The objective of this research was to evaluate responses of water quality and fish biomass in the 16.8-ha Lake Ohinewai to removal of invasive fish species. Lake Ohinewai is a shallow (maximum depth 4.5 m), hypertrophic, riverine lake in pastureland in the Waikato Region, New Zealand. We hypothesised that reduction of invasive fish to below 100 kg/ha would improve water quality, so we removed koi carp (Cyprinus carpio), brown bullhead catfish (Ameirus nebulosus), goldfish (Carassius auratus) and rudd (Scardinius erythrophthalmus) during the recapture phases of four capture-recapture population estimations. We also installed a one-way barrier that allowed adult fish to leave but prevented re-entry of adult koi carp. In 2011, before fish removal, koi carp comprised 97% (308 kg/ha, 95% CL 211–466) of the total biomass of invasive fish (334 kg/ha). We reduced the biomass of koi carp to 39 kg/ha (95% CL 24–67) in 2012 and to 14 kg/ha (95% CL 7–27) in 2014 by a combination of fish removal and the one-way gate. Total invasive fish biomass in 2014 was estimated as 28 kg/ha, well below our target of 100 kg/ha. In 2016, after two years without fishing but with the one-way barrier still in place, koi carp biomass had increased to 94 kg/ha (95% CL 49–197) and total invasive fish biomass was 157 kg/ha, partly because of a strong biomass response by catfish (12 kg/ha in 2011, 36 kg/ha in 2016). The native shortfin eel (Anguilla australis) also showed a strong biomass response (14 kg/ha in 2012, 41 kg/ha in 2016).
Water quality (Secchi depth, suspended solids, and concentrations of chlorophyll a, total nitrogen and total phosphorus) was evaluated before and after fish removal. None of these variables showed changes that were coincident with invasive fish removal except for chlorophyll a concentration, and the lake remained hypertrophic.