New research shows fish need faster flowing rivers for feeding, and could lead to challenges by conservation and recreation groups to water allocation.
New Zealand’s Fish and Game chief executive Bryce Johnson said the study would be immediately brought up with Environment Minister Dr Nick Smith.
Cawthron Institute scientist Dr John Hayes says his research shows taking too much water from rivers is bad for fish because the faster a river flows, the more food becomes available for the fish.
It did not matter whether the fish were introduced trout or salmon, or native species.
“The more power in a stream, the more it lifts invertebrates off the river bed and makes them available to fish. Most invertebrates live in shallow, faster places and big fish like eels or trout live in deeper water and the drift is carried to them by the current,” Hayes said.
He said he had a “eureka moment” while modelling river flows on Southland’s renowned Mataura River.
His research team was running the new model on the river, and up until then he had not appreciated that the concentration of drifting invertebrates increased with water flow, in a similar way that fine sediment concentrations increased with flow.
The research will have global significance. Existing modelling is based on work done in Colorado in the 1970s but the “missing ingredient” was food availability.
“The new knowledge has global implications for irrigation and hydro-electric development, and freshwater fisheries conservation and management. In New Zealand, regional councils may need to revise minimum flows upward and water allocation limits downward,” Hayes said.
“The environmental, social and economic consequences are far-reaching. Fish & Game and the Department of Conservation have a better case for arguing for precautionary flow decisions, but tighter limits on taking water will be challenging for farmers and a government committed to sustainable economic growth,” Hayes said.
Johnson said many South Island rivers, especially in Canterbury, were used for irrigation and were potentially over-allocated.
“It stands to reason that the more water taken from a river, the less life it is able to support.”
He said 10 years ago the Government had set up a working party to establish national environmental standard on ecological flows.
“For some reason it got bagged, I suspect because it was going to come out with outcomes that would get in the road of irrigation expansion,” Johnson said.
Southland Fish & Game manager Zane Moss said fisheries managers had long been concerned that traditional habitat modelling had come up with similar results across a range of rivers that were different in nature.
“This research really is outstanding in an international context and greatly advances the confidence managers have in ensuring trout habitat is not sold down the river in over-allocation,” he said.
A 2003 NIWA study into anglers’ perceptions of New Zealand’s lowland rivers and trout fisheries, commissioned by Fish & Game, shows that angling quality had declined over 70 years.
The report noted that the decline was perceived to be most marked in Nelson, Hawke’s Bay, Marlborough and Canterbury – dryland regions where there is the greatest demand for water for irrigation.
Hayes says freshwater fisheries scientists in the United States were using the model in a multi-million dollar research programme on endangered salmonid populations, and how to restore them, in the Columbia River catchment.
Neil Deans, technical policy advisor to the Minister for the Environment, said the quality of the research was internationally recognised, and he would recommend it be evaluated for the national limit-setting process.
The Ministry for the Environment said the draft national environmental standard for ecological flows has been superseded by the National Policy Statement for Freshwater Management in 2011.
This requires regional councils to safeguard the life supporting capacity, ecosystem processes and indigenous species’ habitats by maintaining or improving water quality and sustainably managing water quantity. It said this would be achieved through regional plans setting limits to safeguard both water quality and quantity and enable use of water within those sustainable limits.
“This can mean using sophisticated science models to determine how much water of what quality is needed to protect fisheries, insect life and food production. This new research will assist that process and needs to be evaluated as to how it will inform those processes,” the ministry said.
2016 Fairfax New Zealand Limited