When more than 1000 fish move through a fishway in one day, you know you’re onto something. Juvenile barramundi, empire gudgeon, rainbowfish, tarpon, long-finned eels – scientists recorded all of these and more during monitoring work at a fishway in Insulator Creek south of Ingham recently.

“To pull in our traps and see all these fish – it’s awesome,’’ Matt Moore, a fisheries ecologist at Catchment Solutions, says. “A lot goes into the design of these structures, so we love seeing a result like that.”

The Insulator Creek fishway is one of six fishways built in the Herbert River and Murray River catchments between Ingham and Tully over the last year. Collectively, they’ve opened up more than one hundred kilometres of waterway to diadromous fish – the type that need to travel upstream from our estuaries to freshwater nursery grounds as juveniles and then back downstream as adults to breed.

Terrain NRM has been working with Catchment Solutions, OzFish Unlimited and Australasian Fish Passage Services on the fishways, which are also described as fish ladders or rock ramps.

“Up to half of the 80+ species of native fish we have in Wet Tropics waterways need to move between salt and freshwater habitats to complete their lifecycles, so barriers like weirs, pipes, culvert crossings and weed chokes can stop these species from thriving,” marine biologist and Terrain NRM project coordinator Deb Bass says.

Fishways are a relatively new solution in the Wet Tropics region. They’re essentially a series of underwater stairs for fish, with deep pools between each one for resting. Rocks are placed precisely to create slower flow conditions, and they need to be large enough to withstand floods. Some fishways have an S-bend, others are built in a straight line, and some need to be longer structures to achieve the right gradient.

A ‘nursery slot’ is a new feature, trialled in one of the Wet Tropics fishways, to help super small juveniles move upstream.

“It needs 20mm or less of water going through it to create the right conditions for weaker-swimming juvenile fish,” Matt says. “We recently recorded the smallest snakehead gudgeon ever documented in Australian fishway studies travelling through the slot. It was 16mm long. That was the kicker for us.”

Geoff Collins, an aquatic ecologist from OzFish Unlimited and Adjunct Research Fellow at James Cook University’s TropWATER, says the new fishways are monitored for five days to assess the diversity and size of fish moving through. Results are showing they will make a big impact on the health of fish species and the waterways themselves.

“We are opening up habitat that many juvenile fish haven’t been able to access, particularly in low-flow situations, for decades in some cases,’’ he says.

“These structures are enabling migration for tens of thousands of fish. And the more habitat we can give these fish access to, the more productive our fish stocks will be. Freshwater wetlands, creeks and rivers are important nursery grounds with abundant food resources that’ll help our native fish to grow fast, and to travel back downstream and breed in the tropical wet season.”

The ‘Fish Homes and Highways’ project began with extensive survey work in the Herbert and Murray River catchments, where more than 3,500 potential fish barriers were identified, leading to on-ground inspections of over 300 sites.

The fishways were built at barriers like causeways, and in other places bed-level crossings were built to replace causeways, or creek crossings were completely removed.

“We’ve also removed aquatic weeds and there has been revegetation in an area where fish passage works were completed,’’ Deb says.

The Fish Homes and Highways project is funded by the Australian Government’s Reef Trust program.