The California Sea Lion is the well-known “performing seal” of zoos, circuses and marine theme parks. It is a sexually dimorphic species, with males reaching three to four times the weight of adult females and 1.2 times their length. Pups are born with a thick brownish-black lanugo that is generally moulted by the end of the first month. The succeeding light brown juvenile coat is shed 4-5 months later and is replaced by adult coloration. Male California Sea Lions reach lengths of 2.4 m, and weights of more than 390 kg. Females only reach 2 m, and weigh an average of 110 kg. Newborn pups are about 80 cm long and weigh 6-9 kg (Peterson and Bartholomew 1967).

Age of maturity for both sexes is about 4-5 years. Females produce one pup each year after a gestation of about 11 months. A long term mark-resighting study (1980-2006) recorded a maximum observed longevity of 19 years for males and 25 years for females (Hernandez-Camacho et al. in press). Age-specific birth rates vary among age classes; 5 yr-old females show 0.59, females between 6 and 15 year-old exhibit 0.79 (Melin 2002; Hernandez-Camacho et al. in press), and between 16-25 year-old show decreased birth rate between 0.35 and 0.11 (Hernandez-Camacho et al. in press).

Pupping and breeding take place from May through July. Pupping starts earlier in the Gulf of California (May 8) than in California (May 20) and the duration of the breeding season is longer in the Gulf (13 weeks) than in California (9.5 weeks) (Garcia-Aguilar and Aurioles-Gamboa 2003). Males are highly polygynous and hold territories both on land and in shallow water near shore for periods up to 45 days. Females stay ashore with their newborn pups for about seven days before they depart for the first of many foraging trips that usually last 2-3 days and are followed by attendance with the pup at the rookery for 1-2 days. Most pups are weaned at 12 months of age. However, some pups continue to receive maternal care as yearlings and 2-3year olds (Newsome et al. 2006).

The diving pattern of lactating adult females is consistent with that of a number of other otariid species. The deepest dive recorded was to approximately 274 m and the longest dive lasted just under ten minutes. Typical feeding dives are shallower than 80 m, and last less than three minutes (Feldkamp et al. 1989; Antonelis et al. 1990). Lactating adult females are active for most of the time they are at sea and feeding bouts occur during the day and at night, with peaks of activity at dawn and dusk. The fact that feeding dives occur in bouts suggests that California sea lions are frequently exploiting patches of prey (Weise and Costa 2007).

California Sea Lions are generally found in waters over continental shelf and slope zones; however, they occupy several islands far offshore in deep oceanic areas, such as Guadalupe Island. They frequent coastal areas including bays, harbours, and river mouths.

California Sea Lions feed on a wide variety of prey, but usually maintain a preference for 4-5 species at each location, often taking what is abundant locally or seasonally in the areas they occupy (Lowry et al. 1990; Garcia-Rodriguez and Aurioles-Gamboa 2004). A lower diversity of prey is taken outside the breeding season, when many animals disperse over large areas, as opposed to during the breeding season, when preferred prey can be reduced by intense foraging activity in small areas within travelling range of the rookeries (Lowry 1991). Principle prey taken by California Sea Lions in the Pacific includes: Pacific whiting, market squid, red octopus, jack and Pacific mackerel, blacksmith, juveniles of various species of rockfish, herring, northern anchovy, and salmon (Antonelis et al. 1984; Lowry et al. 1990; Lowry 1991). Sea lions in the Gulf of California have northern anchovy, Pacific whiting, and rockfish as prey in common with animals in the Pacific and also take various species of midshipmen, myctophids and bass, as well as sardines, largehead hairtail and Eastern Pacific flagfin (Aurioles-Gamboa et al. 1984; Garcia-Rodriguez and Aurioles-Gamboa 2004). Because of their boldness and taste for commercially-important fish species, such as salmon and rockfish that are easily taken from fishing lines (DeMaster et al., 1985) California sea lions are considered a nuisance by many sport and commercial fishermen. They will also ascend rivers following spawning runs of anadromous fish and take advantage of man-made structures such as canal locks and fish ladders that concentrate prey.

Predators of California Sea Lions include Killer Whales, sharks, Coyotes and feral dogs, and until they were recently extirpated from the California Channel Islands, bald eagles were known to take young pups.