Rock ramp fishways | "Stairs" for fish open up more habitat

FISH HOMES AND HIGHWAYS PROJECT

STAIRWAYS FOR FISH WILL OPEN UP MORE UPSTREAM HABITAT

28 JUNE 2022

Native fish will be able to access more habitat for more of the year, when eight of the highest priority fish barriers in the Murray and Lower Herbert catchments are remediated later this year.

These catchments are a diversity hotspot for diadromous freshwater fish species – which means the adults spawn in estuaries, and the juveniles migrate upstream where they mature into adults.

After extensive surveying work, hundreds of barriers preventing fish movement were assessed and prioritised for remediation work as part of the Fish Homes & Highways project being delivered by Terrain NRM and funded by the Australian Government’s Reef Trust.

Now, design work is being finalised with one barrier set for work to start shortly.

Trent Power from Catchment Solutions is a specialist in fish passage designs. He says that working in creeks and rivers is difficult at the best of times.

“Work has to be done in dry season when the water flow is at its lowest, and it needs to be complete before wet season starts which can potentially wash a remediation site away if it’s unfinished.”

One of the remediation sites is a rock weir on the lower part of the Herbert River, known as Gedges Crossing, which cuts off 250km of potential upstream habitat for small fish during certain parts of the year.

“Things that stop fish aren’t necessarily a barrier all year round. For instance, the water level at Gedges Crossing only drops enough to create a barrier in the late wet and through the dry season. But those really small fish like baby barras and empire gudgeon certainly can’t get through when flows are low. They can’t jump, and at this size their swimming ability is poor. Anything more than a 10-20 cm drop can stop them in their tracks” said Trent.

A rock ramp fishway design is being considered for the weir. This is a common solution to barriers under three metres in height.

Rocks are strategically placed to influence the flow of the water in a way that makes it possible for a small fish to swim upstream. If you think of it as an underwater staircase, the fish can rest on top of each stair before tackling the next.

Gedges Crossing creates a fish barrier in low flow conditions

On wide streams where a full width fishway is not feasible, the rock ramp is usually placed towards the edge of a waterway. Fish will naturally find it as they move generally move upstream along the banks where the water is moving slower.

Trent says the design is paramount.

“You can’t just throw rocks in anywhere, but when you implement a well-designed and constructed rock ramp it will bring a lot of benefit to fish populations.”

Rocks must be large enough to withstand flood events and placed in precise locations to create the right flow conditions. To physically execute the design, an excavator works from a temporary pad on the stream’s edge. Some of the natural bed material is removed and replaced with the new rocks.

One of the eight barriers is expected to have work start in July. The remainder are undergoing a permitting application under Queensland’s waterway barrier legislation.

While prioritising the crossings for those that are the biggest barrier to fish movement, budget, approvals and landholder support is taken into consideration in selecting which sites to work on.