Steller sea lion
|Steller sea lion|
|Binomial name||Eumetopias jubatus|
The Steller sea lion (Eumetopias jubatus) is the largest member of the Otariid (eared seal) family, and one of several animals named for zoologist and explorer Georg Wilhelm Steller. Males may be up to 325 cm (10–11 ft) in length and can weigh up to 1,100 kg (2,400 lb). Females are smaller than males, 240–290 cm (7.5-9.5 ft) in length and up to 350 kg (770 lb) in mass. Males and females are light buff to reddish brown and slightly darker on the chest and abdomen; naked parts of the skin are black. Wet animals usually appear darker than dry ones. Pups are about 1 m (3.3 ft) in length and 16–23 kg (35-50 lb) at birth and grow to about 30–40 kg (65-90 lb) after 6–10 weeks. Pups are dark brown to black until 4 to 6 months old when they molt to a lighter brown. By the end of their second year, pups have taken on the same pelage color as adults.
Bulls become mature between 3 and 8 years of age, but typically are not massive enough to hold territory successfully until 9 or 10 years old. Females reproduce for the first time at 4 to 6 years of age, bearing at most a single pup each year. Pups are born from late May through early July, with peak numbers of births during the second or third week of June. Females stay with their pups for about 9 days before beginning a regular routine of foraging trips to sea. Females mate 11 to 14 days after giving birth. Implantation takes place in late September or early October, after a 3-4 month delay. Weaning is not sharply defined as it is for most other pinniped species, but probably takes place gradually during the winter and spring prior to the following breeding season. It is not uncommon to observe 1- or 2-year-old sea lions suckling from an adult female.
Steller sea lions are opportunistic predators, feeding primarily of a wide variety of fishes and cephalopods. Prey varies geographically and seasonally. Some of the more important prey species in Alaska include walleye pollock (Theragra chalcogramma), Atka mackerel (Pleurogrammus monopterygius), Pacific herring (Clupea harengus), Capelin (Mallotus villosus), Pacific sand lance (Ammodytes hexapterus), Pacific cod (Gadus macrocephalus), and salmon (Oncorhynchus spp.). Steller sea lions have been known to prey on harbor seal, fur seal, ringed seal, and possibly sea otter pups, but this would represent only a supplemental component to the diet.
Steller sea lion are distributed across the North Pacific Ocean rim from northern Hokkaido, Japan, through the Kuril Islands, Okhotsk Sea, and Commander Islands in Russia, the Aleutian Islands, central Bering Sea, and southern coast of Alaska, and south to the Channel Islands off California. During the May-to-July breeding season, Steller sea lions congregate at more that 40 rookeries, where adult males defend territories, pups are born, and mating takes place. Non-reproductive animals congregate to rest at more than 200 haul-out sites where little or no breeding takes place. Sea lions continue to gather at both rookeries and haul-out sites outside of the breeding season.
The world population of Steller sea lions includes two stocks divided at 144° W longitude (Cape Suckling, just east of Prince William Sound, Alaska). The stock differentiation is based primarily on differences in mitochondrial DNA, but also on differing population trends in the two regions.
The number of Steller sea lions in the western stock declined by 75% between 1976 and 1990. The extent of this decline led the National Marine Fisheries Service (NMFS) to list the Steller sea lion as threatened range-wide under the Endangered Species Act (ESA) in April 1990. In the 1990s, the decline continued for the western stock in Alaska, which was declared endangered in 1997. The eastern stock, which has increased at about 3% per year since the 1980s, remains listed as threatened.
Many factors could have contributed to the decline of the western Steller sea lion stock in the 1980s and 1990s. These include factors that cause mortality directly, such incidental take in fisheries, illegal and legal shooting, predation or certain diseases, as well as other factors that indirectly would lead to population declines by reducing productivity. Such indirect factors include the effects of climate change or fisheries, which would alter prey abundance, distribution or species composition leading to nutritional stress, as well as the effects of certain diseases or contaminants. NMML, along with its research partners in the North Pacific, has been conducting research to determine how each of these factors may have affected sea lions in the past and may be currently affecting sea lion recovery. This research, summarized here, involves investigations of Steller sea lion population dynamics, foraging ecology, physiology and biology, as well as studies of killer whales, climate change, fisheries impacts, sea lion diseases, and the distribution and effect of various pollutants.
In addition to research, NMFS has promulgated regulations on human activity to promote sea lion recovery. As part of the emergency rulemaking when Steller sea lions were first listed as threatened in 1990, NMFS prohibited shooting at or near Steller sea lions, reduced the number that could be killed incidental to fishing operations, and designated 3-mile no-entry zones around rookeries (breeding locations) in western Alaska to protect sea lions from disturbance. In 1992, under the authority of the Magnuson Fisheries Conservation and Management Act, NMFS established no-trawl zones around the same rookeries to protect sea lion prey resources. NMFS listed all rookeries, major haul-out sites, and aquatic feeding areas in the southeastern Bering Sea and in Shelikof Strait as Critical Habitat under the ESA in 1993. In 2001-2003, a complex suite of regulations to manage Steller sea lion and various groundfish fishery interactions were enacted, and a description of these regulations can be found on the National Marine Fisheries Service/Alaska Region website.
|License:||This work is in the Public Domain in the United States because it is a work of the United States Federal Government under the terms of Title 17, Chapter 1, Section 105 of the U.S. Code|
|Source:||File available from the United States Federal Government .|