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Steller Sea Lions Species Description




Species Description

Picture an adult, male African lion—morph that image slightly to befit flippers, add the rugged Alaskan coastline, and an aquatic lifestyle—but keep the roar. This roaring aquatic lion with broad forehead and thick mane prompted German naturalist George Wilhelm Steller to name Eumetopias jubatus or the Steller sea lion on Vitus Bering’s Alaskan journey in 1742. Truly the sea lion king, the Steller sea lion (also known as the northern sea lion) is the largest sea lion in the world belonging to the family Otariidae (meaning “eared seal”), of which there are 16 other species.

Steller sea lions are widely distributed along the rim of the North Pacific Ocean. They can be found from southern California and Oregon, along British Columbia’s coastline, Alaska, Russian Far East, and northern Japan. The majority of the population, however, reside in the Gulf of Alaskan waters—approximately 70 percent. Typically inhabiting coastal and continental shelf waters (within 10-15 nm from land), Steller sea lions have been known to travel into deeper, offshore waters such as the southern Bering Sea to forage. Steller sea lions have also been known to migrate large distances (>400 nm) but it is presumed their routes remain coastal.

Historically, the global Steller sea lion population was quite healthy before the 1980s. Estimates from 1960 to late 1970s suggest the population may have exceeded 250,000 individuals—all indications being a strong population. After the 1980s, however, the overall population took a turn for the worse—about 80% of the U.S. population crashed by 1990. The National Marine Fisheries Service (NMFS) declared the species threatened under the Endangered and Threatened Species Act of 1972. But declines were still continuing to occur. By 1997, NMFS re-listed the Steller sea lion as endangered in the western part of their range after genetic evidence suggested that there were two distinct populations—a western and eastern stock. The delineation point between these two stocks is the 1440W longitudinal line or the middle of Prince William Sound. More recent genetic evidence suggests that there may even be a third distinct population found west of the Russian Commander Islands (Baker 2004).

For nearly two decades, scientists have been trying to identify the cause(s) of the decline, but to date, no single factor can be determined. It is most likely a combination of causes that have led to the decline and subsequent lack of recovery. Since 2000, fortunately, both recognized stocks may be showing signs of stabilization, or even increasing numbers in specific areas. The threatened, eastern stock for example, has recently shown record numbers for that area—approximately 40,000-50,000 individuals where historic numbers of 20,000-25,000 were typical (draft recovery, NRC 2003).

As marine mammals, Steller sea lions make the sea their home for most of their life—however; these animals are more commonly seen near or on rocky outcroppings or beaches. There are two types of land-based locations they use called “haulouts” and “rookeries.” Rookeries are breeding sites where females give birth and nurse their pups. Adult males will mate with females at these locations. Places used for resting are called haulouts. There are 38 rookeries with hundreds of haulouts scattered about the Alaska coastline including the Aleutians and Bering Sea. It is unclear why Steller sealions have selected these locations but it is suspected that different seasons, the type of substrate, how long they have been using the site, and proximity to food resources all play an important role (Calkins and Pitcher 1982a).

Steller sea lions are polygamous—mating with one or more individuals—with males defending territories on rookeries with or without females, and exhibit strong sexual dimorphism (adult males are significantly larger than adult females). Adult males can weigh over 2,000 lbs (900 kg) while females weigh approximately 570 lbs (260 kg). Steller sea lions typically give birth from mid-May to mid-July. Adult males, known as bulls, will attain sexual maturity as early as age seven, but is too small to compete for territories occupied by larger, older males. Females attain sexual maturity as early as 3 years. Although twin fetuses have been reported, it is extremely rare and uncommon. Females will remain on the rookery after giving birth (referred to as the parturition time) from 5-13 days with males mating about 11 days after females give birth. Females will nurse their pups for 1-2 years or longer. Gestation lasts for about a year, but there is a “delayed implantation” period where the egg is not fertilized until 3.5 months after breeding.

 

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