Effects of the Use of Drainage Canal as Fish Ladder on the Management of Irrigation and Drainage Water in a Paddy Plot

– A case study of lowland paddy fields in Hikone City, Shiga Prefecture –

Abstract

A fish ladder was created to optimize fish habitat in a paddy field area. The ladder was constructed from a series of small weirs built in a drainage canal beside paddy plots to provide staircase-like steps allowing fish to pass between the drainage canal and the paddy ponds. Successive weir steps were progressively higher towards the upstream part of the ladder. Fish could run up these steps to spawn in the paddy ponds, and juvenile fish could return to the drainage canal to disperse. We examined the fish habitat and water balance in a paddy plot with this type of fish ladders and their implications for irrigation and drainage management, and fish conservation. The ponding depth of the paddy water was maintained at the top of the flashboard at the outlet of the paddy plot and water drained efficiently following periods of high rainfall. Fish could swim in the deep paddy pond. The volumes of irrigation and, in particular, drainage water were increased compared with conventional paddies without fish ladders. The amount of ineffective rainfall for cultivation and the lot-management water requirement both contributed to the increase in drainage water. It is necessary to consider both “the water requirements of the fish ladders” and “the effective rainfall required for fish conservation” to optimize paddy water management for fish conservation in paddies with ladders.