Spawning Migration of the Japanese Bagrid, Pseudobagrus tokiensis into a Paddy Field Ditch

Abstract

The Japanese bagrid, Pseudobagrus tokiensis, which is a vulnerable species in Japan, was observed to make spawning migrations into a paddy field ditch in the Naka River system, Tochigi Prefecture, central Japan in 1999 and 2000. A lot of fishes ascended the ditch, of which P. tokiensis accounted for 17.7 and 15.4% in number in 1999 and 2000, respectively. Pseudobagrus tokiensis ascended the ditch from early May to late August and the number of them was highest in early July. For mature P. tokiensis, the body size was larger in males than females and the sex ratio was biased to females. Most P. tokiensis ascended the ditch during the night and the number of them increased on rainy days. These results show that paddy field ditches may play a role as the spawning habitat of P. tokiensis and their spawning migration occurs nocturnally, being stimulated by precipitation.