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Greater River Flow Needed for Fish Growth & Abundance

Cawthron research shows greater river flow needed for fish growth and abundance

Computer modelling by Cawthron Institute freshwater scientists has shown that more river flow is needed for fish growth and abundance than previously realised – findings highlighted in a new video released this week.

Senior Scientist: Freshwater Fisheries, Dr John Hayes says that it is vitally important to maintain the capacity of a river to transport the invertebrate food that drift-feeding fish such as trout consume.

“As the flow declines fewer invertebrates drift in the water column so drift-feeding fish have less to eat. That means fewer fish may be supported or they may grow more slowly.”

“We’re finding that trout potentially benefit from higher flows than we thought. That’s the key point that our computer modelling has been showing us. We’ve come to the realisation that the flows that benefit drift-feeding fish have been underestimated in the past. That applies to trout in particular, but the principles also apply to native drift-feeding fish.”

Dr Hayes and colleagues have been developing their understanding of the impact of changes in river flow on fish for the past 15 years and hope that by sharing their findings in a video they can help improve awareness of the importance of adequate river flow for sustaining river ecosystems.

Recently, Dr Hayes and Cawthron’s Rasmus Gabrielsson have been using the new understanding of invertebrate drift transport and the energetics of drift feeding by trout to determine the effects of water abstraction on the life-supporting capacity of river ecosystems and on valued fish populations.

Their work has become even more critically important as the flow and quality of streams and rivers has become an issue of national significance.

“Local authorities have the difficult task of managing competing river uses and the values that are important to people who use rivers, while maintaining instream life in all its variety and abundance,” says Dr Hayes. “It’s our job as scientists and advisers to help them through that decision-making process and to try and show them how reduced river flow can change a river’s ecology.”

The new modelling method has been applied on the Mataura and Oreti rivers to assist Environment Southland in making minimum flow and water allocation decisions for the Southland Water and Land Plan. Versions of Dr Hayes’s trout energetics models have also been used in the USA; on the Colorado River for determining the effect of flow alteration below a large storage dam on trout growth, and in the Columbia River catchment for assessing habitat restoration options for the recovery of endangered sea-run rainbow trout and salmon populations.

To view the Cawthron Institute river flow video go to:


About Cawthron Institute Freshwater Sciences:

Cawthron Institute is New Zealand’s oldest and largest independent science organisation, specialising in science that supports the environment and development within primary industries. Cawthron is a New Zealand leader in freshwater sciences with expertise in river and lake health assessment, catchment management and fish habitat as well as in-stream flow requirements.

For more information about Cawthron go to

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