Rotenone use for native fish conservation: Macroinvertebrate community recovery and the reintroduction of a native galaxiid (Galaxias fasciatus) following piscicide treatment in two streams

Abstract

The rehabilitation of native communities by means of eradicating unwanted fish species using piscicides is an example of employing disturbance to achieve conservation successes. Such projects provide a valuable opportunity to test the efficiency of the tool and the impacts on the receiving aquatic communities, as disturbance occurs at a known time. The piscicide ‘rotenone’ has been widely used to eradicate invasive or unwanted fish species worldwide, but also results in the depletion of present invertebrate communities. Despite being widely used, there is limited information regarding the recolonisation of aquatic invertebrates and the impact on native fish being re-introduced to a stream after rotenone treatment. The mass depletion of invertebrates due to rotenone dosing is of particular concern, as food-limitation could negatively impact on fish condition, and recruitment, compromising the aims of rehabilitation. For the first time in New Zealand, rotenone was employed to eradicate brown trout (Salmo trutta) from two streams that also supported populations of banded kokopu (Galaxias fasciatus). Impacts on fish and aquatic invertebrates were studied in two treatment and two reference streams in Zealandia Sanctuary, Wellington, New Zealand over a 15 month period. Analysis showed invertebrate densities, taxa richness and community structure were significantly impacted in the treatment streams in the 2 week – 2 month period after dosing, but recovered to pre-treatment levels within 4 - 12 months. Following reintroduction after rotenone treatment, banded kokopu condition declined significantly and levels of fish mobility were variable. One year after rotenone dosing, there was recruitment of banded kokopu juveniles in the treatment streams indicating successful reproduction, with no equivalent increase in the reference streams. Results are a positive indication for the use of rotenone as an effective conservation tool to remove unwanted fish species where they threaten native populations.