The California Sea Lion is a sexually dimorphic species; adult males weigh three to four times more and are up to 1.2 times longer than their female counterparts (King 1964). By two weeks of age, male pups are larger and heavier (75.7 cm, 9.01 kg) than their female counterparts (72.3 cm, 7.6 kg; Le Boeuf et al. 1983). Pups are born with a thick brownish-black lanugo that is generally moulted between the first and second month and is replaced by a darker coat. Nursing may last from one to two years (Newsome et al. 2006). Male California Sea Lions measure up to 255 cm in length and can weigh up to 523 kg (Wright et al. 2010). Females measure up to 200 cm and weigh an average of 110 kg (Peterson and Bartholomew 1967).

Both sexes usually reach maturity by four to five years of age. Females produce one pup each year, with a gestation period of approximately 11 months. A long-term mark-resighting study (1980-2006) recorded a maximum observed lifespan of 19 years for males and 25 years for females (Hernandez-Camacho et al. 2008a). Mean age-specific birth rates for females vary among age classes: for five-year-olds the rate is 0.59, for six- to 10-year-olds it is 0.79 (Melin 2002, Hernandez-Camacho et al. 2008b), 10- to 12-year-olds exhibit the highest rate (0.80), but the rate gradually decreases for individuals >13 years old to 0.06 in 22 to 25 years olds (Hernandez-Camacho et al. 2008b).

Pupping and breeding take place from May through July. Pupping starts earlier in the Gulf of California (May 8) than off the coast of California (May 20); the duration of the breeding season is also longer in the Gulf (13 weeks) than off the coast of California (9.5 weeks; Garcia-Aguilar and Aurioles-Gamboa 2003a). Males are highly polygynous and defend their territories both on land and in shallow waters near the shore for periods of up to 45 days. A study on the plasticity of male territorial behaviour in response to varying environmental conditions showed that males inhabiting high-temperature environments (air temperatures >30°C, e.g., the Gulf of California) defend territories adjacent to the coast while immersed in the water (Bohorquez-Herrera et al. 2014). Females remain on shore with their newborn pups for approximately seven days before they depart for the first of many foraging trips that usually last two to three days, followed by another one to two days of caring for their pups at the rookery (Melin et al. 2000, García-Aguilar and Aurioles-Gamboa 2003b).  

Following the breeding season, large numbers of adult and subadult males and juveniles migrate north from the major rookeries in southern California and Baja California and winter from central California to Washington (Fry 1939, Odell 1975, Mate 1975). Smaller numbers of animals migrate to British Columbia (Wright et al. 2010) and southeast Alaska, reaching the northern Gulf of Alaska, the Alaska Peninsula, and the eastern Aleutian Islands (Maniscalco et al. 2004). Other animals remain in the Gulf of California year round and do not undertake long migrations; however, the arrival of numerous adult and subadult males is observed every year at several rookeries and haul-out areas around the southern tip of the Baja California Peninsula (Aurioles-Gamboa et al. 1983).

During the breeding (warm) season, 96% of the dives by lactating adult females from San Miguel Island, California were to depths of eight to 75 m with 92% of dives lasting less than four minutes. During the non-breeding (cold) season, 77% of the dives were between eight and 75 m and 81% lasted four minutes or less. Most dives during both the breeding (84.5%) and non-breeding (81.2%) seasons occurred less than 150 km from San Miguel Island (Melin and DeLong 2000).

During the breeding season, the average dive depth for lactating adult females in the Gulf of California is 81.7±32 m with average dive duration of 3.5±0.9 min. During the cold season, dive depth and duration average 48.9±18.5 m and 1.7±0.6 min, respectively. The distance travelled during the breeding and cold season varies from 72.7±21.8 km to 101.2±18.5 km, respectively (Villegas-Amtmann et al. 2011). Adult females and males exhibit three distinct foraging behaviors to exploit the benthic, epi-pelagic and meso-pelagic environments (Weise et al. 2010, Villegas-Amtmann et al. 2011).

California Sea Lions are generally found in waters over the continental shelf and slope zones; they also frequent coastal areas, including bays, harbours and river mouths. Their feeding habits are influenced by coastal ecosystems and they consume a wide variety of prey. However, they usually prefer four to five species at each site, often consuming what is abundant locally or seasonally in the areas they occupy (Lowry et al. 1990, Garcia-Rodriguez and Aurioles-Gamboa 2004). Their main prey in California are Pacific Sardines (Sardinops sagax), Northern Anchovies (Engraulis mordax), Pacific Hake (Merlucius productus), Jack Mackerel (Trachurus symmetricus), Pacific Mackerel (Scomber japonicus), various species of the genus Sebastes, Red Octopus (Octopus rubescens) and Market Squid (Loligo opalescens; Antonelis et al. 1984, Lowry et al. 1990, Lowry 1991, Lowry and Carretta 1999). On the Pacific coast of Baja California, Sea Lions consume many of the same prey species as well as north Pacific Argentine (Argentina sialis) and Jumbo Squid (Dosidicus gigas; Aurioles-Gamboa and Camacho-Rios 2007). In the Gulf of California, the main prey species include those listed above and many other fish and squid species, including Pacific Cutlassfish (Trichiurus lepturus), Plainfin Midshipman (Porichthys notatus), Anchoveta (Cetengraulis mysticetus), Eastern Pacific Flagfin (Aulopus bajacali), Bigeye Bass (Pronotogrammus eos) and Deepwater Serrano (Serranus aequidens; Aurioles-Gamboa et al. 1984, Garcia-Rodriguez and Aurioles-Gamboa 2004).

Ontogenetic differences in diving capacities and feeding habits have been documented for California Sea Lions (Weise and Costa 2007, Orr et al. 2011), which can be affected by El Niño and La Niña events (Lowry and Carretta 1999, Weise and Harvey 2008). During the 1993 El Niño, adult females travelled farther from the colony, spent more time travelling, made deeper and longer dives, and terminated lactation earlier compared to 1996. Their mean dive depths varied from 19.5 to 279.3 m, but most females made some dives deeper than 400 m (Melin et al. 2008). During one unusually warm year, adult males that forage almost exclusively over the continental shelf altered their foraging effort by spending more time at sea and venturing up to 450 km offshore (Weise et al. 2006).

Due to their coastal distribution, abundance and feeding habits, California Sea Lions interact with several fisheries, including those for Pacific Salmon (DeMaster et al. 1982, DeMaster et al. 1985, Aurioles-Gamboa et al. 2003, Weise and Harvey 2005, Weise and Harvey 2008), causing economic losses for sport and commercial fishermen and a high incidence of Sea Lion entanglement (Stewart and Yochem 1987, Harcourt et al. 1994, Zavala and Mellink 1996, Aurioles-Gamboa et al. 2003).

Predators of California Sea Lions include Killer Whales, Sharks, Coyotes and feral Dogs. Before their recent eradication from California’s Channel Islands, Bald Eagles were known to take young pups.

California Sea Lion rookeries are found on islands from Los Islotes in Baja California Sur to the Channel Islands in southern California. Some pups have been reported from Año Nuevo Island and the Farallon Islands in central California (Keith et al. 1984). In addition to rookeries, this species also occupies haul-out sites, mainly during the fall and winter when predominately adult and sub-adult males move north to Oregon (USA) and British Columbia (Canada; Fry 1939, Mate 1975, Odell 1975). In the southern part of their range, haul-out areas are known to occur on islets off the coast of Mazatlán and the Islas Tres Marias north of Puerto Vallarta; however, this area has not been surveyed for decades. This species’ overall range extends north through the Gulf of Alaska as far as the Aleutian Islands (Maniscalco et al. 2004), and south around the end of the Baja California Peninsula to the Gulf of California. Between 2005 and 2013, several California Sea Lion sightings were reported as far south as Punta San Pedrillo, Costa Rica (Lenin Oviedo pers. comm.). A California Sea Lion male was sighted off the coast of the Yamsky Islands in the Shelikova Bay in the Sea of Okhotsk in 2007, 2008 and 2011. On each occasion, the animal was seen with a group of Steller Sea Lions. It was not observed during 2013 and 2014 (Vladimir Burkanov pers. comm.). Females, which were only very rarely found north of Point Conception, California in the early 1980s, are now routinely found in northern California, where former breeding sites have been reoccupied. However, adult female California Sea Lions tend to reside at their rookeries (Hernández-Camacho et al. 2008a). Based on estimates of sex biases in gene flow, females disperse on average 6.75 times less frequently than their male counterparts (González-Suarez et al. 2009).