Case study: Queensland’s first attempt at tilapia eradication

Mozambique tilapia (Oreochromis mossambicus) is an invasive fish species thought to have been illegally introduced into Australia in the 1970s. Tilapia have spread both naturally and by people illegally moving fish between water courses. The species is capable of adapting to a wide range of ecological conditions and can therefore live in a variety of habitats. Mozambique tilapia are now common in both northern and southern Queensland, as well as around Carnarvon in Western Australia.

One of the first reports of a feral population of Mozambique tilapia in north Queensland came from the Townsville district in about 1977. Intensive fish surveys by the Queensland Fisheries Service (now part of the Department of Agriculture, Fisheries and Forestry) confirmed tilapia had invaded the area’s waterways. Ornamental ponds in the Townsville Botanic Gardens and surrounding creeks and stormwater drains, including tidal watercourses, were all found to be infested. A decision was made to attempt eradication of the tilapia from these water bodies in 1980.

The aim of this project was to completely remove (ie eradicate) Mozambique tilapia from the Townsville stormwater drainage network and ornamental ponds at the Townsville Botanic Gardens, using a fish poison – liquid rotenone.

Partners and management

The Queensland Fisheries Service worked with staff from the Queensland Museum and Townsville City Council to survey the distribution of tilapia and attempt eradication.

Process

Eradication attempts using liquid rotenone extract began in mid-June 1980. Population hotspots identified in the previous fish surveys were treated, including ornamental ponds in the Townsville Botanic Gardens, accessible sections of the stormwater drainage network (both upstream and downstream of the ornamental ponds), and an adjacent creek. Small hand-operated spray units were used to spray rotenone onto the surface of the water from both the shore and a boat. An outboard motor was used to help mix the poison into the water column. Where the stormwater drain disappeared underground, rotenone was poured directly into drainage water at the upstream entrance. The largest pond in the gardens was retreated after two days to maximise the chances of complete eradication.

Results

In the five days following application of the poison, more than 1000 dead Mozambique tilapia were recovered from the ornamental ponds and accessible sections of the stormwater drainage network.

To gauge the effectiveness of the eradication attempt, follow-up surveillance was done during September 1981. It soon became apparent that the eradication attempt had failed. Large numbers of Mozambique tilapia up to 200 mm long were found in both the gardens’ ponds and in nearby drains. The fish were also present in areas where they had not previously been found. Tilapia nests, some guarded by adult fish, were seen in tidal sections of the adjacent stormwater drain.

Why did this eradication attempt fail?

While this eradication attempt was well-intentioned, there were several factors that weighed heavily against its success. These included:

  • the presence of dense stands of aquatic vegetation that severely hampered efforts to distribute the poison evenly through the water column
  • the inexperience of staff distributing the poison, including a lack of appropriate training on optimum delivery techniques and on dosage rates suitable for Australian conditions
  • the inaccessibility of some sections of the stormwater drains that flowed underground for considerable distances
  • limited knowledge of the true extent of the tilapia infestation and the length of time they had been in the system
  • inadequate post-treatment surveillance, which needed to be more intensive and geared to allow the immediate retreatment of newly colonised areas.

Another major factor in the failure of the eradication attempt was a lack of community awareness of the problems caused by releasing tilapia into the wild. It is not known how much movement of tilapia between water courses was caused by people in the area actively moving them, but there was some evidence of this happening. During the 1981 fish survey, large numbers of tilapia were discovered in the effluent ponds of a piggery at a local prison farm on the outskirts of the city. The only way fish could have arrived at this location was through human release.

What was learnt

  • The full extent of the infestation and possible complications need to be assessed before attempting eradication.
  • Powdered rotenone, if not thoroughly mixed with water and a wetting agent, will clog the spray heads of distribution equipment, particularly small units.
  • A distribution system with a powerful pump is needed to apply rotenone effectively. More powerful distribution systems have proved more effective in recent eradication attempts.

Conclusions

Despite two rotenone applications to the affected Townsville water bodies and the resultant removal of over 1000 Mozambique tilapia, this eradication attempt was not successful. Total eradication of invasive pest fish populations can be difficult even when conditions appear to be ideal. Factors such as the physical dimensions of a water body, the presence of aquatic vegetation, and inadequate dispersal of the poison can affect the success of any eradication attempt using rotenone. Furthermore, if there are pathways open to allow recolonisation of previously treated sites, then the eradication attempt will inevitably fail. In the case described here, the Townsville Botanic Gardens’ ornamental ponds were connected to an estuarine creek that was a probable reservoir for Mozambique tilapia. In hindsight, the size of the creek and tidal exchange made it impossible to adequately treat the entire system with rotenone. However, at the time, this eradication attempt was seen as a unique opportunity to attack a new incursion at a relatively early stage. Had it been successful, the distribution of Mozambique tilapia in north Queensland today may have been different.

Author Invasive Animals CRC
Year 2014
Publisher Invasive Animals CRC
Pages 2
ISBN/ISSN PestSmart code: TILCS6
Control method Poison / Toxin
Region QLD
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