都市域谷戸水田における冬季の管理条件が春季の水生生物に及ぼす影響ついて  [in Japanese] Effects of Paddy Field Management during Winter on the Condition of the Aquatic Animals in Valley-bottom Paddy Fields in Spring  [in Japanese]

谷戸水田において冬季の管理状況の差異が春季の水生生物相に及ぼす影響把握を目的に,方形枠を用いた定量調査を試みた。2002年の4〜6月の調査で7綱の生物群(内昆虫綱は12科15種)が確認された。5月の代掻き作業により一時的に水生生物相は相当のダメージを受け,代掻き直後には極めて少数しか確認されなかった。しかし,代掻きから1週間以上経過すると個体数密度も比較的増加し,水生生物相が速やかに回復していた。一方,代掻きを行わない不耕起水田では高い個体数密度を維持していた。6月になると乾田でユスリカ科の増加に伴い全体に高い個体数密度(約280個体/0.25m^2)となるのに対し,湿田では約50〜150/0.25m^2にとどまった。

This study investigates how the difference of the paddy field management during winter affects the conditions of the aquatic animal in spring in valley-bottom paddy fields. We used a 50cm-sided cubic box to sample aquatic animals in the paddy field. We first placed the box sampler in the targeted paddy field, then, we captured all the aquatic animals in the box sampler through a net. We believe that this method is efficient in order to determine the fixed quantity of the aquatic animal in the paddy fields, though after July it could be difficult to install the box sampler among grown rice plants. With the sampling method, we have affirmed the existence of 7 classes of the taxonomical group, which includes 16 species from 12 families of Insecta in the research from April through June in 2001. The sampling right after reconditioning the fields by pudding resulted in collecting much less aquatic animals. With the results, we suspect that pudding paddy fields in May temporarily damaged the aquatic animal in the paddy fields. However, after more than a week from the pudding we found that the number of aquatic animals increased, and the aquatic fauna in the paddy fields seemed to be recovered from the damage. On the other hand, the paddy fields that were not padded before rice plant transplantation, and the non-tilling of the paddy fields, maintained a highly dense population of individuals, especially, Ceratopogonidae. In June we noticed an increase of Chironominae, and a highly dense population of individuals in the well-drained paddy fields (approximately 280 individuals /0.25m2). In contrast, we sampled only 50 to 150 individuals /0.25m^2 in the poorly-drained paddy fields. The more Sphaerium japonicum japonicum and Buranchiura sowerbyi are sampled, the less drained the paddy fields are. The population density of B. sowerbyi is relative to the moisture condition of the paddy field during winter.