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

Abstract To understand the success of non-native fish invasions, it is important to clarify their tolerance to habitat differences and their adaptations to these habitats with regard to feeding habits. In this study, the invasive characteristics of the non-native fish Gambusia affinis were examined in order to determine its growth and feeding habits in different habitats (lotic and lentic). Fish specimens were collected from October 2021 to November 2022 from the Nakaibori Stream (lotic) and from May to December 2022 from the Nagatsugawa Reservoir (lentic). Growth indices (total length: TL, eviscerated weight: EW, and condition factor: CF) for both habitats showed that TL and EW were significantly lower for lotic than for lentic habitats. However, the CF of the lotic habitat was higher than that of the lentic habitat. The vacuity index (VI) for the lotic habitat was more than five times higher than that for the lentic habitat. The food range of G. affinis in the lentic habitat comprised 14 food items, which mainly included members of Diptera. However, in the lotic habitat, only eight of these were consumed, 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 low niche breadths (0.39). The feeding strategy of G. affinis was determined to be closely related to changes in habitat prey assemblages and possibly indicates competition for food with native fish species.