Tag Archives: Kodiak Island wildlife

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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Sea Otters

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Sea otters evoke strong emotions in humans.  Most of us can’t help but say, “Ahhhh!” when we see a cute furry otter floating on its back or looking at us in surprise, front flippers held high in the air.  Watching a baby sea otter sitting on its mother’s stomach, or hearing one break open a clam on a rock makes most of us smile.  Many fishermen, though, are not fond of sea otters and for a good reason.  Sea otters are so efficient at finding and eating shellfish that they are able to reduce populations of abalones, clams, and sea urchins to the point where a commercial fishery for these species is not viable in areas with large sea otter populations.  In this post and my next two posts, I’ll discuss sea otters and their fascinating biology and behavior.

While sea otters are the second smallest marine mammal, they are the largest members of the mustelid family, which also includes freshwater otters, weasels, minks, skunks and badgers. Sea otters may weigh as much as 100 lbs (45.5 kg).  The average adult California female weighs 44 lbs. (20 kg), and the average male weighs 64 lbs. (29 kg).  In Alaska, the average adult female is 4 ft. (1.2 meters) long and weighs 60 lbs. (27.3 kg), while the average adult male is 5 ft. (1.5 meters) long and weighs 70 lbs. (31.8 kg).

River Otter

Sea otters are the only mustelid in the genus Enhydra, and they are significantly different from all other mustelids. Sea otters are one of nine to thirteen (taxonomists disagree on the exact number) species of otters found around the world. Except for sea otters and the endangered species of marine otters, all other otters live primarily in freshwater, although river otters (Lutra canadensis) travel freely between rivers and the ocean, and on Kodiak, it is common to see river otters swimming near shore in the ocean.  River otters and sea otters resemble each other, but sea otters are larger and weigh two to three times more than river otters.  Sea otters have adapted to a life in the ocean with hind feet that are webbed to the tips of their toes and resemble flippers.  River otters also have webbed feet, but they are small, making it easier for river otters to move on land, while sea otters are very clumsy out of water. A sea otter’s tail is flat and looks like a paddle, while a river otter has a long, round tail that tapers to a point. The claws in the forepaws of a sea otter can be extended, but those of a river otter cannot. River otters swim on their stomachs, and although sea otters can also swim on their stomachs, they usually swim on their backs while paddling with their hind flippers.  River otters give birth to litters of up to four pups, but sea otters, like other marine mammals, usually only give birth to a single pup.

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There are three subspecies of sea otters.  Enhydra lutris lutris ranges from the Kuril Islands to the Commander Islands in the western Pacific Ocean.  The sea otters in this subspecies are the largest and have a wide skull and short nasal bones. The Southern sea otters, or the California sea otters as they are commonly called (Enhydra lutris nereis), are found off the coast of central California.  Sea otters in this group are smaller and have a narrower skull with a long rostrum and small teeth.  The vast majority of sea otters belong to the subspecies Enhydra lutris kenyoni, the Northern sea otters.  This subspecies ranges from the Aleutian Islands to British Columbia, Washington, and northern Oregon.

Before the 1700’s, an estimated 150,000 to 300,000 sea otters inhabited the area from northern Japan to the Alaska Peninsula and along the Pacific coast of North America to southern California. Between 1741 and 1911 when sea otters were aggressively harvested for their luxurious furs, the population dropped to only 1000 to 2000 animals, and they had been eliminated from much of their original range. Many biologists believed the population was headed toward extinction. In 1911, the International Fur Seal Treaty was signed by the U.S., Russia, Great Britain, and Japan, stopping the commercial hunting of sea otters, and slowly, their numbers began to increase.  Sea otters began re-colonizing much of their former range and were reintroduced to other areas.  Sea otters now occupy about two-thirds of their historical range.

 Counts between 2004 and 2007 estimate the worldwide sea otter population at approximately 107,000 animals. Sea otter populations are considered stable in most areas, although California populations have plateaued or slightly decreased, and there has been a drastic decline in sea otter numbers in southwest Alaska, from Kodiak Island through the western Aleutian Islands.  This area once contained more than half of the world’s sea otters, but the population has declined by at least 55 to 67 percent since the mid 1980’s, and in 2005, the U.S. Fish and Wildlife Service listed this distinct population segment as Threatened under the Endangered Species Act. In 1973, the otter population in Alaska was estimated at between 100,000 and 125,000 animals, but by 2006, the population had fallen to approximately 73,000 animals, mainly due to declines in the Southwest Alaska District Population Segment. The cause of this decline is unclear, but evidence suggests that it may be due to increased predation by killer whales.

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