets were most abundant when more euphausiids were found from the coast to well offshore. Murrelets were absent in years when a large prey concentration occurred only close to shore, indicating that the birds were too close to shore for us to census. We suggest that the critical habitat of this population should include the nearshore marine waters within 10 km of Año Nuevo Island.