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国内血統登録書から分析したゴマフアザラシの生存状況
寺沢 文男
新江ノ島水族館
（日本動物園水族館協会種保存委員会ゴマフアザラシ種別調整者）

要約
1）1986年から2010年までのゴマフアザラシ血統登録台帳25年分を用いて，飼育下における生存状況を分析した．対象は，飼育下繁殖個体（以下，繁殖）582頭，野生由来保護個体（以下，保護）487頭，入手経路不明個体（以下，不明）26頭であった．
2）繁殖と保護それぞれを性別で分けた全4群における最高年齢は，保護メスが推定38歳，繁殖メスが34歳，保護オスが推定34歳，繁殖オスが31歳であった．このうち2010年末時点で生存中の個体は繁殖メスのみであった．
3）調査時における個体の生死にかかわらず，30歳以上の頭数は，オス11頭（繁殖3頭，保護7頭，不明1頭），メス10頭（繁殖5頭，保護5頭）であった．
4)25年間の各調査年度における生存個体の増加数は，20～29歳の年齢層において最も顕著であり，保護オスが最多の13頭，繁殖メスが最少の7頭であった．
5）4群それぞれにおける平均余命年数は，繁殖オス13.7歳，保護メス12.5歳，保護オス11.9歳，繁殖メス11.0歳であった．なお，繁殖個体は出生年月日，保護個体は来園年月日を起点とした．
6）5年ごとの級間隔で分けた飼育下個体群全体の平均余命年数は，1991～1995年8.1歳，1996～2000年11.9歳，2001～2005年12.6歳，2006～2010年15.6歳であった．
7)10歳ごとの各級間隔および30歳以上の年齢階層で分けた死亡頭数は，0～9歳の年齢層が最も多く，その中でも繁殖メス0歳は51頭で最多だった．



Survivorship for Spotted Seal, Phoca largha, Analyzed from Internal Studbook
Fumio Terasawa
Enoshima Aquarium
(Coordinator of Spotted Seal Studbook, JAZA)

ABSTRACT
The current status of captive survivorship of the Spotted Seal was surveyed through the Japanese Association of Zoo and Aquarium Studbooks from 1986 to 2010. This analysis was based on 582 animals born in captivity (ABCs), 487 animals rescued from the wild (ARWs) and 26 animals of unknown origin.

The seals were divided into four groups, male ABCs, female ABCs, male ARWs and female ARWs. The maximum age was estimated to be 38 years for female ARWs and 34 for male ARWs, and was 34 for female ABCs and 31 for male ABCs. On 31 December 2010, one female ABC of maximum age was alive while the other three seals died.

Over these 25 years, there were 11 males (3 ABCs, 7 ARWs and 1 of unknown origin) and 10 females (5 ABCs and 5 ARWs) which were over 30 years-old. Among these, 4 male (4 ARWs) and 6 female (3 ABCs and 3 ARWs) were still alive on 31 December 2010.

The survival numbers at the end of every year were calculated from 1986 to 2010. The results showed that the number of seals aged between 20 and 29 years increased the most compared to the numbers between 0 and 9 years, 10 to 19 years and for those over 30 years. In seals aged between 20 and 29 years, the smallest number was 7 female ABCs. However, the largest number was 13 male ARWs. Seals over 30 years old first appeared from 1992, and the first was a female ABC.

The number of days survived in captivity was calculated using a technique presented by DeMastar and Drevenak (1988). In the present study, 167 (including 59 cases of unknown sex) stillbirths and abortions, and 42 seals of unknown arrival date were excluded from the analysis. For both male ABCs and female ABCs, the days from birth to death, or to 31 December 2010 were used. Also, for both male ARWs and female ARWs, the days from arrival date to date of death, or to 31 December 2010 were used.

The expected number of years for the four groups was calculated to be 13.7 (male ABCs), 12.5 (female ARWs), 11.9 (male ARWs) and 11.0 (female ABCs) years respectively.

The expected number of years for four periods (1991-1995, 1996-2000, 2001-2005 and 2006-2010) was calculated. The expected number of years was 8.1 from 1991 to 1995, 11.9 from 1996 to 2000, 12.6 from 2001 to 2005, and 15.6 years from 2006 to 2010.

Twenty out of the twenty-one missing numbers due to double counting for the animals were made in 1989 and 1990. Therefore, in the present study, the data from 1986 to 1990 were only shown, but not analyzed in Figure 2-5.

The number of deaths between 0 and 9 years of age was greater than for deaths between 10 and 19 years, 20 and 29 years and for those over 30 years. In particular, the largest number was 51 animals for 0 year-old female ABCs.

In conclusion, our findings show that the number of seals aged between 20 and 29 years and over 30 years was increasing gradually. In the near future, a reexamination will be required. However, the current survivorship for spotted seals in captivity is thought to be approximately 30 years-old.