Since their establishment in New Zealand in the 1930s, the globally invasive western mosquitofish (Gambusia affinis) has become widespread throughout lowland freshwaters, primarily in the North Island. Despite co-occurring in many ecosystems, interactions between native common bullies (Gobiomorphus cotidianus) and G. affinis are presently unknown. Like G. affinis, G. cotidianus are abundant in lowland waterways, tolerant of a wide range of environmental conditions and can consume similar prey. My research investigated the potential dietary overlap between these two species using a controlled mesocosm experiment and a stable isotope study of co-occurring G. affinis and G. cotidianus in natural lake habitats. Zooplankton were selected as the prey source for the mesocosm experiment. Twenty-one 100 L mesocosms were assigned either four G. cotidianus, four G. affinis, or were left as a control group (i.e., with no fish) and monitored for 14 days. Zooplankton community composition in the fish treatments was significantly different from the control tanks by Day 14 of the experiment yet did not differ significantly from one another. Both G. cotidianus and G. affinis reduced the abundances of larger crustacean zooplankton relative to the controls. This is likely to have reduced the effects of predation, competition, and physical interference by the crustaceans on rotifers, allowing rotifer species to reach numerical dominance in the treatment mesocosms by the end of the experiment. In contrast, zooplankton assemblages in the control mesocosms were dominated by cladocerans by Day 14, indicating that these had a competitive advantage in the absence of zooplanktivorous fish. Four Waikato lakes were selected as sites for the stable isotope study. Gambusia affinis, G. cotidianus, and basal food web taxa were collected from each lake during the austral winter of 2022. Samples were analysed for the stable carbon and nitrogen isotopes ¹³C and ¹⁵N, and results were compared to data published by Collier et al. (2018) from the same taxa and lakes during an austral summer. Mean δ¹³C values of G. affinis were significantly enriched compared to G. cotidianus at three lakes in both summer and winter, while the mean δ¹⁵N of G. affinis was significantly enriched at three lakes in summer and all lakes in winter. These differences were greatest in winter, even when the summer dataset was censored to match the winter fish length distribution. My results indicate that G. affinis and G. cotidianus are likely consuming different prey sources and that their diet is most similar during summer. Additionally, my results showed seasonal variation between lakes in the isotopic enrichment of both fish and basal food web taxa, suggesting food webs and trophic interactions may differ between sites. Overall, my thesis demonstrates the potential for interspecific competition between native G. cotidianus and non-native G. affinis in New Zealand through an example of zooplankton dietary overlap in a controlled experimental context. The stable isotope experiment provided mixed evidence for dietary overlap across both summer and winter in sympatric lake populations. Both experiments offer numerous future research opportunities to investigate the mechanisms behind the findings.