Zealandia reservoir to be drained to remove a tonne of pesky perch

Monday, 22 March 2021

The lower reservoir at Zealandia is being drained this month to remove around a tonne of unwanted fish, which are eating native species and throwing the ecosystem off balance.

Perch were introduced to Roto Kawau, the lower part of an existing man-made reservoir, more 140 years ago, by people wanting to create fishing opportunities.

They preyed on native fish, and altered the food chain by consuming invertebrates (zooplankton), which normally eat algae (a phytoplankton called cyanobacteria), keeping it in check.

Zealandia director Dr Danielle Shanahan says the project has been a long time coming.

Dr Danielle Shanahan, director of Zealandia in Wellington.

Water is being pumped out and emptied down the spillway into the Kaiwharawhara Stream.

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The introduced perch in Zealandia's reservoir were mostly around 5-6cm long, but together made around a tonne of biomass. (File photo)

Already delayed a year by Covid-19, a system of pumps would drain around 109,000 cubic metres from the 180,000m3 reservoir, dropping the water by as much as eight metres by mid-May.

Water would be discharged in controlled amounts down the spillway into the Kaiwharawhara Stream.

More than 100 tuna/eel had been captured and released downstream to complete their life cycle, and banded kokopū and freshwater sponges were temporarily moved to the upper reservoir.

There were more than 22,000 perch in the reservoir, most around 5-6cm in length, equalling around a tonne of biomass.

The team would use nets and a fishing technique called electric fishing to reduce perch numbers and relocate native species, before using rotenone to poison the remaining perch, sprayed by hand by Department of Conservation staff over the Te Mahanga Stream and wetland, and then over Roto Kawau by helicopter.

A rescue team would follow behind, reviving native fish in clean water. Dead fish would be taken to the landfill.

Lowering the water level meant less rotenone would be needed, and prevented issues of stratification, where the top layer is warm and the bottom later is cold (rotenone breakdown is affected by temperature).

It also removed hiding spots for fish amongst overhanging vegetation as the reservoir dropped away from the banks, ensured heavy rain wouldn’t cause overflow during treatment, and allowed dam maintenance to happen simultaneously.

The perch population had been controlled with electric fishing and removal of eggs over the years, but it was time to do it once and for all. “There are just so many fish, it’s not realistic to do this forever,” Shanahan said.

The project was done in conjunction with local iwi. Lead ranger in bicultural engagement Terese McLeod (Taranaki Whānui) said, “We see water as having its own mana, identity, and sovereignty.

“Something useful to explain the concept is the whakataukī, ‘ko au te awa, ko te awa ko au’, ‘I am the river, the river is me.’”

The sanctuary closed on March 18, and would again for one day in May, weather dependent.