Case study: Eureka Creek tilapia infestation — a threat to western drainages

Two species of tilapia, Mozambique tilapia (Oreochromis mossambicus) and the black mangrove cichlid or ‘spotted tilapia’ (Tilapia mariae), have invaded waterways in tropical and sub-tropical Australia. Following their introduction in the 1970s and 1980s, both species have increased in number and have spread rapidly by natural dispersal and human intervention. Black mangrove cichlids are now present in many rivers and creeks in the Wet Tropics region of north Queensland while Mozambique tilapia occupy a large number of catchments in both north-east and south-east Queensland as well as in Western Australia.

In 2008, during a routine pest fish surveillance program, a population of black mangrove cichlids was discovered at Eureka Creek, a tributary of the western flowing Mitchell River in north Queensland. This was a significant discovery as it represented the only confirmed occurrence of a species of tilapia in a western flowing drainage in Queensland. It is possible that this population became established from individuals escaping though the Mareeba-Dimbulah Water Supply Scheme (MDWSS). Although fine mesh screens had been installed on the western flowing channels of the MDWSS, it is possible that tilapia circumvented these screens and made it into the Mitchell River Catchment. The other possible method of introduction was by people releasing fish either directly into the waterway or into nearby waterbodies such as farm dams.

Follow-up surveillance after the initial discovery in Eureka Creek found black mangrove cichlids to be in relatively low numbers and limited to an isolated section of stream. This suggested that the species was only recently introduced and that the population had not had a chance to spread. This, coupled with the proximity of the infestation to a main road and a nearby township, implied it was highly likely that the infestation stemmed from the illegal, local release(s) of fish by members of the general public into the waterway.

Aim

To stop the spread of tilapia into the previously unaffected Gulf of Carpentaria drainage by eradicating the Eureka Creek population using the fish poison (or ‘piscicide’) rotenone.

Partners and management

This eradication effort was managed by Fisheries Queensland (part of the Department of Agriculture, Fisheries and Forestry (DAFF)) in co-operation with other agencies and organisations including water provider SunWater, the Environmental Protection Agency, the former Department of Environment and Resource Management, the Wet Tropics Management Authority, the Tablelands Regional Council, the North Queensland Pest Fish Committee, the Mitchell River Catchment Group, traditional owners of country the BarBarrum, Djungan, Chillagoe, Mitchell River and Kowanyama peoples, and affected landholders.

Process

Before the start of the eradication exercise, extensive consultation was carried out with stakeholders. This included relevant federal, state and local government departments, regional natural resource management groups, farmers, indigenous representatives and the general community. The management options considered were:

  • do nothing and monitor the incursion
  • undertake a ‘fishdown’ operation to reduce population numbers
  • use a fish poison in an attempt to kill all tilapia within the area.

Stakeholders were fully involved in the management and decision making process, and were aware of the potential consequences and outcomes of each of the management options. Stakeholders all agreed that the best option was to use rotenone to attempt to totally remove  tilapia from Eureka Creek. This decision was based on the fact that water flow in Eureka Creek could be regulated, and that sand barriers could be used to create a ‘closed’ water body, which would assist the poisoning process.

Features of the program

Pre-eradication surveillance found that the population of black mangrove cichlids was confined to a 3 km section of Eureka Creek. In October 2008 that section of creek, plus a 1 km buffer both upstream and downstream of the known incursion area, were targeted for poisoning. Water flow in Eureka Creek during this time of year is regulated by the MDWSS. The water supply to the creek was subsequently ‘switched off’ prior to the application of rotenone. Sand dams were constructed to block upstream and downstream movements of fish and possible rainfall inflows. Attempts were also made during this time to reduce the impact on native fish by capturing individuals via electrofishing and moving them to pools outside of the treatment area.

Powdered rotenone mixed with a wetting agent was sprayed onto the water surface, producing a lethal concentration of 0.25 mg/L. This slurry was applied from the creek banks using backpacks and petrol-powered spraying apparatus mounted on all terrain vehicles and four wheel drive trucks. A small boat was used in the largest of the pools to help distribute the piscicide. During poisoning, officers attempted to relocate distressed native fish and turtles that had been partially affected by rotenone into untreated parts of the creek.

The whole operation, including preparation and cleanup, took up to 20 people a week to complete. The actual creek poisoning was carried out over 1.5 days. Water and soil samples were taken and analysed 2 days after the poisoning, to ensure safe levels of rotenone before water flow in the creek system was allowed to resume.

Results

The poisoning program removed 34 black mangrove cichlids and unexpectedly a single Mozambique tilapia. The four largest pools in the treatment area were re-surveyed via electrofishing two days after the treatment was completed. No fish of any species (including natives) were found alive in the pools. Post-treatment recovery of native fish populations in Eureka Creek was relatively swift, with many species recolonising the affected area within a month of the treatment. Up until mid 2011, post-treatment surveys had not found any tilapia in Eureka Creek or in other areas of the Mitchell River catchment.

What was learnt

  • Conventional fish sampling techniques, including electrofishing, are not always 100% effective at detecting pest fish species — especially when pest fish populations are at very small numbers. For example: a single specimen of Mozambique tilapia (a species which had not previously been found in electrofishing surveys) was found during the Eureka Creek poisoning operation.
  • Fish poisons such as rotenone are non-selective. Stakeholders need to be aware that when piscicides are used, there will be deaths of both native fish species and possibly other vertebrates. Coordinated and intensive efforts need to be made prior to piscicide application to limit the amount of non-target species deaths.
  • Rotenone needs to be mixed thoroughly into the water column, to maximise the poisons’ effectiveness. This includes distributing the poison under root mats and in shoreline grasses and reeds. Using paddle wheels and outboard motors can be effective ways of distributing the poison into these densely vegetated areas.
  • Laboratory tests to determine residual concentrations of the poison in water and soil samples are costly and can take a long time to process — particularly if they need to be shipped interstate.
  • Appropriate workplace health and safety precautions need to be put in place to ensure the personal safety of all involved. During this eradication exercise a strict dress code including appropriate footwear, disposable overalls and use of canister masks was enforced for all staff distributing the poison. Other mandatory precautions should include the use of cleansing facilities before the consumption of food or drink and a ban on smoking.
  • All stakeholder groups in the broader geographic area must be informed of the implications of a planned eradication attempt. Without their support, an eradication attempt may be stopped at any point. Early identification of key spokespeople, particularly for local indigenous groups, helps to ensure the required information is communicated to all parties.

What didn’t work and why

Efforts to relocate distressed native fish from the poisoned section of creek to untreated areas were not effective. A number of individuals were moved upstream to clean areas but died soon after. These included species such as sooty grunter (Hephaestus fuliginosus) and rainbow fish (Melanotaenia spp). Attempts to rescue native fish living in areas to be treated with a piscicide should be done over several days before poisoning operations begin. A relatively mild method such as electrofishing should be used to capture fish for relocation.

Conclusion

This is the only known example where a piscicide has been successfully used in a flowing creek system to eradicated tilapia in Australia. Effective coordination of a multidisciplinary team of up to 20 people and early engagement with local stakeholders contributed to the success of this operation.

Author Invasive Animals CRC
Year 2014
Publisher Invasive Animals CRC
Pages 4
ISBN/ISSN PestSmart code: TILCS5
Control method Poison / Toxin
Region QLD
Documents Eureka Creek tilapia infestation — a threat to western drainages   [455 kb PDF]