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Water flow model short-changes fish

NZ NewswireNZ Newswire 17/06/2016

We might be taking too much water from our rivers and fish are struggling because the low flows aren't providing them with enough food, scientists say.

The research, by Nelson's Cawthron Institute, is based on 15 years of work and the institute says it has global implications for irrigation, hydro-electricity schemes and recreational fishing.

"In New Zealand, regional councils may need to revise minimum flows upward and water allocation limits downward," it said on Friday.

The most common model for estimating minimum flow requirements for fish was developed in Colorado in the 1970s, says project leader John Hayes.

However, it didn't take into account food availability in the form of drifting invertebrates.

The team had its "Eureka moment" on the drifting insects when studying Southland's Mataura River.

They found the concentration of invertebrates decreased as stream flows declined, because the river had less power to pick them up from the river bed.

"A river acts like a conveyor belt delivering the drifting food to the waiting fish, Dr Hayes said.

The research showed less water meant less drifting food and modelling showed that translated to fewer, or more slowly growing, fish.

A lot of current minimum flows and allocation rates around the country could be having adverse effects on trout and native fish.

Regional councils should be more cautious about allocating water.

The environmental, social and economic consequences are far-reaching."

The research confirmed anglers' gut feeling about a decline in the fishery over 70 years - based on a Fish & Game survey in 2001 - with the biggest declines being in the 1940s, 50s and 80s.

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