Tag Archives: Steller Sea Lion

Steller Sea Lions, Part 3

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This week I will discuss recent and current research on Steller sea lions as well as theories to explain why their numbers have decreased so rapidly over the last several years.

Steller sea lion females live up to thirty years, while males have a maximum life span of twenty years.  Males have a much higher mortality rate than females, probably at least in part due to the stresses incurred by securing and maintaining territories.  By the time they are ten years old, there is a three to one ratio of females to males.

Stellers die from a number of causes; many are well-understood, but the underlying reasons for their dramatic population decline are still a mystery.  A high number of aborted Steller sea lion fetuses are found in the wild, and it is estimated that less than one-third of all pups reach sexual maturity.  Pups may be washed off the rookery by storm waves or killed by adults tossing, biting, or crushing them.  A pup may also be abandoned by his mother or die from disease or starvation.  Threats to Steller sea lions of all ages include disease, loss of habitat, contaminants and pollutants, boat strikes, shooting by humans, entanglement in fishing nets and ocean debris, and indirect impacts, such as competition with fisheries for important food sources, including walleye Pollock.

It is known that sea lions are preyed upon by killer whales and sharks, but a recent study by a biologist at Oregon State University and a biologist with the Alaska Sea Life Center pinpointed a surprising possible predator of sea lions.  Pacific sleeper sharks are a large, slow-moving species of shark that until recently were believed to be scavengers or to prey on fish.  Pacific sleepers can grow to twenty feet (6.1 m) long, and there is now evidence that they may prey upon sea lions, although the incidence of this predation is unknown.  Biologists inserted “life-history transmitters” into the abdomens of thirty-six juvenile Steller sea lions.  These transmitters record temperature, light, and other properties during the sea lions’ lives.  When a sea lion dies, the tags either float to the surface or fall out on shore and transmit the data by satellite to researchers.  Seventeen of the original thirty-six tagged sea lions have died.  Fifteen of the transmitters indicated the sea lions had been killed by predation.  Usually when a sea lion is killed, the tag is ripped out of the body and floats to the surface, recording a rapid temperature change and exposure to light.  Three of the predation deaths were different, though.  They recorded an abrupt drop in temperature, but they did not float to the surface and sense light, indicating that they were still surrounded by tissue.  The obvious explanation is that they were eaten by a cold-blooded animal such as a shark.  The only other possible shark candidates in the area are great white sharks and salmon sharks, both of which have counter-current heat exchangers in their bodies, giving them higher body temperatures than those recorded.  Biologists believe the only possible predator in the area that is large enough to eat a sea lion and has a body temperature as low as those recorded is a Pacific sleeper shark.

While still much more research is needed to definitively identify Pacific sleeper sharks as predators of sea lions and to understand how many sea lions sleeper sharks actually kill and eat, the possible ramifications are troubling.  Ground fish harvests in some area of the Gulf of Alaska have been limited in recent years to reduce competition for fish that are preferred by Steller sea lions.  It is possible, though, that limiting fishing has led to more fish, providing a food base for a larger population of Pacific sleeper sharks, and adult sleeper sharks may in turn prey on sea lions.  If this is true, then management directives may have harmed rather than helped the Steller sea lion population in the Gulf of Alaska.

The relationship between Pacific sleeper sharks, sea lions, and ground fish is still not well understood, and it is a good example of the complexities of the North Pacific food web.  Understanding why Steller sea lion populations, as well as populations of other pinnipeds, are decreasing in certain areas is not an easy undertaking.  Several factors have been suggested to explain the decline of the western Steller sea lion population in the last three to four decades.  Possible reasons are described as “top down” processes and “bottom up” processes.  Top down processes include predation by killer whales or sharks; killing by humans, either directly such as by shooting, or indirectly by entanglement in fishing gear or ocean debris; and harassment of sea lions, especially at rookeries.  Bottom-up processes include reduced prey quality and abundance, either due to competition with commercial fisheries or for some other reason; long-term shifts in their environment, such as changes in ocean temperature or an increase in contamination; and disease.  At the present time, no one or combination of these factors sufficiently explains the decline of the western population of Steller sea lions.

There are currently a number of scientific studies examining the nutritional and biological needs of Stellers.  An interesting result from a study by Carla Gerlinsky at the University of Washington showed that under-nourished sea lions are able to dive for a slightly longer period of time than unstressed sea lions when foraging for food.  However, while the nutritionally-stressed sea lions are able to dive and therefore forage longer, they need more time on the surface to recover between dives, leading to longer foraging trips requiring more energy.  These longer foraging trips also increase the risk of predation at sea and reduce the amount of time a female can spend feeding and taking care of her pup.

Biologists and fisheries managers are also working on practical solutions to decrease human/sea lion conflicts, such as non-lethal ways to deter sea lions from raiding commercial fishing nets, signage near harbors and fish-cleaning stations to remind people that feeding sea lions is a federal offense, and methods of keeping fish-cleaning stations tidy, so sea lions can’t help themselves to fish scraps.  In Kodiak, sea lions were hauling out on an old breakwater float in the boat harbor, causing continual conflicts with humans at the harbor.  When the old float was replaced with a new one, the old float was moved away from the dock, and the sea lions that had already staked claim to the float, moved with it, leaving the new float sea-lion free for human use.

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Steller Sea Lion, Part 2

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This week I’ll tell you a little about Steller sea lion reproductive behavior and biology.

Steller sea lions use both haul-outs and rookeries.  Rookeries are breeding colonies where sea lions mate, and females give birth; and haul-outs are areas where sea lions rest.   Rookeries are used during the mating and pupping season by adults and pups.  Haul-outs are sites used by some non-breeding adults and sub-adults throughout the year and by adults during times other than the breeding season.

Female Steller sea lions reach sexual maturity between the ages of three and six, and most breed every year.  Males are sexually mature between the ages of three and seven, but they are not physically mature and large and strong enough to hold territories until they are nine to ten years old.  Male Stellers are very territorial, and holding and defending a territory is physically exhausting.  Not only must they sometimes engage in fierce, often bloody, fighting with other bulls, but a male often goes without eating for one to two months while he stays on the rookery to defend his territory.  Because of these exhaustive physical demands, males hold territories for an average of only two years, which means they only have a few mating seasons.  It is probable that most males never breed, but the largest, strongest, most successful bulls are those that hold territories, and they mate with many females, passing on their genes to the next generation.

Bulls come ashore at rookeries in mid-May, and they use vocal and visual displays to establish territories, sometimes fighting with other males.  Bulls defending a territory will remain on the rookery until mid-July without eating or drinking.  Females arrive soon after the males and give birth to a single pup within three days of their arrival.  Females remain with their pups for five to thirteen days before leaving the rookery every one to three days to feed, and feeding trips generally last less than 24 hours.  Pups usually nurse for one year, but unlike other pinnipeds for which weaning is predictable, Steller pups may continue to nurse for up to three years.  Mothers use smell and vocalizations to create a bond with a newborn pup.

Approximately two weeks after giving birth, a female Steller will mate again.  Like many other animals, Steller sea lions exhibit delayed implantation.  While a female breeds in June, the fertilized egg does not implant on the uterine wall until October, making the gestation period, from implantation until birth, approximately seven to eight months.    Pups are able to swim and crawl soon after they are born.  They are approximately 3.3 ft. (1 m) in length and weigh between 35 and 50 lbs. (16-22.5 kg). 

 Next week’s post will cover some surprising new research about Steller sea lions and a possible predator that may be at least partially responsible for the decrease in Steller populations in the North Pacific.

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Steller Sea Lion

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Steller sea lions are impressive animals, and you wouldn’t want to run into one in a dark alley, or even on a fishing dock.  A large bull can way over a ton, and they have a have nasty attitudes to go along with all that blubber.  For all you Star Wars fans, I’ve always imagined that Jabba the Hutt was created with a Steller sea lion in mind.  For this post and the next two, I will write about Steller Sea lions, their biology, distribution, social structure, and some amazing new research pinpointing a surprising possible predator of Stellers.

The Steller or Northern sea lion (Eumetopias jubatus) is a member of the order Pinnipedia, which includes harbor seals and walruses, and it is the largest species in the family Otariidae, the “eared seals”.  This family also includes the California sea lion and the Northern fur seal. Otariids, unlike phocids (the “true seals”), have external ear flaps, an elongate neck, long fore flippers used for propulsion, and hind flippers that can rotate, allowing sea lions to use all four limbs for movement on land.  They are called sea “lions”, because adult males have thick necks with long fur on the neck, resembling a lion’s mane.  Steller sea lions were named after German physician Georg Steller, who was the naturalist on the 1741 Russian expedition led by Vitus Bering.

Steller sea lions are found from southern California, along the coastline of the Pacific rim to northern Japan, but most of the breeding rookeries are located from the Gulf of Alaska to the Aleutian Islands.

Steller sea lions exhibit marked sexual dimorphism.  Males, on the average, are 1.3 times longer than females, but they weigh 2.5 times more than females.  Adult male Stellers average 1500 lbs. (750 kg) and are 9 ft. (2.7 m) in length.  A maximum-sized male can weigh as much as 2500 lbs. (1120 kg) and be 10 -11 ft. (3-3.4 m) in length.  Females average 600 lbs. (272.7 kg) and are 7 ft (2.1 m) in length, but may weigh as much as 770 lbs. (350 kg).

A Steller sea lion has a hefty body and a blunt snout.   A male has a distinctive forehead and a mane of long hair on the back of his neck, shoulders, and chest.  This mane not only protects him from cold air and water temperatures and from jagged rocks on his haul-outs and rookeries, but it also protects him when fighting with other males.  Pups are dark brown at birth, and since the tips of their hair are colorless, they appear frosty.  Their hair lightens after their first molt.  Adults are blonde to reddish- brown with dark- chocolate-brown on their undersides and flippers.  Females are usually lighter in color than males.  A Steller’s fur is thick and coarse, and they shed or “molt” their fur every year.  The molt takes approximately four weeks and occurs in the late summer or early fall.

A sea lion has a streamlined body shaped like a torpedo, which reduces drag when moving through the water.  This streamlining is due to a thick layer of blubber under the skin.  Stellers have long, wing-like fore flippers that they stroke up and down to thrust themselves through the water in a movement that resembles flying.  They use their hind flippers for steering.  Unlike harbor seals, sea lions are able to fold their hind flippers under their bodies to walk on land.  They are quite agile on land, and an adult male Steller can easily out-run a human.

Stellers, like all eared seals, have small, external ear horns. Biologists believe that hearing is one of the most important senses for a sea lion, and they probably have acute hearing under water and fairly good hearing in air.

Steller sea lions are very vocal.  At a haul-out, you may hear growls, roars, and grumbles from the older sea lions, along with lamb-like vocalizations from young pups.  Unlike California sea lions, Stellers do not bark.

I’ll have more about Stellers next week. Meanwhile, if you haven’t already done so, visit my home page and sign up for my Mystery Newsletter.