Where the fish flow

The fight to save a tiny fish from extinction could hang on efforts to halt the upstream spread of its invasive foe.

It rained a lot on Edgbaston Reserve, Bidjara Country, in early January: 100 mm – about a quarter of the reserve’s average annual rainfall. Most ecologists would be ecstatic to see so much rain fall in an arid region. But Dean Gilligan is not most ecologists, and though the rain was good news for the landscape, it made Dean concerned about one species.

“The water is great for the environment,” says Dean, Bush Heritage’s Freshwater & Wetlands Ecologist, “however when you want to eradicate a pest fish that occupies almost every puddle, you really want it to be dry.”

Dean is talking about the invasive Gambusia or Mosquitofish. Native to freshwaters of the south-eastern United States, Gambusia were shipped around the world in the early 1900s as a mosquito control.

In Australia, as in most countries where they were introduced, Gambusia have caused widespread harm to native aquatic species. At Edgbaston, an 8100-hectare Bush Heritage reserve in central Queensland, they’re placing immense pressure on one fish, the Red-Finned Blue-eye.

This species only inhabits the springs at Edgbaston and without Bush Heritage’s active management would cease to exist.

Gambusia can persist everywhere from near-freezing alpine tarns to brackish marshes and even the shallow, freshwater springs that dominate the eastern third of Edgbaston. Of the 50 springs occurring at Edgbaston that can support fish species, only 12 are Gambusia-free and these contain the last remaining populations of Red-finned Blue-eye fish.

When it’s dry, the springs are isolated from one another and naturally protected from the advances of Gambusia. But after decent rains, the flood gates open.

“Edgbaston is a very flat landscape. When it rains more than 20 mm in a day, the whole place goes under a sheet of water and Gambusia can go wherever they like – between springs and along waterways to find new vacant habitats that they can colonise,” says Dean.

To protect the Red-finned Blue-eye from Gambusia invasion, Bush Heritage have installed 40 cm-high shade-cloth barriers. The flip side is that they also pen Red-finned Blue-eye in – hindering immigration, emigration and gene sharing.

As a result, Dean and his colleagues must conduct regular genetic testing on top of the already resource-intensive task of constructing and maintaining hundreds of metres of barriers.

The current work at Edgbaston adds to a long legacy of contributions made by many dedicated scientists, volunteers, staff and partners to bolster the population and improve habitat for the Red-finned Blue-eye.

The latest addition comes in the unlikeliest of forms: laser beams from the sky.

Edgbaston might be flat to the naked eye, but the ground is in fact etched with almost imperceptible runnels and folds. Like miniature river valleys, these features form drainage lines between the spring mounds, and it’s by swimming upstream along those lines during floods that Gambusia are able to move across the landscape.

Figure out where those drainage lines converge, and you can potentially protect multiple springs in one fell swoop.

“To fence one individual spring, we might need 400 m of fencing, but with this approach we might be able to protect a cluster of six springs with a 50 m fence in a strategic location,” explains Dean.

To map the drainage lines, Dean and his colleagues have turned to LiDAR (Light Detection and Ranging) a remote sensing method that uses hundreds of laser beams per square metre to take measurements from the air.

The resulting point clouds can be used to generate 3D topographic maps accurate to 2 cm vertical increments.

“Using modern GIS spatial analysis, we can then generate a flow-path analysis to understand the path taken by a drop of rain falling anywhere on the reserve.

And by doing that you can map out all the separate micro-catchment areas on the reserve and understand the pathway that Gambusia are going to follow if they move against the flow to get from one spring to another,” says Dean.

It’s no small task. Edgbaston’s spring complex spans 6 km from north to south and 2 km across. The springs themselves range in size from small puddles to reservoirs the size of a football field, and new springs are popping up all the time.

A paper published last year found that the Edgbaston springs have trebled in size over the last decade as water pressure in the Great Artesian Basin rises due to the Queensland government’s bore-capping scheme. This has added another layer of not just complexity, but also urgency to Bush Heritage’s work.

With the LiDAR mapping already complete, Dean hopes to begin construction on the new barriers in early 2024. When that happens, it won’t just be the Red-finned Blue-eye that see the benefits. Many other endemic species call the Edgbaston springs home, too: the Edgbaston Goby, a dozen snails, a flatworm, a shrimp, an amphipod, a dragonfly and many plants.

All are impacted by the presence of Gambusia, which occupy the ‘Goldilocks zone’ of life in the springs. This is the area between the spring vents that millennia-old, deoxygenated water from the Great Artesian Basin bubbles to the surface and the outer periphery, which is oxygenated, and also more exposed to the extreme temperature fluctuations of the arid environment.

Long-term, Bush Heritage’s vision is to remove Gambusia from Edgbaston entirely. Starting in the northern, ‘upstream’ cluster of springs, Dean and the team will slowly work their way downstream – eradicating Gambusia and constructing barriers as they go until Gambusia are pushed off the reserve’s southern boundary.

“I can envisage the great Gambusia wall,” he says. “It might take a decade, two decades or even 50 years, but if we keep pushing them south, eventually we’ll have none left on the reserve.”

Bush Heritage gratefully acknowledges funding from WIRES National Grants Program which has made LiDAR mapping at Edgbaston Reserve possible.