Elephant seals on the Farallones: population structure of an incipient breeding colony.

-The colonization of Southeast Farallon Island, California, by northern elephant seals, Mirounga angustirostris, began in 1959 with the appearance of a single animal. By 1971, over 100 juveniles were counted during the fall and spring haulout. Breeding began on the island in 1972. The age of juveniles and their pattern of island use for the two years preceding the birth was similar to that of Afio Nuevo Island, a nearby rookery. The majority of seals onshore during the fall and spring peaks were juveniles less than two years old. Tagged individuals sighted on Southeast Farallon Island were born on rookeries to the south: Afio Nuevo Island, San Miguel Island, and San Nicolas Island. Males were observed more frequently than females. Utilization of the hauling-out place by juveniles was not determined primarily by availability of space. During the first breeding season, adult females arrived on the island before the first male was observed. The purpose of this paper is to describe some aspects of breeding colony formation in northern elephant seals, Mirounga angustirostris. Specifically, we describe events and the individuals involved in initiating breeding on a California island, and the pattern of island use prior to the commencement of breeding as well as the age, sex, and origin of the immigrants. The population structure of an incipient breeding colony of pinnipeds has not been studied previously. On 20 January 1972, a northern elephant seal was born on Southeast Farallon Island, one of a group of islands west of San Francisco, California. This is the first elephant seal birth recorded here and illustrates that this species is continuing to expand its breeding range northward since its near-extermination during the last century. It has been speculated that elephant seals once bred here over 100 years ago, a reasonable speculation since the island lies within the former breeding range of the species (Radford et al., 1965). Northern elephant seals were once found from Cabo San L'azaro in Baja California, Mexico, to Point Reyes peninsula north of San Francisco, a range of approximately 1621 kilometers (km) (Scammon, 1874). Intensive exploitation by man during the early part of the 19th century reduced the species to near extinction by 1869 (Scammon, 1874). From 1884 to the middle 1930's the species bred only on Isla de Guadalupe, an isolated island 243 km west of central Baja California; in 1892 the total population was estimated at less than 100 animals (Bartholomew and Hubbs, 1960). With protection provided by the Mexican government in 1922, joined later by the United States government, the elephant seal began a remarkable comeback which continues to the present day. According to Rice et al. (1965), initial observations of elephant seals on islands within their former range occurred as follows: Islas San Benito (1918),