Figure 3. Preliminary analysis of direction of dispersal and mixing of Steller sea lions on rookeries during the reproductive period, 2002–07 (females of age 4+).
Figure 4. Preliminary data on stock structure of Steller sea lions in Asia: W = Commander Islands (western population); E = East Kamchatka; K = Kuril Islands; O = Sea of Okhotsk; O-i = Iony Island; O-y = Yamsky Islands.
- Dispersion Among Rookeries:
We used the annual sightings of sea lions on the rookeries to examine the level of mixing and dispersion among those rookeries (Fig. 3). On average, 21% (0%-33%) of the marked population resighted on rookeries during the breeding season were nonresident animals (of all ages and sexes born at different rookeries). Approximately 24% (0%-44%) of the mature females (age 4+) were nonresidents, while only 9% (0%-19%) of females that gave birth were nonresidents.
More nonresident individuals of all ages and sexes were recorded on rookeries that were within 100 km of another rookery (Lovushki and Raykoke Islands) or located at specific geographic areas close to common migration routes (Antsiferov and Tuleny Islands and Kozlov Cape). The minimum distance between natal and breeding rookery among nonresident marked females was 45 km, and the maximum was over 1,400 km.
A high level of isolation was recorded for the remote sites of Medny Island (Commander Islands) and the Yamsky Islands in the Sea of Okhotsk. A newly established rookery was discovered on Tuleny Island in the 1980s, and we found that sea lions on this site primarily dispersed from the northern part of the Sea of Okhotsk (Iony and Yamsky Islands) but not from the Kuril Islands, as was previously believed.
During the breeding seasons of 2002–07, 332 marked sea lions (of all ages and sexes) from seven rookeries were encountered on Tuleny Island. Approximately 71% (236) of those animals came from the northern part of the Sea of Okhotsk (mainly from Iony Island) and only 29% (96) from the Kuril Islands.
Steller sea lions from Kamchatka or the Commander Islands were not encountered on this rookery. An interesting phenomenon of a one-way dispersal of sea lions between the Commander Islands and Kamchatka Peninsula was also noted. Of 721 sea lions branded on Medny Island (Commander Islands), 19% (134) repeatedly traveled to Kozlov Cape (Kamchatka Peninsula) in 2002–07. The distance between these rookeries is 415 km, but some animals visited both rookeries during one season.
Three branded females from Medny Island gave birth to pups and copulated on Kozlov Cape. Of 350 Steller sea lions branded on Kozlov Cape during the same time, none were ever observed on Medny Island. For unknown reasons, dispersion of animals between these rookeries occurs in only one direction—from Medny Island to Kozlov Cape.
We used repeated encounters of 5,242 marked Steller sea lions to examine the current metapopulation structure of Steller sea lions in Asia (Fig. 4.) Currently, there are four relatively independent reproductive groups (subpopulations) in the Russian Far East: Commander Islands, Kamchatka, Kuril Islands, and the Sea of Okhotsk.
The Commander subpopulation inhabits waters near the Commander Islands, the east coast of Kamchatka, and the western part of the Bering Sea. Steller sea lions from this subpopulation are also widespread to the east from the Commander Islands and have been observed along the Aleutian Islands, near the Pribilof Islands, in Bristol Bay, near Kodiak Island, and along the Kenai Peninsula coast in the Gulf of Alaska.
The home range of the Kamchatka subpopulation mainly covers the waters along the east coast of Kamchatka and the western part of the Bering Sea. Some juvenile males from Kozlov Cape have been observed in the northern half of the Kuril Islands. The Kuril subpopulation breeds on the Kuril Islands, but its home range stretches from the Olutorsky Gulf in the western part of the Bering Sea to the coast of China, including the coast of Hokkaido Island and all of the Sea of Okhotsk.
The Sea of Okhotsk subpopulation breeds mainly in the northern part of the Sea of Okhotsk and near the Sakhalin coast. Possibly, it consists of two smaller, relatively isolated reproductive groups, located on Iony and Yamsky Islands. The home range of this subpopulation includes all of the Sea of Okhotsk, the coast of Hokkaido, and the northern part of the Sea of Japan (up to the Tatar Strait).
Juvenile males are regularly encountered near the Kuril Islands and southeast Kamchatka coast. A thorough genetic analysis of over 3,000 skin biopsy samples from the Russian population is being used to examine the levels of genetic isolation that may be present within the population. (continued)
