Fish Passage: How man-made barriers are threatening our fish species

The Wet Tropics boasts some of the most diverse assemblages of freshwater and estuarine fish species in Australia, but did you know that the survival of many of these species is threatened by man-made barriers that prevent them from moving between freshwater streams and wetlands and the estuaries downstream?

One of the key reasons that waterway health is so important is to protect the wellbeing and diversity of our fish communities. The number and type of fish present in our waterways can tell us a lot about the health of the whole ecosystem.

Of the more than 70 species of freshwater fish known to live in Wet Tropics basins it’s estimated that around 60 percent make annual migrations to the estuaries to spawn. The juveniles of these species then migrate upstream to the relative safety of shallow freshwater wetlands and floodplains to grow and mature. Without access to these areas the survival rate of juvenile fish is greatly reduced.

And whilst the most obvious (and insurmountable, for a fish) examples of these are our major dams, there can be thousands of other obstacles taking their toll on fish before they even get as far as a upstream as a dam or natural waterfall.

Road culverts, crossings, engineered structures and small weirs are examples of potential fish barriers.

Barriers to fish passage are widely considered to be the reason why jungle perch have all but disappeared from Queensland's southern catchments where they were once prevalent.

The issue has not gone unnoticed among recreational anglers, as the iconic barramundi, mangrove jack and jungle perch all spawn in estuaries with juvenile fish moving into freshwater wetlands to mature.

It’s considered likely that the stocking of waterways with barramundi fingerlings has become necessary due to fish barriers as much as fishing pressure. The loss of these peak predators in freshwater systems can help pest species like tilapia get more of a foothold.

The key fact to understand when it comes to migration of Australian fish is that it’s the juveniles that move upstream. Barramundi, for example, may be only 25-40mm long when they make this migration and jungle perch may be 15-20mm.

At this size these fish are not strong swimmers and even a 100mm hurdle can prevent their passage to safety – especially if the flow is high or there is a length of relatively smooth concrete to traverse on the upstream side.

Of course, fish passage of all migratory species, not just the iconic ones, is the key to healthy, flourishing fish communities, but more is known about the needs of those species with economic and recreational value.

The team at Wet Tropics Waterways is keen to better understand the risks posed by barriers to fish passage in our region, and we’re investigating options to identify these barriers and prioritise them for mitigation based on those that will give the best outcomes for our fish populations.

You can find out more about fish barriers in the Wet Tropics in Season 2, Episode 9 of the Reef and Rivers Podcast.

 

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