Invasive fish have a variety of effects on indigenous fish communities and freshwater ecosystems generally, and the magnitude of these effects is partly dependent on invasive fish biomass. For example, a koi carp biomass of 120–130 kg/ha was sufficient to depress macroinvertebrate and plant biomass, and to elevate chlorophyll a concentrations (Haas et al. 2007; Bajer et al. 2009; Hicks et al. 2011). The purpose of this section is to provide (i) estimates of the relative biomasses of invasive fish that have been established by boat electrofishing in a range of lake and riverine habitats in the North Island; and (ii) some estimates of absolute biomass derived from mark-recapture studies in shallow lakes. Collectively, these data provide a basis for future comparisons of invasive fish monitoring information in a region where coarse fish have proliferated.