事例研究　CASE STUDY

白川 北斗^１）＊, 柳井 清治^２）, 後藤　晃^１）

１）北海道大学大学院水産科学院　〒041-8611 函館市港町3-1-1
２）石川県立大学生物資源環境学部環境科学科　〒921-8836　石川県石川郡野々市町末松1丁目308番地

Hokuto SHIRAKAWA^１）＊, Seiji YANAI^２）, Akira GOTO^１） : Spawning redds selection of Arctic lamprey Lethenteron japonicum in the Ishikari River. Ecol. Civil Eng. 15(1), 71-79, 2012.

1) Graduate School of Fisheries Sciences, Hokkaido University, 3-1-1 Minato-sho, Hakodate, Hokkaido 041-8611, Japan
2) Department of Environmental Science, Ishikawa Prefectural University, 1-308 Suematsu, Nonoichi-machi, Ishikawa, Ishikawa 921-8836, Japan

Abstract: We examined spawning preference of the Arctic lamprey Lethenteron japonicum in order to establish an effective conservation measures for the critically endangered natural populations. Investigations were carried out in the Oshirarika River. The spatial distribution of spawning site was monitored by DGPS and environmental variables (current velocity, substarate composition and water depth) were measured at each spawning redds in 2005, 2006 and 2009. The generalized additive models (GAM) were employed to extract a determinant variable for presence/absence of spawning redds. The spawning redds were made at water depths between 35 and 60 cm, and current velocity between 25 and 70 cm/s, with cobbles of 10 cm in diameter according to field observation. Spatial distribution of spawning redds differed by each research year, but they tent to show aggregate distribution. Water depth were a significant factor to determine the location of spawning redds throughout the three years, but current velocity was the only significant in 2006. The GAM analysis demonstrated that critical environmental factors influencing on spawning redds of L. japonicum was water depth, especially with 35-50 cm depth. Since water level of Oshirarika River decreases rapidlu from May to July after snow-melt (decreasing 25 cm in two weeks). Our results suggest an adaptive behavior that L. japonicum selects to spawn in midium water depth in order to prevent the depth of fertilized eggs by dry during water discharge decreasing periods.

Key words: adult lamprey, conservation, reproductive strategy, spawning preference, spawning,redds

摘要

絶滅が危ぶまれているカワヤツメの産卵場選択に重要な環境要因を明らかにするため，北海道石狩川の支流であるオシラリカ川において， 2005年， 2006年， 2009年の三カ年調査を行った. 調査は全ての年度で6月に行い，産卵床(在/不在)と物理環境(流速，水深，河床材料)を測定し，同時に高精度GPSを用いて産卵床の空間位置を記録した.産卵床と環境要因の関係を調べるため，解析には一般化加法モデル(GAM) を用いた. 調査の結果，産卵床の空間分布は年度ごとに異なっていたが，その分布は一様ではなく集中分布する傾向を示した. またカワヤツメの産卵場選択に影響する要因として， 3カ年通して水深35-50cm前後が統計的に有意に重要であったが，流速は年度ごとに異なることが示された. これらのことから，カワヤツメが水深35-50cm前後に産卵するのは，河川の水位低下による卵の斃死を防ぐためと考えられた.

2010年10月9日受付, 2010年12月31日受理
*e-mail: dipper@fish.hokudai.ac.jp