There is widespread concern about the effect of introduced species on native species. The San Francisco Estuary (SFE) is a highly invaded system (Cohen and Carlton 1995), with a mix of native and introduced species that didn’t evolve together. Humans introduced non-native species in a variety of ways, ranging from recreation to ship ballast water to aquarium and pond releases (Hanak et al. 2013). … However, not all non-native species disrupt an ecosystem. Many introduced fishes do not have clear negative impacts on populations of native species, and some may provide benefits to humanity in the form of recreation opportunities, ecosystem services, or as indicator species (Moyle et al. 1986, Bork 2018, Grossman 2016). In this blog, we’ll review the complex roles of non-native fishes within our novel estuary and propose a systematic framework evaluating the “invasiveness” of these introduced species.