STELLER SEA LION

 

Adult male, female and pup on Yam sky Islands in the northeast Sea of Okhotsk

The Steller sea lion is also known as Steller’s sea lion and northern sea lion is a near –threatened species of sea lion in northern pacific. It is the sole member of the genus Eumetopias and the largest of the eared seals. Among pinnipeds, it is inferior in size only to the walrus and the two species of elephant seals. The species is named for the naturalist Georg Wilhelm steller, who first described them in 1741.The steller sea lion has attracted considerable attention in recent decades, owing to significant and largely unexplained declines in their numbers over an extensive portion of their northern range in Alaska.

Distinctive characteristics

Steller sea lions are the largest otariid and the fourth largest pinnped. Both sexents are robust and powerfully built at all ages. They are sexually dimorphic, with adult males weighing three times as much as females, and grow 20-25% longer. In addition to being larger, males have a mane of longer guard hairs extending from the back of the head to the shoulders and all around the neck. Breeding bulls are also very thick and wide in the neck and shoulder area. There is a sagittal crest on the skull that imparts a small to moderate forehead to adult and sub adult males. Adult females and juveniles have no crest and have only a minor forehead dip in front of the eyes and appear to be almost flat from the crown to the tip of the nose. Males also have larger canines that are both longer and thicker than those of females.

                                                                   Steller sea lions have a massive wide head. The muzzle is thick, wide, long, and blunt on the end. The eyes are widely-spaced apart, and set well back from the end of the muzzle and like the ear pinnae, appears small when compared with the size of the head. The vibrissae are pale, conspicuous, and long. Both the fore and hind flippers are long and broad. The fore flippers have serparse, short fur that extends beyond the wrist onto the middle of the dorsal surface of the flipper in a “V” pattern that does not reach the round tip. The rest of the dorsal surface, and the palms of both fore flippers are covered with a hairle black leathery skin. The first digit is the longest, widest and thickest, and curves posteriorly, giving the flipper a swept back look. Digits two- four are successively shorter. There is a small opening in the skin at the end of each digit for a claw that is usually reduced to a vestigial nodule, and rarely emerges above the skin. The claw openings are set back from the free edge of the flippers by cartilaginous rods that extend the length of each toe. The bones of the three central toes terminate at the position of the small nails that emerge through the skin on the dorsal surface, set back from the end of the flipper. The first and fifth toes are longer than the three middle toes, and the first toe, or hallux, is longer and wider than the fifth toe. The hind flippers have short hair covering part of the proximal end of the flipper, and the rest of the dorsal surface, and the entire sole is covered in black leathery hairless skin.

                                                                   Coloration in adults is pale yellow to light tan above, darkening around the insertion of the flippers to brown and shading to rust below. Unlike most pinnipeds, when wet adult stellers are paler, appearing light grayish-tan. Juveniles are darker than adults and are dark tan to light brown. Pups are born with a thick blackish-brown lanugo that is molted by about six months of age. Scars from bites and healed wounds are darker than the background color. The dental formula is I 3/2, C 1/1, PC 5/5.There is a wide diastema (gap) between the 4^th and 5^th post-canines.

Dental formula    I 3/2, C 1/1, PC 5/5.

Size

The maximum length of adult males is about 3.3 m and average weight is 1000 kg. The maximum length for adult female is about 2.5 m and average weight is 273 kg. Pups are born at an average of about 1m and 18-22 kg.        

Life Span    Males live up to 20 years old and female live up to 30 years old.

Gestation Period    274 days.

Can be confused with

Steller Sea lions are readily separated from northern and Guadalupe fur seals that occur within their range by their massive size, large blocky head and muzzle, and pale colour. California sea lions are the most likely species to be confused with stellers sea lions. Careful attention to head and muzzle size and shape. Overall coloration, and length and width of fore- and hind- flippers permits separation. Smaller steller sea lions in the size range of large California sea lions, look like they are more muscular and powerfully built than the California’s, which look more rounded and streamlined. Also, smaller stellers have little or no sagittal crest development and a nearly flat-topped head whereas comparably- sized adult and sub adult California sea lion males have a moderate to large sagittal crest and more pronounced forehead. Steller sea lion eyes also seem smaller and set farther apart, due to the proportionately larger head and wider muzzle.  

Distribution

Steller sea lions are found from central California( formerly southern California), north to the Aleutian islands, and west along the Aleutian chain to Kamchatka, and from there south along the kuril islands to northern japan, the sea of japan, and Korea. Stellers also occur in the sea of Okhotsk. From the Aleutians they range north across the Bering Sea to the Bering Strait. Throughout their range they are usually found from the coast to the outer continental shelf and slope. 

Ecology and behavior

Steller sea lions are polygynous, and breed in the late spring and summer. Adult males arrive at the rookeries before females and those that are nine years or older, claim territories, which they aggressively and vociferously defend. Stellers have deep voices and produce powerful low- frequency rolling roars and can be heard for long distances over the noise of wind and waves. Roaring males often bob their head up and down while vocalizing. This is in contrast to the side to side head wave of California sea lion males when they produce their characteristic repetitive bark.

                                                                   Pups are born from May through July, and females stay continuously ashore with their new-borns for the first week to ten days after giving birth. Following this period of attendance, females make foraging excursions, primarily at night for periods of 18-25 hours, followed by time ashore to nurse their pup. Females come into estrus and mate about two weeks after giving birth. Weaning often takes place before the next breeding season, but it is not unusual to see females nursing yearlings, older juveniles, or multiple offspring.

                                                                  Steller sea lions can leave haul outs in large groups. Sightings at sea are most often of groups of 1-12 animals. They aggregate in areas of prey abundance, including near fishing vessels, where they will feed on netted fish and discarded by-catch. They are not considered migratory, and juveniles and sub adults make most long distances trips. Adults usually forage and live near their natal colonies and return to these sites to breed. The area used by adult females for foraging in winter is considerably larger than the area used in summer. Diving is generally to depths of 200 m or less and dive duration is usually two minutes or less, with both parameters varying by season and age of the animal. Adult females tend to dive deeper in winter than summer. Diving ability of pups and juveniles increases with age, and they routinely dive to depths of around 140 m for periods of two minutes as yearlings. The diving of adult males has not been studied.   

Predation

Steller sea lions are top- tier carnivores, but are susceptible to predation, primarily by killer whales. Shark species also possible predators: sleepers and great whites may prey on juvenile sea lions.

Feeding and prey

Steller sea lions feed on many varieties of fish and invertebrates. Much of the information on diet comes from animals living in Alaska, where they feed on walleye Pollock, pacified cod, Atka mackerel, herring, sand lance, several varieties of flatfish, salmon and rock fish, and invertebrates such as squid, octopus, bivalves and gastropods. Adult females with young pups feed extensively at night, switching to foraging at any time after the breeding season. Adult males are known to occasionally kill and consume young northern fur seals.     

Exploitation

Historically this sea lion was sporadically harvested for fat, meat and “trimmings” (vibrissae and testes). There was a government-sponsored cull in this century that was pursued with the hope of limiting population size and its impact on commercially important fish species. Currently however, the steller sea lion is suffering a massive range-wide population decline for as yet unknown reasons. The chief suspected cause is the over-exploitation of stocks of Pollock in the high north pacific. Other reason suggested for the decline include long term change in ocean temperature, accumulation of human produced toxins and disease. It may be that the decline is attributable to the subtle interaction of several of these factors. In 1990, the steller sea lion was declared a threatened species under the United States endangered species act.

Threats and status

Steller sea lions have been important to the native people living near them for millenia. Native Alaskans currently take between150-300 a year for food and other uses. The worldwide population of steller sea lions declined by 64% during the period from 1960 to 1989, and is currently estimated to be approximately 100,000 animals. The decline has been most dramatic in the large populations from the Gulf of Alaska (-54%), west throughout the Aleutian island chain (-81%), to the moderately sized Russian population (-74%). During the same period, the moderately-sized south eastern Alaska population increased (+70%). The reasons for these changes and the overall decline are unclear, but are the subject of intensive ongoing investigations.    

Factors hypothesized include

The direct and indirect effects of large-scale commercial fisheries on key prey species, long –term ecosystem shifts, and changes in behavior by a primary predator, the killer whale, or a combination of these factors.

IUCN status    Endangered