The California Sea Lion (Zalophus californianus) is a coastal sea lion of the northern Pacific Ocean. Their numbers are abundant (188,000 U.S. stock 1995 est.), and the population continues to expand at a rate of approximately 5.0% annually. They are quite intelligent, can adapt to man-made environments, and even adult males can be easily trained. Because of this, California sea lions are commonly used for entertainment in circuses, zoos and marine parks; and are used by the US Navy for certain military operations.

Male California sea lions grow to 390 kg (860 lb) and 2.4 metres (8 ft) long, while females are significantly smaller, at 110 kg (240 lb) and 2 metres (6.5 ft) long. The have pointed muzzles, making them rather dog-like. Males grow a large crest of bone on the top of their heads as they reach sexual maturity, and it is this that gives the animal its generic name (loph is "forehead" and za- is an emphatic; Zalophus californianus means "Californian big-head"). They also have manes although they are not as well developed as the mane of adult male South American or Steller sea lions. Females are lighter in color than the males, and pups are born dark but lighten when they are several months old. When it is dry the skin is a chocolate brown colour.

As its name suggests, the California sea lion is found mainly around the waters of California. It also lives around Oregon, Washington, and British Columbia to the north and Mexico to the south. The Galápagos Sea Lion and the extinct Japanese Sea Lion were once considered subspecies of the Californian sea lion. Now these two populations are generally considered as distinct species.

California sea lions breed from the Channel Islands off Southern California, to Mexico. Major breeding sites are San Miguel and the San Nicolas islands. Non-breeding populations live as far north as British Columbia. The Galápagos subspecies breeds exclusively on the Galápagos Islands and rarely travels far.

California sea lions prefer to breed on sandy beaches in the southern part of their range. They usually stay no more than 10 miles out to sea. On warm days they stay close to the water's edge. At night or on cool days, the sea lions will move inland or up coastal slopes. Outside of the breeding season they will often gather at marinas and wharves, and may even be seen on navigational buoys. These man-made environments provide safety from their natural predators: orcas and white sharks.

California Sea Lions feed on a wide-variety of seafood, mainly squid and fish. Commonly eaten fish and squid species include salmon, hake, Pacific whiting, anchovies, herring, and market squid. They feed mostly around the edge of the continental shelf as well as sea mounts, the open ocean and the ocean bottom. Average annual food consumption of males in zoo increased with age to stabilize at approximately 4,000 kg/year by the age of 10 years. Females showed a rapid increase in average annual food consumption until they were 3 years old. Thereafter, females housed outdoors averaged 1,800 kg/year.

California sea lions may eat alone or in small to large groups depending on the amount of food available. They will cooperate with dolphins, sharks and seabirds when hunting large schools of fish. Sea lions from the state of Washington will wait at the mouths of river to for the salmon run. They also have learned to feed on steelhead, and white sturgeon below fish ladders at Bonneville Dam and at other locations in the Columbia River, Willamette River, and in Puget Sound.

Adult females forage between 10 and 100 km from the rookery, and dive to average depths of 31.1 to 98.2 m, with maximum dives between 196 and 274 m. They travel at an estimated speed of 10.8 km/h, and that young sea lions have an initial defecation time averaging 4.2 hours. Adult females spend 1.6-1.9 days on land and 1.7-4.7 days at sea.

California sea lions are highly social and breed around May to June. When establishing a territory, the males will try to increase their chances of breeding by staying on the rookery for as long as possible. During this time, they will fast, using their blubber as an energy store. Size is a key factor in winning fights and well as waiting. The bigger the male the more blubber he can store and the longer he can wait.

A male sea lion can only hold his territory for up to 27 days. Females do not become receptive until 21 days after the pups are born, thus the males do not set up their territories until after the females give birth. Most combat fights occur during this time. Soon, the fights go from violent combat to ritualized displays such as barking, head-shaking, stares and bluff lunges. There can be as many as 16 females for one male.

The females have a 12-month gestation period and give birth around June and August. Mothers may give birth on land or in water. The pups are born with their eyes open and can vocalize with their mothers. Pups may nurse for up to six months and grow rapidly due to the high fat content in the milk. California Sea Lions are the only mammals whose milk does not contain lactose. At about two months the pups learn to swim and hunt with their mothers.

After the breeding season, female California sea lions normally stay in southern waters while the adult males and juveniles generally migrate north for the winter.Social organization during the non-breeding is unstable. However a size related dominance hierarchy does exist. Large Males use vocalization and movement to show there dominance and smaller males always yield to them. Non-breeding groups are gregarious on land and often squeeze together. Most sea lion found in man-made environment are males or juveniles since sea lions don’t breed there and it is mosty those groups that migrate to those places during the non-breeding season.

Most of the iconic trained seals of circuses and marine parks are California sea lions. They are highly intelligent and can be trained to do different tricks such as throwing and catching balls on their noses, running up ladders, or honking horns in a musical fashion. Even adult males can be trained. Some people object to using sea lions for entertainment. They claim the animals are used as "clowns" in unnatural environments and are simply gimmicks with no education value. On the other side, people have claimed that training captive sea lions has allowed then to experience just how intelligent they are.

The California sea lion is used in military applications by the U.S. Navy Marine Mammal Program. These include detecting underwater landmines and equipment recovery. Sea lions can be highly useful as they can reach places inaccessible to human divers. They can dive to 1,000 feet and swim up to 25 miles per hour in short bursts. They can also swim silently in the water.

Sea lions have been deployed in the Persian Gulf to protect US ships from enemy frogmen. The animals are trained to swim behind a diver approaching a ship and attach a clamp, which is connected to a rope, to the diver's leg. Navy officials say that the sea lions can get the job done in seconds and the enemy doesn’t know the clamp was attached to his leg until it’s too late.

California sea lions are protected under the U.S. Marine Mammal Protection Act. However their population has been increasing and conflicts with humans and other wildlife has increased. California sea lions have damaged docks and boats, stolen fish from commercial boats and have injured swimmers in San Francisco Bay. Because of this, they have been shot at by locals and fishermen.

In 2007, legislation was introduced (HR 1769: Endangered Salmon Predation Prevention Act) that would amend the Marine Mammal Protection Act to permit lethal California sea lion removal from near salmon runs when their population exceeds a determined maximum sustainable level. The purpose of HR 1769 is to relieve pressure on the precipitously declining salmon populations of the Pacific Northwest. Officially, pinnipeds account for only an estimated 4% of salmon loss in 2007. However, the 4% predation number comes from actual observation of predation; since much of the predation occurs underwater, marine biologists believe that the true rate is far higher .

Short of lethal removal, attempts have been made to identify California sea lions that are aggressive salmon predators and to relocate these members from the area near salmon runs. However, relocation generally fails as the sea lions simply return. In January 2008, at the request of Washington and Oregon, the National Marine Fisheries Service has drafted a proposal to euthanize approximately 30 animals annually at Bonneville Dam . That proposal is open for public comment until 19 February 2008.