The Mareeba-Dimbulah Water Supply Scheme (MDWSS) is a network of piped siphons, balancing storages, weirs and concrete or earth-lined channels located on the Atherton Tablelands in north Queensland. The network was built during the 1950s to supply irrigation water to the Barron, upper Mitchell and Walsh river catchments. Water moves via this distribution network from Tinaroo Falls Dam on the Barron River in an easterly direction, while the Mitchell and Walsh river catchments flow in a westerly direction towards the Gulf of Carpentaria.
Tinaroo Falls Dam is now home to a number of introduced fish species, including the invasive Mozambique tilapia and the black mangrove cichlid or spotted tilapia. Mozambique tilapia were first discovered in Tinaroo Falls Dam during the late 1990s and black mangrove cichlids were discovered a few years later. Since their introduction, both species have rapidly colonised Tinaroo Falls Dam as well as most of the Barron River and its associated tributaries.
The presence of tilapia in Tinaroo Falls Dam has increased the risk of these species invading westerly-flowing Gulf of Carpentaria catchments through the MDWSS network. Due to increased connectivity of Gulf systems during monsoonal flooding, tilapia have the potential to rapidly colonise other parts of the Gulf drainage, including environmentally sensitive wetlands. Exclusion screens were trialled as a way to prevent the spread of tilapia species from Tinaroo Falls Dam into western catchments via the MDWSS.
A range of stakeholders contributed to a tilapia risk assessment and management plan for the MDWSS in 2003 including several local councils now operating as the Tablelands Regional Council, the Barron River Integrated Catchment Management Group, Mitchell River Watershed Management Group, Tablelands Fish Stocking Society, Sunfish, Fisheries Queensland, the former Agency for Food and Fibre Sciences (now part of DAFF), Queensland Conservation Council, Queensland Seafood Industry Association, and SunWater. These stakeholders also collaborated during the screen design and construction phase. Screens were funded and installed by SunWater.
A stakeholder committee with representatives from local and state government departments, community and industry groups and water supply managers formed to work out the level of risk and potential management options for preventing the spread of tilapia from Tinaroo Falls Dam. The committee determined the risk of tilapia being spread via the MDWSS network to be very high and recommended a multi-faceted management approach. They recommended increasing public awareness of the tilapia issue, further research into the biology and ecology of both tilapia species, continued monitoring of westerly-flowing catchments for tilapia, and the installation of exclusion device(s) to stop the movement of tilapia through western parts of the irrigation network.
A number of investigations and feasibility studies were done to determine the best exclusion device for the task. Physical and electrical barriers were evaluated and possible installation sites were identified. The cost, volume of water to be filtered, velocity of flow, physical size of the barrier, maintenance and the size of the fish were important factors to be considered. A decision was made to install a static inclined screen, known as a Coandã-effect screen, which was found to be the most efficient and cost-effective screening device.
The main feature of this program was the use of Coandã-effect self-cleaning screens. This was the first time that screens of this type had been used in Australia for the exclusion of pest fish. The screens are made up of horizontal, stainless-steel wedge wires spaced at 0.5 mm intervals. Wires are tilted at an angle of 3–6 degrees toward the bottom of the screen panel to encourage the movement of debris down the screen face. This configuration enables the self-cleaning function of the screen, and reduces the need for mechanical cleaning.
Coandã-effect screens are typically installed on the downstream face of an overflow weir, so that water flows over and down the face of the screen panel. Filtered water falls through the screen mesh and is collected in a channel underneath the screen. Overflow water, debris and fish are transported to a collection area at the base of the panel and disposed of away from the original water source.
A 0.5 mm Hydro-Shear Coandã-effect self-cleaning screen was installed on the West Barron main channel in December 2004 at a cost of $1.3 million. The 0.5 mm mesh was selected to effectively exclude all tilapia life stages from the water, including eggs. An interpretive sign was put up near the fish exclusion screen to give visitors a basic explanation of how the screen works and why it was installed.
So far, the exclusion screen has been successful in preventing the spread of tilapia through the MDWSS into western catchments. Recent fish surveys found black mangrove cichlid juveniles immediately upstream of the screens. However, no tilapia have been found in the channel system or the associated balancing storages below this point. Ongoing annual fish surveys will continue to monitor for tilapia both up and downstream of the exclusion screen.
The mesh size and careful placement of the screen have been two important factors in the success of this program. Together, these worked to prevent the movement of tilapia eggs and larvae, as well as juveniles and adults.
The self-cleaning function of the screen has helped keep ongoing maintenance costs to a minimum.
The feeding of filtered material collected by the screen into eastwards flowing sections of the MDWSS has also worked to minimise the chance of tilapia spreading downstream into western-flowing catchments.
Extensive community consultation during the screen design and construction phase kept stakeholders informed of developments. This helped to increase community confidence and support for the screening project.
Screens are relatively expensive to design, construct and install.
Material filtered by screens needs to be disposed of at a sensible location to stop possible downstream spread of fish.
Screens need regular cleaning to prevent the build-up of calcifying algae that clog the screen and cause overflows.
Ongoing surveillance programs are needed to ensure screens are working effectively.
In this case study, installation of a Hydro-Shear Coandã-effect screen successfully prevented the spread of tilapia through the Mareeba-Dimbulah Water distribution network. This method may be applicable in other circumstances where there is a need to control the movement of pest fish.