原著論文 ORIGINAL PAPER

北海道東部河川におけるシロザケの死骸が森林‐河川生態系に及ぼす影響

柳井　清治^1)*・ 河内　香織¹⁾・ 伊藤　絹子²⁾

1)北海道工業大学環境デザイン学科　〒006-8585　札幌市手稲区前田７条15丁目
2) 東北大学大学院農学研究科　〒981-8585　仙台市青葉区堤通雨宮町１-１

Seiji YANAI^1)*, Kaori KOCHI¹⁾ and Kinuko ITO²⁾: Effects of chum salmon carcass (Oncorhynchus keta) on forest and stream ecosystems, in eastern Hokkaido, Japan.. Ecol. Civil. Eng. 9(2), 167-178, 2006.

1) Dept. of Environmental Design, Hokkaido Institute of Technology, 7-15-4-1, Maeda, Teine, Sapporo, 006-8585, Japan
2) Graduate school of Agricultural sciences, Tohoku University, 1-1, Tsutsumidoriamamiya, Aoba, Sendai, 981-8555, Japan

Abstract:The effects of salmon carcasses on forest and stream ecosystems were determined by nitrogen stable isotope analysis in natural streams in eastern Hokkaido, Northern Japan, where numerous chum salmon (Oncorhynchus keta) were migrated upstream from ocean to spawn in autumn. The leaves and soils surrounding riparian forest and aquatic animals were collected at salmon present and absent stream before and after migration. The nitrogen stable isotope ratio (d15N) of riparian trees (Salix spp. and Ulmus davidiana var. japonica) were not significantly different between the two sites. The grass leaves of Petasites japonicus, collected within 5m at salmon present stream showed higher d15N value (6‰) than another site (0.5〜3‰), however, this was not significantly different. The d15N of A0 soil layer at the salmon present site was 3 ‰ higher than absent site, but this was not significantly different. The C-N ratio was the lowest within 5m of salmon sites, which was also not significantly different. The movement of carcasses was traced by telemetry, and three out of the five carcasses were removed within 10 m. However, one carcass was transported to 500 m distant hillslope ridge. The d15N in four taxa of stream dwelling invertebrates and juvenile masu salmon (Oncorhynchus masou) were significantly different between the salmon present and absent stream. The temporal change of d15N values of aquatic invertebrates were compared before and after migration, 0.5 to 3.5‰ increases were observed in grazer, collector-gatherer and predator, but not shredder. These results suggest that the salmon carcasses did not significantly effect on riparian vegetation, however, they strongly influenced on stream ecosystems by enhancing trophic levels of aquatic consumers.

Key words: marine derived nitrogen, nitrogen stable isotope, riparian vegetation, salmon carcass and stream invertebrates

摘要

サケの死骸が河畔林生態系・および渓流生態系に及ぼす影響を明らかにするため，豊富にシロザケの遡上回帰が観察される，北海道東部網走管内のモコト川上流域において，サケの分解移動過程，および窒素安定同位体比を用いサケが河畔植生と水生動物へ及ぼす影響を推定した．この河川と同規模，同土地利用の支流域でサケが遡上しない河川を対照河川とし，比較を行った．
　この結果，サケの遡上は11月をピークとし，死骸は河川内と陸域で観察され，その比率は３対１程度であった．河川内に滞留しているものはほとんど水生菌が繁茂し水中で分解されるのに対し，陸上に持ち上げられた死骸の多くは大型動物類に被食を受けたものが多かった．河岸の死骸の５体に電波発信機を装着し移動距離を調べたところ，３体は10m 以内，１体が500m 程度移動したことが判明し，少数であるが遠方まで運ばれている可能性が示された．
　次に河畔植生のハルニレ，アキタブキおよびヤナギ属葉の窒素同位体比を測定したところ，ハルニレ，アキタブキは対照河川に比べて高かったが有意な差とはならなかった．逆にヤナギ属では対照河川と比べて低い傾向があった．一方，水生動物類（ガガンボ科，コカゲロウ属，トウヨウマダラカゲロウ属，アミメカワゲラ科およびサクラマス）の窒素同位体比は対照河川に比べていずれも高く，有意な差が見られた．遡上前と遡上後の同位体比値を比較したところ，ガガンボ科を除き増加する傾向があった．
　以上の結果から，サケの影響は本調査河川においては河畔植物に関しては有意には現れず，逆に水生動物群には明瞭に現れた．しかし陸上に持ち上げられた死骸の多くは大型哺乳類や鳥類の摂食を受けており，一部は遠くまで運ばれている可能性があった．今後はサケの死骸が陸上生態系の中でどのように利用されているかを明らかにすることが重要となる．

2005年8月23日受付，2006年5月11日受理
* e-mail: yanai@hit.ac.jp