Abstract
The purpose of this study was assess the ability of frogs to escape from agricultural concrete channels. The escape capabilities of four frog species from vertical concrete wall surfaces of different abrasive characteristics (＂abration degrees＂) were examined in a laboratory. The arithmetical mean roughness index (Ra) was used as a criterion for measuring the abration degree of the wall surfaces. Walls with Ra values of 0.23, 0.28, 0.55, and 1.23 were used for laboratory tests. More than 7.5％ of the individuals (comprising the Indian rice frog, Fejervarya kawamurai and the black spotted pond frog, Pelophylax nigromaculatus) escaped from the four experimental devices. The wrinkled frog, Glandirana rugosa escaped from the three experimental devices of abration degree values of Ra 0.28, 0.55, and 1.23. The daruma pond frog, Pelophylax porosa brevipodus only escaped from the device of abrasion degree value of Ra 1.23 at low rate (12.5％ of the individuals). We proposed that P. porosus brevipodus should be regarded as an indicator for frog escape tendencies from concrete channels. If such a standard is adopted, it will be necessary to take coutermesasures in channel walls with Ra vaule 1.23. These criteria would be applicable to drainage basins with orthogonal walls or a type of channel in which channel branches project from the sides of the stream (referred to as Water Channel Fabricated I.). Because, every species showed a preference for routes containing ＂corners＂(i.e., ＂surface where two walls come together at an angle＂) it was assumed that climbing would be very difficult in a channel with a flat wall surface. These results suggested that measures should to be taken to create escapable conditions in such channels.
