Fundamental Experiments to Develop Eco-Friendly Techniques for Conserving Frog Habitat in Paddy Areas: Escape Countermeasures for Frogs Falling into Agricultural Concrete Canals

Abstract

Frogs often drown in agricultural canals with deep concrete walls that are installed commonly in paddy areas after land consolidation projects in Japan because they cannot escape after falling into the canal. We propose a partial concrete canal with gently sloped walls as a countermeasure for frogs to escape the canal and investigated the preferable angle of the sloped walls, water depth and flow velocity for Rana porosa porosa. Our experiments showed that only 13 individuals (2%) escaped by leaping out of the canal, indicating that climbing up is the main escape behavior of R. p. porosa. Walls with slopes of 30-45 degrees allowed 50-60% of frogs to escape from experimental canals, the frogs especially easily climbed up walls with a 30 degree slope. Adjusting water depth to 5 cm or more would assist the frogs in reaching the escape countermeasures because at such depths frogs are not able to stand on the canal bottom and to move freely about. Flow velocity should be slower around the countermeasures because R. p. porosa is not good at long-distance swimming and cannot remain under running water for a long time. The broadened inlets of this new canal design provide water of slower velocity near the shores of the sloped walls, which encourages frogs to escape.