Brown Trout Eradicated from Karori Reservoir

Wellington, NZ – Wellington, NZ – Brown trout are now confirmed to be completely removed from the Karori Upper Reservoir and its tributary streams, allowing native fish species and koura (freshwater crayfish) to flourish. The trout, an introduced species, were removed four years ago as part of a joint operation between Department of Conservation and ZEALANDIA to restore a completely indigenous aquatic community. The operation was the first in New Zealand to use the piscicide Rotenone in flowing streams.

In February 2011 the reservoir and streams were treated with the Rotenone, a natural toxin found in the roots of many tropical plants and used for centuries by indigenous people in Southeast Asia and South America for the harvesting of fish to eat. It has also been widely used overseas and increasingly in New Zealand to eradicate undesirable fish, as it is highly toxic to fish but has low toxicity to birds and mammals.

Manager Conservation, Research, Learning & Experience at ZEALANDIA Raewyn Empson said that while no trout have been detected in the operational area since the eradication, time was needed to confirm that there was not a small remnant population present.

“Several potential breeding seasons later and still no sign of trout, we can now be confident that the operation was a success,” Empson said.

The effects of the successful removal of trout have been dramatic. Hundreds of juvenile banded kokopu have been detected in follow-up surveys where only a few were found before trout were removed. Koura too became much more visible, even before they had time to breed, indicating their behaviour had changed – in the presence of trout they stayed hidden to avoid predation; in the absence of trout this was no longer necessary. The success of this operation now means that Zealandia can explore options for introducing other native fish species to the upper reservoir, continuing the successful restoration programme that has seen significant benefits to Wellingtonians.

ENDS

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