At-sea distribution, abundance and habitat affinities of Xantus's Murrelets

We used shipboard and aerial surveys at sea to study distribution, abundance and habitat affinities of Xantus's Murrelets Synthliboramphus hypoleucus within their range, including waters from British Columbia to southern Baja California, and to 500 km offshore. We recorded 1628 murrelets during strip-transects conducted in most years from 1975 to 2003. Densities were highest over the continental slope (depths 200-1000 m) at distances 25-150 km offshore. Murrelets were most numerous in warmer waters of lower salinity, a pattern consistent each year regardless of El Nino-Southern Oscillation or Pacific Decadal Oscillation anomaly fluctuations. During the breeding season, murrelets concentrated in the Southern California Bight (SCB), with lower densities off Baja California and from Point Conception to Bodega Bay, California. During the nonbreeding period, they dispersed north as far as northern Vancouver Island, British Columbia, although densities were highest from central Baja California to central Oregon. We used generalized additive models to estimate the abundance of this species at sea. We observed no trends in abundances across years, 1975-2001 (SCB), and 1985-2003 (central California). After adjustment for biases in survey data, our estimate for the total number of Xantus's Murrelets in North America during the nonbreeding season (1975-2003) is 39 700 birds, consisting of an estimated 17 900 breeding birds (95% confidence interval = 13 900 to 21000) and 21 800 subadults/nonbreeders.