The great Lakes debate

• Published date: 24 July 2021

Kaipara District Council (KDC) released the Kai Iwi Lakes (Taharoa Domain) Reserve Management Plan 2016, which states they want to ban trout fishing and noted: “Exotic fish releases cease in all lakes by 2018. Monitor changes in other species as the trout population decreases.”

The plan identified trout as a pest fish in Northland dune lakes. However, it stated it would consider re-releasing trout if native species were seen to decline in their absence.

“Consider using Lake Kai Iwi as a ‘control’ lake to aid research,” the report said.

The Taharoa Domain Governance Committee (TDGC) had established a Department of Conservation-led working group to monitor and advise on the management of native species in the Lakes and to better understand the ecology of species.

DoC freshwater science adviser Dr Dave West said the decision to stop the release of trout was part of community discussion when the TDGC developed its management plan which went through a very formal process to develop in 1999 and again at its review and update in 2015/16, which had public support.

“Trout is a large, efficient predatory fish which are not native to Aotearoa’s fresh waters. They eat the young and adults of native freshwater species and compete with native species for food and habitat.

“New Zealand science journals contain many articles documenting the impact of trout on native freshwater species, but I am not aware of any that show trout benefit or have no effect on native freshwater species.

“Trout and native fish can co-exist, but native fish are unlikely to thrive.

“Additional factors can impact native fish more than exotic ones, such as poor water quality and habitat, so those native fish that might have been able to co-exist with trout under good conditions may become extinct at that location.

“Basic fish biology suggests that when a fish does not have to compete with another fish for food or habitat it will have better growth. Removal of trout from a lake in the Zealandia sanctuary in Wellington benefited the resident native banded kokopu.”

Speaking about the effect of trout on mosquito fish, West said he was not sure if the presence of gambusia was factored into TDGC’s decision.

“There is a risk that when trout are no longer present in the lakes gambusia numbers will increase and they will impact native freshwater species such as Dune Lakes Galaxias.

“Few gambusia have been found in trout guts sampled from the lakes, which suggest trout are not currently playing a large part...