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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