Weirs, stream condition affect fish diversity

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A new study reveals fish passage facilities can reduce the impact of weirs on fish migration, but benefits depend on the condition of habitat upstream.

Commissioned by Queensland water business Seqwater through its partnership with Griffith University, the study found that two impassable weirs in the Logan-Albert river network in southeast Queensland contributed to the loss of fish diversity in upstream reaches.

This was because species that required movement between estuary and freshwaters experienced restricted access.

Research Fellow Dr Robert Rolls led the study through Griffith’s Australian Rivers Institute and says the results – published in Biodiversity and Conservation – demonstrated that weirs fitted with operating passage facilities allowed migratory species to pass weirs used as a source of water.

“While many migratory species, such as sea mullet, varied in abundances throughout the year in reaches upstream of weirs with fishways, fish communities upstream of Luscombe Weir and Wyaralong Dam supported low abundances of migratory species at all times,” says Dr Rolls.4. Luscombe Weir C

“Our findings also indicated that even in reaches upstream of fishways, fish were in extremely low abundance and diversity when vegetation from the riparian zone had been cleared, thus leading to widespread erosion and sedimentation of the river channel.

“This tells us that we need to understand the interactions between multiple human impacts to rivers and the subsequent effects for aquatic biodiversity, especially if we are to prioritise how best to spend resources and ensure the most effective conservation outcomes, including benefits for people through recreational fisheries and tourism.”

Seqwater Senior Scientist Dr David Roberts says better management and operation of fishways to mitigate the impact of weirs and dams can be achieved through regular maintenance – such as ensuring they are not clogged by sand or debris – and improving reliability through better technology.

“It is also important to operate fishways so as to maximise benefits. For example, Seqwater has altered operating settings to accommodate peak fish migration times, thus enabling more fish to travel through.”

Dr Roberts says these actions will improve the overall health of the river over time, adding that other factors identified in the study, such as riparian conditions, require attention to fully realise the benefits of fishways on a degraded river system.

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