First Symposium on Steller’s and White-tailed Sea Eagles in East Asia pp. 111-116, 2000
UETA, M. & MCGRADY, M.J. (eds) ©Wild Bird Society of Japan, Tokyo Japan

Estimation by a computer simulation of population trend ofSteller’s Sea Eagles
Mutsuyuki Ueta1 & Vladimir Masterov2
1. Research Center, Wild Bird Society of Japan. 2-35-2 Minamidaira, Hino, Tokyo 191-0041, Japan.
e-mail: mj-ueta@netlaputa.ne.jp
2. Dep. Vertebrate Zoology, Laboratory of Ornithology, Moscow State University, 199899 Moscow, Russia.

Abstract. The population trend of Steller’s Sea Eagles Haliaeetus pelagicus was estimated using acomputer simulation model. The simulation was conducted using an “individual based model”. Thesimulation breeding success was estimated from 308 nests in the Amur River basin 1996-1998 andmortality of each age class estimated from the number of eagles in each age class in Hokkaido 19971999 (N = 332) to the simulation model. The model predicts that the Steller’s Sea Eagle population
will slowly decrease, and that adult mortality has the strongest affect on the population trend. The
result suggests that lead poisoning in Hokkaido is a major factor in the predicted population decrease.

INTRODUCTION
In order to conserve endangered species, it is important to understand what are importantfactors that might cause population decline. This information is important when constructing
an effective management plan. Population models are an effective method to better understandthe factors that might cause population decline. For raptor species, population models havebeen constructed for Northern Spotted Owls Strix occidentalis (Lande 1988), White-tailed Sea
Eagles Haliaeetus albicilla (Green et al. 1996), and Bonelli’s Eagles Hieraaetus fasciatus (Real& Manosa 1997). In this paper, we estimate the population trend of Steller’s Sea Eagles Ha.
pelagicus by computer simulation model.

METHODS
The population trend of Steller’s Sea Eagles was estimated using a computer simulation
run for a 100 years time span. The wintering population of Steller’s Sea Eagles in Hokkaido,
northern Japan has been estimated 1,500 to 2,000 individuals (WGWS 1996). The simulation
was started from 1,500 individuals.
The simulation used an “individual based model”. This model uses the mean probabilityof life history factors, such as breeding success and mortality rates, and a table of random

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Table 1. Demographic parameters used for simulationof population trend of Steller's Sea Eagles.
Breeding success
Percentage of pairs with no fledgling
Percentage of pairs with a fledgling
Percentage of pairs with two fledglings

47.1%
26.8%
26.1%

Mortality rate
First year
Second year
Third year
Thereafter

68.7%
24.8%
21.5%
6.6%

numbers. Life expectancy of each individual within was decided by the table and theprobability. For example, if the annual mortality rate was 20%, a random number between 0
and 100 would be generated and assigned to each individual bird, and those individuals whoassigned number was < 20 would survive, which those ≥ 20 would die in a given year.
Assembling the results of annual mortality on individuals over many years the trend of the total
population was determined. This model is suitable to show the trend of a small population
because it includes the chance fluctuations in probability, such as successive hatching of one of
the sexes (Horino & Miura 1997).
Parameters used in this simulation model were mean breeding success, and mortality rateof each age class.
Steller’s Sea Eagles wintering in Japan are mainly summering in north Sakhalin, lower
Amur region, Shantar Islands to Magadan (Ueta et al. 2000, McGrady et al. 2000). The
mean breeding success was estimate from 308 nests in Amur in 1996-1998. The Amur Riverbasin was surveyed mainly by small boat. Because almost all large nestlings (≥ 8 weeks ofage) will fledge (Masterov unpublished data), we assumed 100% survival of them to fledging.
To estimate mean mortality rate of each age class, a survey was conducted on thewintering grounds on Hokkaido. We assumed there are no age-specific wintering areas. The
identification method of age was depending on Morioka et al.(1995) and Morioka (2000). We
counted the number of individuals of each age within about 200 m of observation points
because we could not confidently determine age from long distance. The survey was conducted
on Nov. 23 1997, Dec. 17-18 1997, Jan. 16-20 1998, Jan. 18 and Feb 21, 1999. If the exact age
could not be determined, these birds were put into grouped age classes (e.g. 1-2 years old). At
the end of the count, the numbers of individuals in these groups were shared between the ages
they included according to the ratio of known age birds.

RESULTS AND DISCUSSION
Breeding success and mortality rate
Table 1 summarizes breeding success and mortality estimation used in the simulation.
Breeding success was recorded for 121 nests in 1996, 91 nests in 1997, and 96 nests in 1998. It

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2000

1500

1000

500

0025

50

75

100YearsFig. 1. Computer simulation of Steller’s Sea Eagles population. Lines show each 100 times simulations data.

was estimated that 47.1% of pairs raised no fledgling, 26.8% raised a single fledgling, and26.1% raised two fledglings.
In total, 433 individuals were observed in Hokkaido. The percentage of first wintereagles was 23.3%, 7.3% second winter eagles, 5.5% third winter eagles, 4.3% fourth wintereagles, and 59.7% older than fifth winter eagles. From this, we estimated annual mortality rateas: 68.7% for first year, 24.8% for second year, 21.5% for third year, and 6.6% thereafter.
Because we counted the number of individuals in each age class in winter, mortality between
fledging to first winter could not be determined. Therefore, first year mortality was probably
underestimated. We do not believe the underestimation to be great because in the period fromlate summer when eagle fledge to autumn, food for eagles is abundant, because there aremany dead salmon on the river, and because the results of satellite-tracking (McGrady et al.
2000), suggest low mortality. Therefore the underestimation of first year mortality is notthought to be great.
That mortality rate of young eagles is high and low in adult eagles has been shown in
species closely related to Steller’s Sea Eagles. The mortality of Bald Eagles Ha. leucocephalus
in the Aleutians was estimated at around 95% up to the acquisition of adult plumage in theforth to sixth year, and at around 5% per year thereafter (Sherrod et al. 1977). In this study,mortality was 81.5-82.7% up to the acquisition of adult plumage in the forth to fifth year, and6.6% thereafter.
Simulation of population trend
The results from running the simulation 100 times are shown in Figure 1. Within the 100year time period, there was no case of the simulation showing a population increase, and allsimulations showed population declines. The mean simulated population size 100 years in thefuture was 934.6. A slow decline of population that predicted by the simulation agree with

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2000
Simulation
Adult mortality increase

1500

Young mortality increase
Breeding success decrease

1000

500

0025

50

75

100YearsFig. 2. Computer simulation of Steller’s Sea Eagles population under different rates of mortality and breeding
success. Lines show averages of 100 simulations. Solid line show the mean simulation using parameters
shown in Table 1. “Adult mortality increase” shows the simulation with adult mortality rate increased 2%.
“Young mortality increase” shows the simulation with 1-3 years old mortality rate increased 2%. “Breedingsuccess decrease” shows the simulation with 51.1% of pairs failing to breed, 24.8% producing a single
fledgling, and 24.1% producing two fledglings.

observed results in Magadan area (Potapov et al. 2000).
In order to illustrate which factor affects the population trend most, we simulated thepopulation trend increasing adult mortality by 2% (add 2% mortality to older than 5 years oldbirds), again by increasing sub-adult and juvenile by 2%, and again by decreasing breeding
success (51.1% of pairs failing to breed, 24.8% producing a single fledgling, and 24.1%
producing two fledglings). The mean population trends from 100 simulations are shown in
Figure 2. These simulations suggest that adult mortality has the biggest effect on thepopulation trend. A similar result has been shown in Bonelli’s Eagle (Real & Manosa 1997).
In Hokkaido, Steller’s Sea Eagles and White-tailed Sea Eagles are exposed to the threatof lead poisoning (Kim et al. 1999). In the winter of 1997-98, 18 Steller’s and three Whitetailed Sea Eagles were found dead from lead poisoning in Hokkaido, due to ingestion ofbullet fragments from hunted deer (Kurosawa 2000). In the winter of 1998-99, 16 Steller’s andnine White-tailed Sea Eagles were found dead from lead poisoning in Hokkaido (Kurosawa2000). The number of dead eagles is almost one percent of the wintering population ofSteller’s Sea Eagles in Japan. We don’t know what percent of total lead poisoned eagles wefind but it is thought to be low. For the Red-crowned Crane Grus japonensis, it was estimatedthat 18.2% of dead cranes were found (N. Kurosawa per. comm). Because of the large bodysize and color of Red-crowned Cranes, it likely that they are be easier to find than deadeagles. It seems reasonable that perhaps over 5% of the Steller’s Sea Eagle that winter in
Hokkaido may be dying of lead poisoning.
Of further concern is the age structure of dead eagles. Of all eagles killed by lead,percentage of first winter eagles was 23.8%, 14.3% for sub-adult birds, and 61.9% for adult

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birds (N. Kurosawa personal comm.). Because sub-adult birds are second and third wintereagles, and adult eagles are older than forth winter, the age structure of dead eagles is similar topopulation structure, and suggests that mortality by lead poisoning affects all age classes
equally. Lead poisoning’s effect of increased juvenile mortality is likely to cause a slow
decline in the population, but an increase in adult mortality will likely cause a rapid decline inthe population.
The Hokkaido Local Government will forbid to use lead bullet for hunting of deer fromon November 2000 in Hokkaido. Although this is welcomed and is positive in conservation
terms, we need further effort to reduce the mortality by lead poisoning.

ACKNOWLEDGEMENT
We would like to thank N. Kurosawa and K. Saito of Hokkaido Veterinary Medical for informationon lead poisoning, NEC for constructing the computer simulation and funding.

LITERATURE CITED
Green, R.E., Pienkowski, M.W. & Love, J.A. 1996. Long-term viability of the re-introduced population of theWhite-tailed Eagle Haliaeetus albicilla in Scotland. J. Appl. Ecol. 33: 357-368.
Horino, S. & Miura, S. 1997. Population model and its application in wildlife management. Jap. J. Ecol. 47:
189-191. In Japanese.
Kim, E-U., Goto, R. Iwata, H., Masuda, Y, Tanabe, S., & Fujita, S. 1999. Preliminary survey of leadpoisoning of Steller’s Sea Eagle (Haliaeetus pelagicus) and White-tailed Sea Eagle (Haliaeetus albicilla)in Hokkaido, Japan. Environmental Toxicology and Chemistry 18(3): 448-451.
Kurosawa, N. 2000. Lead poisoning in Steller’s Sea Eagles and White-tailed Sea Eagles. In: Ueta, M. &McGrady, M.J. (eds). First Symposium on Steller’s and White-tailed Sea Eagles in East Asia. pp. 107109. Wild Bird Society of Japan, Tokyo, Japan.
Lande, R. 1988. Demographic models of the Northern Spotted Owl (Strix occedentalis caurina). Oecologia
75: 601-607.
McGrady, M., Ueta, M., Potapov, E., Utekhina, I., Masterov, V.B., Fuller, M., Seegar, W.S., Ladyguin, A.,
Lobkov, E. & Zykov, V.B. 2000. Migration and wintering of juvenile and immature Steller’s Sea
Eagles. In: Ueta, M. & McGrady, M.J. (eds). First Symposium on Steller’s and White-tailed Sea
Eagles in East Asia. pp. 83-90. Wild Bird Society of Japan, Tokyo, Japan.
Morioka, T., Yamagata, M., Kanouchi, T. & Kawata, T. 1995. The Birds of Prey in Japan. Bunichi Sogo
Shuppan Co. Ltd., Tokyo Japan. In Japanese with English summary
Morioka, T. 2000. Aging by molt patterns of the flight feathers Steller’s Sea Eagle. In: Ueta, M. & McGrady,M.J. (eds). First Symposium on Steller’s and White-tailed Sea Eagles in East Asia. pp. 11-16. Wild
Bird Society of Japan, Tokyo, Japan.
Potapov E., Utekhina, I. & McGrady, M.J. 2000. Steller’s Sea Eagle in Magadan District and in the North of
Khabarovsk District. In: Ueta, M. & McGrady, M.J. (eds). First Symposium on Steller’s and Whitetailed Sea Eagles in East Asia. pp. 29-44. Wild Bird Society of Japan, Tokyo, Japan.
Real, J. & Manosa, S. 1997. Demography and conservation of western European Bonelli’s Eagle Hieraaetus
fasciatus population. Biol. Conserv. 79: 59-66.

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Sherrod, S.K., White, C.M. & Williamson, F.S.L. 1977. Biology of the Bald Eagle (Haliaetus leucocephalusalascanus) on Amchitka Island, Alaska. Living Bird 15: 143-182.
Ueta, M., Sato, F., Nakagawa, H. & Mita, N. 2000. Migration routes and differences of migration schedule
between adult and young Steller’s Sea Eagles. Ibis 142: 35-39.
Working Group for White-tailed Eagles and Steller’s Sea Eagles (WGWS). 1996. Wintering status of Steller’sSea Eagles and White-tailed Eagles in northern Japan. Survey of the status and habitat conditions ofthreatened species, 1995. pp. 1-9. Environment Agency, Tokyo, Japan. In Japanese with Englishsummary.