Survey of the Number of Seals Incidentally Caught by Salmon Trap Nets Along the Coastal Waters of the Nossapu Cap in Nemuro Peninsula From 2002 to 2003 Compared With the Survey From 1982 to 1983

27 Mar Survey of the Number of Seals Incidentally Caught by Salmon Trap Nets Along the Coastal Waters of the Nossapu Cap in Nemuro Peninsula From 2002 to 2003 Compared With the Survey From 1982 to 1983

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Abstract

北海道東部から襟裳岬に至るゼニガタアザラシの1970年以来の個体数減少と納沙布岬周辺で混獲され大量死亡する個体の関係を知るために，1982年と1983年に納沙布岬周辺のサケ定置網におけるアザラシ類の混獲数調査が行われた（以下，80年調査）．80年調査からちょうど20年後，海洋環境の変化や北海道東部のゼニガタアザラシの個体数が増加傾向にある中，アザラシ類の混獲数や特性の変化を知るために，過去と同じ要領で納沙布岬周辺のサケ定置網におけるアザラシ類の混獲数調査を2002年と2003年に実施した（以下，00年調査）．その結果，80年調査と比較して，00年調査の結果では，歯舞漁協管内の15ヶ所の定置網におけるアザラシ類の混獲数は，ほぼ同数（80年調査272頭，00年調査261頭）で，また，アザラシ類がもっとも多く混獲される定置網の位置や混獲されたアザラシ類の種構成［最多がゼニガタアザラシ（Phoca vituli stejnegeri）で，次いで多いのがゴマフアザラシ（Phoca largha）］に違いは見られなかった．しかし，全アザラシ類の混獲数に占めるゼニガタアザラシおよびゴマフアザラシの割合（80年調査ではゼニガタアザラシおよびゴマフアザラシの割合はそれぞれ77.6％，20.6％，00年調査では91.2％，7.7％）には有意な差が見られた．また，ゼニガタアザラシの混獲時期は，80年調査では9月中旬と11月中旬に混獲数が多くなる2山型を示したのに対し，00年調査では，定置漁業の開始直後の9月上旬に混獲数が多く，定置漁業が終わるにつれ減少する傾向が見られた．千島列島全域におけるゼニガタアザラシとゴマフアザラシの分布の中心は，択捉島以南の，特に歯舞群島・色丹域に集中して分布していると考えられているため，今後の保全管理を考える上には，歯舞・色丹グループと道東グループとの関係が重要と考える．

English Abstract

A total of 261 seals were incidentally caught by 15 salmon trap nets of the Habomai Fisheries Cooperative Association from September to November in 2002 and 2003 (00 years’ survey). No difference was found between the number of seals in the 00 year’s survey and the number (272 seals) in the survey from September to November in 1982 and 1983 (80 years’ survey). No differences were found in species ratio [domi nt and the next species were harbor (Phoca vituli stejnegeri) and spotted seals (P. largha)], and in the positions of salmon trap nets in which most of the seals were incidentally caught between both surveys, respectively. However, there was a significant difference between the rates of harbor and spotted seals of all incidentally caught animals between both surveys (91.2% and 7.7% in the 00 years’ survey and 77.6% and 20.6% in the 80 years’ survey, respectively). The frequency distribution of incidentally caught seals showed two peaks in the middle of September and November in the 80 years’ survey, the other side, a peak in the beginning of September when the salmon trap net fishery started with gradual decrease until November in the 00 years’ survey. The distribution centers of harbor seals and spotted seals are thought to be in the southern area of the Kuril Islands, especially concentrated in Habomai and Shikotan Islands. Therefore, further research on the relationship between the East-Hokkaido group and the Habomai-Shikotan group are necessary to their conservation.