Steller Sea Lion

The Steller (or northern) sea lion (Eumetopias jubatus) is the largest member of the family Otariidae, the “eared seals,” which includes all sea lions and fur seals. Steller sea lions are named for Georg Wilhelm Steller, the German surgeon and naturalist on the Bering expedition who first described and wrote about the species in 1742. While they are the only living member of their genus, they share parts of their range with a smaller related species, California sea lions. Steller sea lions' impressive low-frequency vocalizations sound more like roars than California sea lions’ barks. They also share parts of their range with another otariid: northern fur seal.

Historically, Steller sea lions were highly abundant throughout many parts of the coastal North Pacific Ocean. Indigenous peoples and settlers hunted them for their meat, hides, oil, and other products, and today sea lions are an important subsistence resource for Alaska Natives.

Because of unexplained widespread population declines in Alaska, Steller sea lions were first listed under the Endangered Species Act (ESA) in 1990. In 1997 NOAA Fisheries recognized two distinct population segments, listing the eastern distinct population segment (DPS) as threatened and the western DPS as endangered. The eastern DPS has since recovered and is no longer listed, which is a significant achievement under the ESA. The western DPS remains endangered.

Learn more about the eastern and western populations of Steller sea lions

Steller sea lions are sexually dimorphic—adult males are much larger than females. Adult males may be up to 11 feet long and can weigh up to 2,500 pounds. Adult females are 7.5 to 9.5 feet long and weigh up to 800 pounds. 

Adult males are further distinguished by long, coarse hair on the chest, shoulders, and back. An adult male’s chest and neck are also more massive and muscular than a female’s. Both adult males and females have light blonde to reddish brown coats that are slightly darker on the chest and abdomen. The light coloration is still visible when the body is wet. Like other pinnipeds, they molt, or shed their fur, every year.

Both sexes have long light-colored whiskers (vibrissae) on their muzzles, which they use to sense prey and feel their way underwater. The flippers and other hairless parts of the skin are black. On land, sea lions, unlike "true" seals, can turn their hind flippers forward for walking. They can climb and are sometimes found on rocks or cliff faces high above the water. When they swim, Steller sea lions use their broad, long front flippers to propel themselves and are highly maneuverable.

Steller sea lions are predatory and consume  a wide range of prey, foraging and feeding primarily at night on over a hundred species of fish (including Atka mackerel, walleye pollock, salmon, Pacific cod, Pacific sand lance, arrowtooth flounder, Irish lords, rock sole, capelin, eulachon, Pacific sandfish, Pacific herring, rockfish, smooth lumpsucker, and Pacific hake) and cephalopods (including squid and octopus). Their diet varies in different parts of their range and at different times of the year, depending on the abundance and distribution of prey species. To meet their energy requirements to grow, survive, and reproduce, Steller sea lions likely depend on predictable prey that are readily available and sufficiently abundant so that they may forage efficiently throughout the stages of their lives and during different times of the year. 

Steller sea lions forage near and off shore, and in both benthic and pelagic zones. Different individuals may have different foraging strategies. For example, NOAA Fisheries’ research shows that in the non-breeding season, some adult females may spend long periods of time foraging well off the continental shelf while others forage much nearer to terrestrial sites. During the breeding season, a female must forage close enough to her rookery to return often and nurse her young, and her foraging range and trip duration will expand greatly as the pup grows. An adult female has very high energy demands—especially in the winter, when she must find enough food to feed herself and nurse her pup, possibly while pregnant with the next year’s pup. 

Steller sea lions, especially males, can travel long distances in a season. Diving ability changes with age. The deepest dive documented is about 1,400 feet in depth. 

Steller sea lions need undisturbed land habitat to rest, molt, socialize, mate, give birth, and nurse small pups during the breeding season. They are highly social and may rest in large groups, overlapping their bodies. At sea, they can be seen alone or in small groups, and sometimes gather in large "rafts" at the surface near areas of large prey aggregations. While at sea, they can sometimes be seen holding one or more flippers out of the water to thermoregulate. Flippers are a poorly insulated part of the body because the blood vessels are close to the surface of the skin. When floating on the surface of the water, sea lions can extend their flippers into the air to absorb heat, which is then circulated to the rest of the body, including areas near important seasonal prey resources.

Steller sea lions inhabit the colder temperate to subarctic waters of the North Pacific Ocean. They need both terrestrial and aquatic habitats. They mate and give birth on land, at traditional sites called rookeries. Typically, sea lions come back to mate on their natal rookery site, each year. Sites where animals haul-out to rest, where breeding does not occur, are referred to as haulouts. Haulout and rookery sites usually consist of beaches (gravel, rocky, or sand), ledges, and rocky reefs. In the Bering Sea and Okhotsk Sea, sea lions may also haul-out on sea ice.

Steller sea lions are distributed mainly around the coasts along the North Pacific Ocean rim from northern Hokkaido, Japan through the Kuril Islands and Sea of Okhotsk, the Aleutian Islands and Bering Sea, the southern coast of Alaska, and south to central California. While they are most typically found in coastal waters on the continental shelf, they also occur and sometimes forage in much deeper continental slope and pelagic waters, especially in the non-breeding season.   

The western DPS includes Steller sea lions that originate from rookeries west of 144° west longitude (Cape Suckling): those in the Gulf of Alaska, the Aleutian Islands, the Bering Sea, and Asia. The eastern DPS includes sea lions originating from rookeries in southeast Alaska, British Columbia, Washington, Oregon, and California. Data show that the foraging ranges of the two DPSs overlap, especially in the non-breeding season. For example, females from the eastern Gulf of Alaska regularly occur in parts of southeast Alaska and males from the eastern DPS have been observed as far north as the Bering Sea. In recent years, a “mixing zone” has also become established in northern southeast Alaska on at least two new rookeries partially established by western DPS females.

Learn more about the eastern and western populations of Steller sea lions

Steller sea lions are colonial breeders. They have a polygynous mating system, in which only a small proportion of the sexually mature males father most of the pups in a given season. 

Adult males, also known as bulls, arrive early on rookeries to establish breeding territories that they defend through the breeding season. Bulls become sexually mature between 3 and 8 years of age, but typically are not large enough to hold territory successfully until 9 or 10 years old. A mature male may go without eating for 1 to 2 months while he defends his territory. Not all males will successfully hold a breeding territory for one or more breeding seasons.

Females begin to arrive on rookeries in mid-May. Females typically give birth to their first pup between 4 to 6 years of age, usually giving birth to a single pup each year. However, they may not pup every year. Pupping occurs from about mid-May to mid-July and peaks in June. Females usually mate within 2 weeks after giving birth. Steller sea lions are mammals; adult females, also known as cows, stay with their pups for a few days after birth before beginning a regular routine of foraging at sea, nursing pups on land, then going back to forage. Female Steller sea lions use smell and distinct vocalizations to recognize and create strong social bonds with their newborn pups. While most pups likely wean before their first birthdays, some pups are nursed for as long as three years.

At birth, pups are about 3 feet in length and weigh 35 to 50 pounds. Pups have a thick, dark brown to black "lanugo" coat until 4 to 6 months old, when they molt to a lighter brown. By the end of their second year, pups are the same color as adults. Males can live to be up to 20 years old, while females can live to be approximately 30 years old.

Steller sea lions are exposed to a variety of human-caused and natural threats. Some of the most pressing threats are discussed below.

Effects of Fisheries on Prey 

Cumulative and annual commercial fishery removals may result in temporal and seasonal changes in distribution and abundance of primary prey, prey reduction, and  changes in prey size; they may also cause ecosystem effects. All of these may affect Steller sea lions' ability to reliably access sufficient prey to sustain the health, reproduction, and survival of individuals and support sustained increase and eventual recovery of the population.  

Climate Change

Global climate change is expected to have profound impacts on arctic and sub-arctic marine ecosystems. This may affect the composition, spatial and temporal distribution, and abundance of prey available to Steller sea lions. 

Sea Level Rise

Sea level rise caused by climate change will directly affect terrestrial rookery and haulout sites currently used by Steller sea lions as well as those that may be used by a recovering population. This may result in more deaths among small pups, and traditional sites on some islands with low relief may be submerged.

Temperature Changes

Decadal scale regime shifts, and shorter-term oceanographic anomalies such as  El Niño Southern Oscillation (ENSO) events and marine heatwaves, also can have large effects on distribution and abundance of Steller sea lion prey. Marine heatwaves have already shown to reduce Steller sea lion survival.

Ocean Acidification

Ocean acidification effects on Steller sea lions are uncertain but are likely to include serious impacts on ecosystems and may have adverse effects on specific species prey through food web effects. 

Harmful Algal Blooms

Harmful algal bloom (HAB) toxins cause sickness and death in humans and Steller sea lions. Warming water temperatures increases toxic algae growth, prevents water from mixing, allowing algae to grow thicker and faster, and algal blooms absorb sunlight, making water even warmer and promoting more blooms.

Disease and Parasites

Steller sea lions are exposed to a variety of diseases and parasites. Adult females and pups are likely the age-classes most vulnerable to disease and parasitism. Infectious disease and parasitism likely have a relatively low impact on the recovery of the Steller sea lion's western distinct population segment. However, climate-change-related shifts in distribution of other species may expose Steller sea lions to novel disease vectors or parasites that could have large-scale impacts. NOAA Fisheries works with a variety of scientists and veterinarians to monitor disease in animals that are found dead and to sample live animals. 

Toxic Substances

Contaminants that might harm Steller sea lions can occur naturally in the environment and enter ocean waters from many sources, such as oil and gas activity (tankering and pipeline transport, production, etc.), vessel accidents and sinkings (leading to loss of cargo, fuel, etc.), local industrial development, atmospheric transport, wastewater discharges, at-sea processing, runoff, toxic waste sites, nuclear testing, industrial accidents, and natural sources. Once in the environment, some contaminants move up the food chain and accumulate in top predators. Certain pollutants that Steller sea lions are exposed to have been shown to damage the immune and reproductive systems of other mammals and to negatively impact their health and survival. Of particular concern now is information showing that mercury contaminant loads of some western DPS Steller sea lion pups are relatively high in some parts of the range where declines are still occurring. Exposure to organochlorines known to affect health impacts in other species have been found in parts of the western and eastern DPSs, though impacts on Steller sea lions are not clear. Emerging environmental contaminants, such as Polybrominated diphenyl ethers (PBDEs) are known to bioaccumulate in marine mammals and thus are also a concern, though Steller sea lion exposure to these compounds is not well understood.

Human-Caused Injuries and Mortality

Steller sea lions may be disturbed by humans in vessels (power, kayak, etc.), manned and unmanned (e.g. drone) aircraft, and on land. When disturbed, Steller sea lions may stampede into the water, which can injure or kill pups and juveniles when they are crushed by adults, and in extreme cases cause pup abandonment. Steller sea lions that are illegally fed can cause them to become habituated, aggressive, lead to more fisheries interactions, entanglements, and illegal shootings as a result of increased frustrations.

Interactions with Active Fishing Gear

Historically, commercial fishing in the Bering Sea and the North Pacific Ocean incidentally killed many Steller sea lions. Changes in fishing techniques and areas and times fished are thought to have significantly reduced incidental take. Amendments to the Marine Mammal Protection Act in 1988 and 1994 required observer programs to monitor marine mammal incidental take in some domestic fisheries and NOAA Fisheries provides estimates of mortality and serious injury of Steller sea lions due to US commercial fisheries in marine mammal stock assessments for the western and eastern DPSs. These estimates are likely an underestimate of the actual levels, since large segments of the fishing industry, (including fisheries near Steller sea lion rookeries, haulouts, and feeding areas, and fisheries with known past interaction with sea lions) have either no or very limited observer coverage. Thus, we are highly uncertain about the impact of this threat. 

Disturbance

Steller sea lions may be disturbed by vessels (power boats, kayaks, paddleboards, etc.) approaching from the water, by manned and unmanned (e.g., drones) aircraft, and by approach from the land. Disturbance to Steller sea lions on terrestrial haulout sites can lead to individuals fleeing toward the water, causing mass stampedes during which pups and juveniles may be injured or killed when crushed by adults. Disturbance can also cause sea lions to flee from the tops of steep rocks or cliff faces, also causing injury or death. Disturbance can lead to interruption of important behaviors such as nursing, resting, breeding, and socializing. Repeated disturbances that cause lactating females to abandon rookeries or use them less often could impact the health and survival of pups by reducing normal nursing cycles. Steller sea lion feeding can also be disturbed by vessel traffic or underwater noise

Entanglement

Entanglement in marine debris and/or ingestion of fishing gear is known to contribute to Steller sea lion injury and mortality. Steller sea lions can become entangled in marine debris or fishing gear, either swimming off with the gear attached or becoming anchored. Once entangled, sea lions may drag and swim with attached gear for long distances, ultimately suffering fatigue, compromised feeding ability, or severe injury that may lead to reduced reproductive success and death. 

Available data indicate that the incidence of entanglement is greater in Southeast Alaska than in areas west of 144° west longitude. Entanglement rates to the west however are likely underestimated due to fewer observations in a less-populated area. Not all entangled animals strand (e.g., they may drown) and not all stranded animals are found or reported. This is true especially in the most remote parts of the range of this species.

Illegal Feeding

Feeding of sea lions is illegal and can lead to close interactions between humans and sea lions that pose risks to both. Feeding-related problems include changes in sea lion behavior; habituation; aggression toward humans; negative impacts to fisheries; and entanglement, injury, and death of animals. Learn more in the  “Take the lead, do not feed” 45-second video.

Illegal Shooting

It is illegal to shoot Steller sea lions unless you are an Alaska Native and the take is for subsistence purposes (food or handicraft) and is not accomplished in a wasteful manner. Historical accounts, especially associated with fisheries, documented substantial mortality to the western DPS due to illegal shooting. While there is likely much less illegal shooting than during the period of sharp decline in the 1970s and 1980s, documented illegal shooting of sea lions in the Copper River Delta area, as well as fishermen’s reports of greater interactions with fisheries are cause for concern. Subsistence use stuck and lost Steller sea lion carcasses further complicate efforts to estimate illegal shooting. Most parts of the range of the western DPS are not systematically monitored for carcasses, so there are no current studies evaluating levels of illegal shooting; large parts of the fishing industry, including fisheries near Steller sea lion rookeries, haulouts, and feeding areas, have either no or very limited observer coverage.

Vessel Strikes

Inadvertent vessel strikes can injure or kill Steller sea lions. There have been four documented cases of Steller sea lions being injured or killed due to nearshore vessel strikes in Alaska since 2007 (PDF: see Table 10). Nearshore vessel strikes are most likely to occur in areas where Steller sea lions are concentrated for feeding or rafting, or near large haulouts or rookeries from which large numbers of animals will be in transit.    

Predation

The primary predators of Steller sea lions are killer whales, sharks, and humans.