Abundance of California Sea Lions on San Nicolas Island, California

Abstract

The seasonal changes in numbers and structure of the California sea lion (Zalophus californianus) population on San Nicolas Island, California, were studied between August 1968 and August 1971. During this period, 39 censuses were taken at approximately monthly intervals. With the exception of 1971, 2 annual population peaks occurred: a winter peak with 9,000-11,000 animals in January and February, and a breeding season peak with 10,000-12,000 animals in July. The maximum numbers of females counted on the breeding areas during the breeding seasons in 1969, 1970, and 1971 were 5,981, 5,163, and 5,187, respectively. The total numbers of territorial males present in late June and early July were 332, 374, and 387, and the total numbers of pups counted in July were 2,857, 2,271, and 3,500. The structure of the breeding population had changed since it was last documented in 1965. The female-to-pup ratios averaged about 2.0 and the female-to-territorial male ratios averaged about 16.0. The data suggested that the San Nicolas sea lion population may have stabilized with regard to total numbers. J. WILDL. MANAGE. 39(4):729-736 The population of California sea lions on the coast of California has attracted attention since the early 1900's when the California Department of Fish and Game began receiving complaints from commercial fishermen that sea lions were damaging the fishing industry (Bonnot 1929). Since then, the California Department of Fish and Game has censused all pinnipeds on the coast of California about once every five years (e.g., Bonnot and Ripley 1948, Carlisle and Aplin 1966, 1971, Ripley et al. 1962, Frey and Aplin 1970). More detailed studies of the seal and sea lion populations on the islands off the coast of southern California, particularly San Nicolas Island, have been made by Bartholomew (1951), Bartholomew and Boolootian (1960), Peterson and Bartholomew (1967), and Odell (1971, 1972, 1974). The California sea lion occurs along the Pacific coast of North America from British Columbia south at least as far south as Mazatlan, Sinaloa, Mexico, including the coasts of Baja California. The breeding range is from San Miguel Island, off the oast of Southern California, southward. The southern end of the breeding range is not precisely known but may be near Mazatlan (Peterson and Bartholomew 1967, Lluch B. 1969). In addition, a few pups may be born from time to time on some small islands north of San Miguel (Bonnot 1929, Peterson unpublished data, Braham 1974). The purpose of this paper is to report the seasonal changes in structure and size of the California sea lion population on San Nicolas Island, California. Until this study began, California sea lions had not been censused on any of the major rookeries off the coast of southern California more than a few times in any one year. The longest series of regular censuses of the pinnipeds on any of the California Channel Islands prior to the present study are data from aerial photographs taken in 1964 and 1965 (see Odell 1972:43). However, most 1 Supported in part by the National Aeronautics and Space Administration, The National Institute of Mental Health (MH 42,750), NSF Grant GB 3871, and NSF Grant GB 18744 to G. A. Bartholomew. 2 Present address: Division of Biology and Living Resources, School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149. J. Wildl. Manage. 39(4):1975 729 This content downloaded from 157.55.39.45 on Fri, 02 Sep 2016 05:39:05 UTC All use subject to http://about.jstor.org/terms 730 ABUNDANCE OF CALIFORNIA SEA LIONS Odell