Growth and diet of the invasive mosquitofish Gambusia affinis in lotic and lentic habitats in Japan

To understand the success of non-native fish invasions, it is important to clarify their tolerance to habitat differences and adaptations to these habitats with regard to feeding habits. In this study, the invasive characteristics of the fish Gambusia affinis were examined to determine its growth and feeding habits in different habitats (lotic and lentic). Fish specimens were collected between May and November 2022 from Nakaibori Stream (lotic) and Nagatsugawa Reservoir (lentic). Growth indices (total length: TL, eviscerated weight: EW, and condition factor: CF) showed that the TL and EW were significantly lower in the lotic habitat than in the lentic habitat. However, the CF of the lotic habitat was higher than that of the lentic habitat. The vacuity index (VI) of the lotic habitat was approximately five times higher than that of the lentic habitat. The food range of G. affinis in the lentic habitat comprised 14 food items, which mainly included members of dipteran adults. However, only six of these were consumed in the lotic habitat, with members of Formicidae being the most common, followed by plants (mostly filamentous algae). Zooplankton were not consumed in the lotic habitat. Both the lotic and lentic habitats had relatively small niche breadths (< 0.4). The feeding strategy of G. affinis is closely related to changes in habitat prey assemblages and may indicate competition for food with native fish species.