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

Good connectivity, or “fish passage” enables fish to migrate to and from breeding and spawning grounds, gives them access to deep water holes and wetlands for refuge in the dry season, enables them to respond to changing water quality such as dissolved oxygen levels, helps them find new areas for feeding, and also lets them evade predators.

It’s estimated that there are thousands of fish barriers in the Wet Tropics, which prevent fish from moving between different habitats. This can mean there is less overall habitat available for fish to live in, which can lead to reduced fish populations. Barriers also impact fish diversity and disrupt lifecycles.

Unfortunately, invasive fish species like tilapia and gambusia can survive and reproduce in degraded and polluted waterways much more successfully than even our hardiest native fish species.

Enhancing waterway connectivity and health by removing barriers to fish movement is one way we can support native fish.

What has been done so far?

To better understand the risks posed by barriers to fish in our region, a number of projects and organisations are identifying the barriers and prioritising them for mitigation based on those that will give the best outcomes for our fish populations.

Experts use high resolution imagery and desktop analysis to identify the potential barriers. The top priority ones are then physically investigated by a field team.

Factors including stream movement, flow rate, distance from the coast, physical specifications of barriers, and ease of modification are then considered, to work out which interventions will get the best ecological outcomes for fish whilst being financially feasible.

So far, data is available for the Daintree, Mossman, Lower Barron, Murray and Herbert catchments. Further funding is required to undertake studies in Trinity Inlet, Russell-Mulgrave, Johnstone and Tully.

Some fish barriers have been remediated in the Murray and Lower Herbert but there is a lot more work to be done.