Population densities and microhabitats of exotic mosquitofish and medaka in irrigation ditches near metropolitan areas

Abstract

In this study, we attempted to evaluate the impacts of an introduced North American fish, the mosquitofish (Gambusia affinis), on populations of medaka (Orizias latipes), a native fish of Japan that is a red databook species. We estimated fish population densities and evaluated microhabitats in small irrigation ditches at 20 sites in Aichi Prefecture, central Japan. Mosquitofish and medaka were found together at 12 of these sites. The densities of mosquitofish and medaka were 9 m^-2 and 5 m^-2, respectively; the difference between these two densities was not statistically significant. However, medaka individuals in sympatry with mosquitofish had injured caudal fins and the severity of the injuries was correlated to the density of mosquitofish; this figure was statistically significant. Despite this, the density of the medaka populations was not explained by any of the microhabitat variables measured or by the density of mosquitofish. Based on the circumstantial evidence, we speculate that effects of interference competition by mosquitofish on medaka on the population level were limited; it is more likely that structural changes to paddy fields, including loss of habitat connectivity via irrigation ditches, contributed to determining the density of medaka.