California Sea Lion Distribution: A Statistical Analysis of Aerial Transect Data

Sightings of California sea lions (Zalophus californianus) recorded on 23 aerial transect surveys of the Southern California Bight were pooled by 30' x 30' grid cells. Contour maps generated from celldensity estimates were used to represent seasonal distributions. The mean number of animals at sea in 3 seasons was calculated by summing cell population estimates, and the jackknife technique was used to place confidence limits on the at-sea component of the summer breeding population. California sea lions concentrate near their rookery islands and at specific open-water locations in summer. The center of their distribution shifts to the northwest in autumn and to the southeast in winter and spring, probably in response to prey availability. J. WILDL. MANAGE. 51(1):13-20 The California sea lion population in California is large and growing. Most breeding occurs on the southern California Channel Islands, where counts of pups show that the population doubled every 10 years since 1950 (Bonnell et al. 1984). Some of this growth may have resulted from immigration because counts of California sea lions at the 2 largest Mexican rookeries (Islas San Benito and Cedros) declined 40% between 1976 and 1981 (comparison of Jun counts) (Mate 1977, LeBoeuf et al. 1983). As of 1982, California sea lions on their southern California rookery islands totalled >50,000 animals (Heath and Francis 1983, Bonnell et al. 1984, Stewart and Yochem 1984). As the population in California has grown, California sea lions have been increasingly associated with fisheries conflicts, prompting an assessment of this species' status (see Demaster et al. [1982] for a review). Most knowledge of California sea lions is derived from studies of populations on land. Until recently, little was known about their habitat utilization and distribution at sea. Beginning in 1975, study was undertaken to map the distributi n at sea of all marine mammal species, including California sea lions, found in the Southern California Bight (SCB). Data were collected on 23 aerial surveys conducted over a 3-year period; each survey required about 20 hours in the air (Bonnell et al. 1980). From 1975 to 1978, 30,996 km (17,047 nautical miles) of trackline were flown. Our paper examines the pattern of California sea lion distribution in the SCB and places confidence limits on density values and population size estimates. Data were collected as part of the BLM-OCS Survey Program, supported by contracts between the U.S. Bur. Land Manage., U.S. Dep. Interior, and the Univ. California, Santa Cruz. The development of the distributional model was supported by a contract to WoodwardClyde Consultants (WCC), Walnut Creek, Calif., from the U.S. Minerals Manage. Serv. We are grateful to the many individuals involved in the collection of these data: D. H. Dettman, G. D. Farrens, C. B. Heath, B. J. LeBoeuf, and M. O. Pierson. We thank T. P. Winfield and R. This content downloaded from 207.46.13.111 on Tue, 09 Aug 2016 06:17:17 UTC All use subject to http://about.jstor.org/terms 14 SEA LION DISTRIBUTION * Bonnell and Ford J. Wildl. Manage. 51(1):1987 121