Different suitability of improved irrigation channels as reproductive sites for Cyprininae and Silurus asotus.

Irrigation channels around paddy fields have been drastically improved over the past several decades in Japan. This has been accomplished exclusively by covering all the sides of channels with concrete. Although it is conventionally accepted that channel improvement has deteriorated quality of spawning and nursery sites for fish, to date, there is little evidence to support this claim. We examined the degree to which an improved irrigation channel with pools functioned as a spawning and nursery site for Silurus asotus and Cyprininae species within the Ezuragawa creek system that pours into Lake Biwa, Hikone, Shiga Prefecture, Japan. More eggs were observed for S. asotus than for Cyprininae species, but the opposite tendency was observed for the larvae and juveniles. Moreover, S. asotus juveniles were extremely rare. Therefore, our data suggest that improved irrigation channels are not suitable spawning and nursery sites for some species.