小河川におけるカワニナ個体群の時空間的変動 : カワニナトラップによる調査

カワニナを個体サイズや底質に関わらず採取できるトラップを作成し，それを用いて広島県東広島市を流れる二つの小河川を対象に，カワニナ個体群の時空間的変動に及ぼす環境要因について検討を行った。半自然河川の角脇川にトラップを仕掛け，5月～ 11月の経時変化を調べたところ，採取された個体数は期間を通じ地点により大きく異なっていた。この差は，水温や流速，BOD，pH では説明できず，河床材料との関係も明瞭ではなかったが，礫質の地点では個体数変動が少なく，殻径3mm未満の稚貝が多く捕獲された。コンクリート三面護岸化された半尾川では，角脇川に比べ非常に少数のカワニナしか捕獲されず，稚貝も1匹しか捕獲されなかったため，ほとんど繁殖が行われていないと考えられた。カワニナの生息に適した環境を評価するためには，個体サイズを考慮した調査を行うことが重要であり，本調査で用いたようなトラップは有効な手段となりうると思われる。

This study aimed to elucidate the environmental factors that determine spatiotemporal fluctuations of populations of the freshwater snail Kawanina (Semisulcospira libertina) in two streams in Higashi-Hiroshima City based on a field survey using a trap designed for sampling regardless of the individual snail size or sediment. The changes from May to November in the semi-natural stream Kadowakigawa were investigated, and the number of individual snails captured by traps varied greatly depending on the site throughout the period. Differences in the numbers of captured snails could not be explained by differences in water temperature, water flow, BOD, pH, or riverbed materials between sites. However, the number of snails tended to be stable, and small individuals (< 3 mm) were frequently captured in the site with a stony riverbed. In the Hannogawa, a concrete-lined stream, only a very small number of Kawanina were captured by the trap compared to the Kadowakigawa, and only one young snail was captured by the trap during the investigation period, so the snail reproduction in this stream was considered to be unsuccessful. Quantitative studies considering the size structure of the population are important in assessing the suitability of a stream for the habitat of the snail. The “Kawanina trap” can be a useful tool for such research.