High-resolution mapping has revealed the remarkable extent of damage caused by spotted tilapia, with an estimated 147 tonnes of substrate excavated across just five assessed sites. Now Traditional Owners, scientists and cane growers have demonstrated a practical, targeted and cost-effective approach to controlling them.

Beneath the murky water of Far North Queensland’s cane drains, an invasive fish has been quietly reshaping the landscape.

High-resolution drone mapping in the Russell River catchment, south of Cairns, has revealed extensive excavation of agricultural drains by spotted tilapia (Pelmatolapia mariae), as the fish feed and construct nests in the sediment.

At five sites examined in detail, tilapia activity was estimated to have excavated 92 cubic metres of material – equivalent to approximately 147 tonnes of substrate – over time. Earlier three-dimensional mapping showed some excavations extending up to 50 cm below the expected drain bed, while surveys in August 2026 identified holes as deep as one metre.

But the significance of the excavation may extend beyond the amount of sediment being disturbed.

An invasive fish that changes its own habitat

Spotted tilapia don’t simply occupy agricultural drains. They modify them.

In softer sediments, breeding fish excavate conspicuous circular nests. In harder clay drains, repeated feeding and nesting creates pits and trenches that can merge into larger excavated areas.

For cane growers, that activity has implications for the condition and function of drainage infrastructure. For waterways downstream, it represents another potential source of loose sediment that can be mobilised during the Wet Tropics’ powerful wet-season flows.

But the excavations may also benefit the fish themselves.

Deeper areas can retain water as surrounding drains dry out, potentially creating precisely the kind of permanent refuge tilapia need to survive until the next wet season. It raises the prospect of a damaging feedback loop: tilapia excavate the drain, the excavation creates better tilapia habitat, and continued occupation leads to further disturbance.

Understanding how the fish use the drainage network, however, has also helped reveal a vulnerability.

Over the first two dry seasons of the study, Madjaybana Rangers working with scientists and local landholders removed more than 1,500 spotted tilapia from agricultural drains using an approach that combined high-resolution drone surveillance with carefully timed, targeted removal.

At successfully controlled sites where conditions allowed appropriate methods to be deployed, more than 95% of visually observed tilapia were removed.

Turning the dry season into an advantage

The work formed part of the Australian Government-funded Reef Coastal Restoration Project, or RCREST, led by the Reef and Rainforest Research Centre in partnership with the Madjandji Aboriginal Corporation RNTBC, local landholders and regional organisations.

Project scientist Dr Rickard Abom said the team began by looking closely at how tilapia were using the extensive drainage networks running through the cane-growing landscape.

During the wet season, drains connect and tilapia can move throughout the system. As water levels fall during the dry season, however, the network contracts and fish become concentrated in the remaining pools. Existing sediment traps provided an unexpected additional advantage by restricting movement between sections of drain during low-flow periods.

Rather than attempting to control tilapia throughout the year, the team targeted these isolated populations during the driest months and before the warmer wet-season breeding and dispersal period.

High-resolution drone surveys were central to making that approach more efficient.

Using an RTK-equipped enterprise drone, the team produced imagery with a resolution of less than 2 cm per pixel – detailed enough to identify tilapia schools at the surface, breeding nests and the characteristic scars left by feeding and excavation.

Instead of spending large amounts of field time manually searching drainage networks for tilapia aggregations, researchers could identify priority locations from the air and direct control effort to where fish were concentrated. Combined with strategic dry-season timing and relatively inexpensive capture methods, this precision targeting reduced search effort and concentrated people and equipment where they could achieve the greatest result.

That combination is central to the project’s cost-effectiveness: rather than attempting to manage tilapia across the entire drainage network, the approach uses high-resolution surveillance to find priority populations and relatively simple equipment to target them when they are most vulnerable.

Simple tools, strategically deployed

Once a tilapia hotspot had been identified, the solution was deliberately practical.

Madjaybana Rangers trialled combinations of inexpensive funnel traps, barrier nets, cast nets, scoop nets and drag nets, adapting the technique to the depth, width, vegetation and structure of each drain. In favourable conditions, a single drag-net operation could capture more than 30 large tilapia in about 15 minutes, while funnel traps baited with ordinary tinned corn and peas proved useful for smaller fish.

The approach became more effective as the Rangers and research team learnt how the fish behaved. Slow movements worked better than chasing them. Removing dense invasive vegetation reduced hiding places, barrier nets prevented escape, and targeting the fish when water was lowest concentrated effort where it could achieve the greatest result.

Suppression that survives the wet season

The Wet Tropics wet season reconnects the drainage network, allowing tilapia to move freely within and between drains. That makes the latest results from some previously treated sites particularly interesting.

At one site where no tilapia were detected by the end of the second dry season, none were detected when surveys resumed in July 2026 – despite an intervening wet season lasting around eight months. Early third-season surveys have also found no tilapia at several other monitored sites, while only small numbers have been detected at others.

The 2026 dry-season surveys have only recently begun, so the results remain preliminary. But they provide an important opportunity to test whether intensive, targeted suppression at priority sites can remain effective even after seasonal reconnection allows fish to move through the broader drainage network.

Clearer water within a day

One of the most striking changes happened almost immediately.

Tilapia continually disturb fine sediments while feeding and excavating nests. At controlled sites, field teams observed substantial improvements in water clarity within 24 hours of removing the fish.

That was environmentally encouraging, but it also made subsequent control easier.

At one site, only 34 tilapia could be removed in 2024, when poor visibility restricted the techniques available. In 2025, improved conditions allowed barrier and drag nets to be deployed and 468 fish were removed from the same site.

From catching fish to changing habitat

The findings suggest the next stage of management may involve more than simply removing tilapia.

Researchers are considering whether some tilapia-created dry-season refuges could be modified after fish removal, while native vegetation could help stabilise drain banks, reduce erosion and improve habitat for native species.

The approach is not being proposed as a one-off solution capable of eradicating tilapia from the entire Russell River catchment. Wet-season connectivity means fish can move through the drainage network, and ongoing monitoring and management will be required.

What the work demonstrates is that strategic suppression at priority locations can be achievable with relatively simple equipment when good science, local knowledge, precision technology and careful timing are combined.

For the Madjaybana Rangers, the project has also left behind practical skills in invasive species management, ecological monitoring and safe field operations that can be applied to future work on Country. For participating growers, it offers a way of tackling an invasive species that is not only an ecological problem but may also be contributing to the deterioration of the drainage infrastructure on which productive cane farms depend.

The two-year RCREST project concluded in April 2026, but dry-season monitoring of tilapia is continuing. The results point towards a control model that could potentially be adapted to other agricultural drainage systems across the Wet Tropics – targeting the right fish, in the right places, at the right time.