ISSN 10630740, Russian Journal of Marine Biology, 2011, Vol. 37, No. 7, pp. 549–557. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © A.E. Kuzin, 2011, published in Izvestiya TINRO.
The formation of the reproductive group of Steller
Sea Lions on Tyuleniy Island is believed to have begun
in the 1980s [8, 12]. For a relatively short period, it
turned into the second largest reproductive group in
Far East after the one on the Iony Islands . It should
be noted that the Tyuleniy group is not completely iso
lated. Animals from the Kuril, Iony, and Yamsky
Islands also contribute to its formation . One of the
largest northern fur seal populations, which number
over 130 000 individuals, also inhabits Tyuleniy Island.
The abundances of both Steller sea lions and northern
fur seals have grown rapidly during the recent years,
being promoted, in addition to other factors that
account for dynamic processes in populations of these
mammals, by a low degree of interspecific territorial
competition on breeding sites because of the mis
match in their reproductive cycles  and also by a
high capacity of the environment that is used by these
animals as a feeding area
While the historical maximum of fur seal abun
dance on Tyuleniy Island is known , the potential
limit of the demographic processes in the Steller sea
lion reproductive group is difficult to guess. Thus, the
need to monitor the condition of both the species and
dynamics in their abundances on the island is beyond
doubt and the scope of studies must cover the expan
sion of these processes.
The purposes of this research were (1) the study of
the spatial and structural organization of the Steller
sea lion community during the breeding season, as
Feeding habits in northern fur seals and Steller sea lions are
rather similar; their diets include mainly walleye pollock,
salmon, greenlings, and cod [3, 11], which are the predominant
biological objects in the epipelagic zone of the Southwestern Sea
of Okhotsk during the summer and fall [2, 13–15].
The Contemporary Condition and Some Demographic
Characteristics of the Steller Sea Lion (
Reproductive Group on Tyuleniy Island, Sea of Okhotsk
A. E. Kuzin
Pacific Research Fisheries Center (TINRO Center), per. Shevchenko 4, Vladivostok, 690091 Russia
Received January 28, 2011
—In 2010, the largest part of the Steller sea lion breeding community on Tyuleniy Island was located
on the harem rookery of northern fur seals, which occupied the eastern beach, as well as on the western side
of the island, which was free of fur seals. At the culmination of harem activity on June 29, 26.5% of the ani
mals at the age of 1+ concentrated on the eastern beach and 41.1%, on the western beach in the daytime.
However, 52.3% of the pups were born on the eastern beach and only 30.4% were born on the western beach.
Pups were also present on the capes: 9.1% of the pups were observed on the northern cape and 8.2% on the
southern cape, while the main population on these sites consisted of nonharem bulls, bachelors, and young
animals. At the peak of harem activity, the number of females per one harem bull was 13.1 at sites 1 to 3 of the
eastern beach and each of them, on average, had 1.05 pups; on sites 7
12 there were, respectively, 9.1 females
and 1.42 pups per female, and on the western beach, 21.7 females and 0.64 pups. The resulting abundance of
sea lions on Tyuleniy Island in 2010 exceeded 1500, which was almost ten times as many as their number in
1989. A total of about 100 bulls, 60 harem bulls, 1000 females, and 700 pups were recorded there. Halfbulls
and young animals amounted to onethird of the entire population. Meanwhile the overall sex ratio at the cul
mination of harem activity was 11.5 females per one bull and 18.8 per one harem bull. About 75% of the
females belonged to the parous group. The mortality rate among newborns reached 5.4%. No mortality was
observed in adults. As many as 133 previously branded Steller sea lions were found and 109 of them (81.9%)
were immigrants. Among immigrants, 29% were branded individuals of reproductive groups from the Kuril
Islands, 54% were from the Iony Islands, 16% were from the Yamsky Islands, and about 1% were from Kam
chatka. Fouryearold individuals predominated among the branded immigrants (23.8%). The oldest Steller
sea lion (21 years of age) was one that was branded on the Srednego Islands in 1989. The rate of marked ani
mal return from 175 pups that were branded on Tyuleniy Island the year before was 13.8%.
Steller sea lion, pups, bulls, females, distribution, abundance, mortality, quantity of immigrants.