Researchers from NOAA’s Alaska Fisheries Science Center have published the results of their 2009 count of Steller sea lion pups in Alaska, which shows improvement in some areas and further decline in numbers in others.
Doug DeMaster, Director of the Alaska Fisheries Science Center said:
“We expected to see the increased Steller sea lion numbers in Southeast Alaska again. The mixed results in the western population, however, indicate that some areas have improved in numbers while others continue to decline, especially the Western Aleutian Islands.”
Steller sea lions, also known as northern sea lions, are the fourth largest pinniped in the world, behind the northern elephant seal, southern elephant seal, and walrus. The males may grow to 11 feet (3.25 m) in length and weigh almost 2,500 pounds (1120 kg). Females are much smaller, and may grow to nine feet (2.9 m) in length and weigh 1,000 pounds (350 kg). Stellers are found throughout the North Pacific Rim from Japan to central California.
Pupping and breeding occur late May to early July throughout their range. Males set up territories on rookeries in late May, females arrive shortly thereafter and give birth to a single pup. They alternate between nursing their pup and feeding trips; most pups are weaned by the end of their first year. The species is not known to migrate, but individuals disperse widely outside of the breeding season, particularly adult males and juveniles.
While Steller sea lion pup production by Alaska’s western stock increased from 9,950 in 2005 to 11,120 in 2009 overall, there was considerable variability. During this period, pup counts increased 18% throughout the Gulf of Alaska and in the eastern Aleutian Islands, but were 6% lower in the western and central Aleutian Islands.
The number of Steller sea lion pups counted in 2009 in Southeast Alaska (7,462) — part of the threatened eastern stock — exceeded any previous counts going back to the 1960s. The new data indicate that pup production has increased at a rate of almost 4% per year at Southeast Alaska’s five major rookeries since the late 1970s.
Steller sea lion biologists also watch the ratio of pups to older animals, since it provides an indication of changes in birth rates. The 2009 numbers suggest that the western Alaska Steller sea lion population has a lower pup to non-pup ratio than the population that inhabits Southeast Alaskan waters.
The number of Steller sea lions in the western stock declined by 75% between 1976 and 1990. The extent of this decline led NOAA’s Fisheries Service to list the Steller sea lion as threatened range-wide under the Endangered Species Act in April 1990. In the 1990’s the decline continued for the western stock in Alaska, which was declared endangered in 1997. The eastern stock remains listed as threatened.
One suspected cause of their precipitous decline is overfishing of Alaska pollock, herring, and other fish stocks in the Gulf of Alaska. Other hypotheses include increased predation by orcas, indirect effects of prey species composition shifts due to changes in climate, effects of disease or contaminants, shooting by fishermen, and others.
In order to safeguard their critical habitat protective zones, catch/harvest limits, various procedures and other measures have been implemented around major haul-outs and rookeries. The IUCN Red List of Threatened Species considers this species to be “Endangered.”
Details of the 2009 pup count can be found at NOAA’s Alaska Fisheries Science Center report.
Copyright © 2009 by Marine Science Today, a publication of OceanLines LLC