Re-introduction of a native galaxiid (banded kokopu), after piscicide treatment, in two small streams and a reservoir, Wellington, New Zealand

Summary

Banded kokopu (Galaxias fasciatus), a culturally significant native New Zealand fish classified 'At risk-declining' due to habitat loss and predation by invasive salmonids, had been reduced to a skewed, juvenile-poor population in two small Wellington streams and a decommissioned reservoir within the Zealandia sanctuary, where introduced brown trout had virtually eliminated recruitment. The Department of Conservation, University of Otago and Zealandia partnered to test whether the piscicide rotenone could reliably eradicate stream-dwelling trout, a first for flowing New Zealand waters, securing multiple resource and fisheries consents and managing the reservoir's water level and valves to contain dosed water until it broke down to safe levels. PIT-tagged adult kokopu were tracked for condition, mobility and spawning before and after dosing, while aquatic invertebrate communities were sampled monthly for a year following treatment. Results showed invertebrate communities recolonized to pre-disturbance levels within a year, and while tagged kokopu suffered a temporary decline in condition after dosing, mobility and spawning were unaffected; five years of surveys since confirmed no trout returned and healthy numbers of juvenile kokopu, leading to the eradication being declared successful in 2015. Difficulties included accurately mapping trout distribution, underestimating how long the piscicide would take to flush from the system, and correcting public misconceptions about the scope of trout removal. The project rates itself successful, crediting careful seasonal timing of dosing, an untreated upstream source of recolonizing fish and invertebrates, and documentation establishing rotenone as a viable tool for removing invasive salmonids from flowing streams.