Species Spotlight: Steller Sea Lions

If you were on Reddit, watched the news, or were in San Francisco last month, you likely heard about (or literally heard) “Chonkers”, the 2,000-pound Steller sea lion who went viral for frequenting Pier 39. While sea lions, notably California sea lions, are not rare to the pier, Steller sea lions are known to live in the cold waters of the North Pacific, and seeing one in the bay is an uncommon treat.

If you’ve ever seen a California sea lion, imagine one that spent the winter bulking up for a strongman competition and now has the face of a grizzly bear.

Steller sea lions are the largest sea lions in the world and among the largest pinnipeds, surpassed only by elephant seals and walruses. Adult males can weigh up to 2,500 pounds and reach lengths of 11 to 12 feet, while females are noticeably smaller at up to 1,000 pounds and around 9 feet long. At birth, pups weigh just 35 to 50 pounds, making them tiny by Steller standards.

Size alone is often enough to identify a Steller sea lion, but there are several other distinguishing features. Unlike California sea lions, whose coats appear dark brown when wet, Steller sea lions retain their pale blond to reddish-brown coloration even after emerging from the water.

Adult males are particularly unmistakable. As they mature, they develop a thick neck, broad chest, and coarse mane-like hair extending from the back of their head to their shoulders, giving them an appearance that more closely resembles their terrestrial namesake than many other sea lion species.

Like all sea lions, Stellers rely on long, sensitive whiskers called vibrissae to help locate prey underwater. Combined with their powerful fore flippers, these adaptations allow an animal weighing more than a ton to move through the water and on land with surprising agility.

Being the largest sea lion on Earth comes with some unique challenges. A fully grown male Steller sea lion can weigh as much as a small car but still needs to be agile enough to catch fast-moving prey in the cold waters of the North Pacific.

Fortunately, Steller sea lions aren’t particularly picky eaters. They feed nocturnally on a variety of over 100 fish and cephalopods, including pollock, herring, salmon, cod, squid, and octopus, by routinely diving hundreds of feet in search of prey. At up to an astonishing 1,400 feet below the surface (the deepest recorded dive by a Steller sea lion), the water becomes dark and murky, limiting their ability to see prey, so they use their sensitive whiskers, to detect subtle water movements from nearby fish to aid their hunts. Their diet shifts throughout the year depending on location and prey availability, and adults may consume 6% to 8% (12% for pups) of their body weight each day just to meet their energy demands.

However, come breeding season between May and July, mature adult males (bulls) can go up to two months without eating as they protect their breeding territories. Only a few bulls will father the entirety of the pups in a given mating season, and the battles to establish territory and defend from rival males can be violent, involving charging, biting, and loud vocal displays that can be heard across the colony.

Each season, the adult females return to rookeries where they give birth to pups conceived the previous year. Remarkably, females often arrive already pregnant, give birth within days of reaching the rookery, and then mate again just weeks later.

For the next year or more, mothers invest heavily in raising their pups. They alternate between nursing on shore and foraging trips at sea, returning to locate their offspring among hundreds or even thousands of other sea lions using a combination of vocal and scent cues. Pups may continue nursing for more than a year, and in some cases even longer.

When they’re not hunting, mating, and taking care of young, Steller sea lions are highly social animals. They gather in large groups on rocky coastlines, islands, and beaches known as haul-outs. These gatherings can be surprisingly noisy, filled with roars, growls, and bellows, unlike the high-pitched rhythmic barking from California sea lions.

Like many marine mammals, Steller sea lions have experienced dramatic population changes over the past century.

The species is divided into two distinct populations: an eastern population that ranges from southeast Alaska to California and a western population that stretches from the Gulf of Alaska into Russia. While the eastern population has increased in recent decades, the western population experienced a steep decline beginning in the 1970s, losing more than 80% of its individuals. By 1990, the species was listed under the Endangered Species Act, and for years scientists worked to understand what was driving the decline.

Unlike many conservation stories where a single threat is clear, the decline of Steller sea lions likely resulted from multiple factors like changes in prey availability, shifting ocean conditions, commercial fishing, disease, contaminants, and predation by killer whales. While no single explanation fully accounts for the decline, the widely accepted understanding is that several pressures likely acted together to stress populations.

The mystery surrounding their decline has made Steller sea lions one of the most studied marine mammals in the North Pacific. Researchers have used satellite tags, aerial surveys, genetics, diet analysis, and long-term population monitoring to better understand their movements, behavior, and health. These efforts have not only produced Steller sea lion conservation data but have also improved our understanding of broader ecosystem changes occurring throughout the region.

After decades of these conservation efforts and population monitoring, Steller sea lion populations have stabilized and even increased. In 2013, the eastern population was removed from the Endangered Species Act, and in 2025 the western population was reclassified from endangered to threatened.

For a species whose future once seemed bleak, Chonkers’ appearance at Pier 39 provided a welcome reminder that Steller sea lions remain a thriving part of North Pacific ecosystems. While a single sea lion doesn’t represent an entire population, his brief stint as a local celebrity introduced countless people to a species with a remarkable story of resilience and conservation efforts.

Thumbnail & Hero: Male Steller sea lion among a colony of females, courtesy of Andrew Trites.