Electrofishing control of Mozambique tilapia in the Herberton weirs

Introduction

Mozambique tilapia (Oreochromis mossambicus) is an invasive fish species illegally introduced into Australia during the 1970s. Tilapia have since spread rapidly — partly due to the movement of fish between water courses by humans, and they are now found in many locations throughout eastern Queensland and in Western Australia.

In late 2003, Mozambique tilapia were found near Herberton on the Atherton Tablelands, 66 km southwest of Cairns in North Queensland. Individual fish were recovered from the larger of two weirs, which is the source of Herberton’s water. These weirs are located on the headwaters of Wild River in the Herbert River catchment. This catchment is the largest in the Wet Tropics bioregion and drains into coastal wetlands of high conservation value in the Hinchinbrook/Ingham area.

To assess the extent of the tilapia infestation at Herberton, Queensland fisheries researchers conducted electrofishing surveillance in early 2004. Almost 1000 fish were removed, most of which were juveniles¹. No further surveys or control measures were undertaken. By late 2004, tilapia were found to have spread about 500 m downstream into the second weir in the system. Following this discovery, the Department of Agriculture, Fisheries and Forestry (DAFF) held a meeting with the former Herberton Shire Council to discuss the threat that tilapia posed to the greater Herbert River catchment if they were to escape from the weirs. In 2006, with Council support, DAFF and the Invasive Animals Cooperative Research Centre (IA CRC) initiated a trial electrofishing program to remove Mozambique tilapia from the weirs.

Aims

• To reduce the risk of Mozambique tilapia spreading downstream from Herberton into areas of high conservation value in the greater Herbert River catchment.
• To assess the use of electrofishing as a method for controlling or eradicating Mozambique tilapia populations in small closed water bodies.

Partners and management

DAFF conducted the initial electrofishing operations in 2003 and 2004. Intensive electrofishing control efforts between 2006 and 2011 were supported by the IA CRC, with the approval of the former Herberton Shire Council, now Tablelands Regional Council (TRC).

Process

Management options available to stop the spread and/or eradicate Mozambique tilapia from the weirs were limited. Completely draining the weirs or applying fish poisons were not feasible due to the weirs’ use as a drinking-water supply. As an alternative, a research program was initiated to control the population of tilapia by physically removing the fish through monthly boat-mounted electrofishing. The cost of this control method was estimated to be about AUD $27 600 for each of the three years of the study’s duration.

Features of the program

The main feature of this program was the use of electrofishing, a technique for collecting fish from freshwater systems² commonly used by scientists to collect data on fish populations. Electrofishing works by passing an electrical current (usually pulsed direct current) through water to create an electrical field.

The current can be produced either by a battery (using a backpack unit) or a generator (bankmounted or boat-mounted unit). This sampling technique is not suitable for use in a marine environment as the fish have a lower conductivity than the surrounding saltwater (see More Info).

Boat-mounted electrofishing units send an electrical current into the water via trailing wires attached to poles (anodes), which extend from the front of the boat. The boat hull then acts as a cathode and completes the circuit. As the boat travels through the water, any fish within a few meters of the anode wires are drawn towards the front of the boat (electrotaxis) and subsequently stunned (electronarcosis). Stunned fish float to the surface where they are collected using a large dip net. Fish quickly recover once removed from the electrical field. Data such as length and weight are recorded before the fish are released unharmed, or in the case of pest fish, humanely euthanised.

Electrofishing as a management and control tool has some important limitations. It is:

• intensive and requires ongoing application to be effective
• expensive to implement and involves the use of specialised gear and trained staff
• unlikely to lead to total eradication, as some fish will always evade capture
• only likely to be effective in small, contained systems such as weirs and farm dams.

Results

• Over the three-year duration of the control program, the number of adult tilapia caught by electrofishing in the Herberton weirs declined dramatically³. In the first year (2006), the total number of sexually mature fish caught declined by 64% (from 0.2 fish to 0.05 fish caught/min). Over the entire three years (2006–2009), the number of sexually mature fish caught declined by almost 90%. In contrast to this, the number of immature fish caught over the same period increased by 366% (from 0.18 to 0.62 fish caught per minute). This increase in juveniles is likely due to a compensatory change in population structure, brought about by the removal of adult fish, which normally compete with and may cannibalise younger fish. Such a rapid rise in juveniles also indicates that the population would likely rebuild its numbers very quickly if electrofishing efforts were halted.

What worked

• Electrofishing successfully reduced the number of mature tilapia in the weirs.
• Water temperature significantly affected the efficiency of electrofishing, with cooler water temperatures resulting in higher catch rates.
• Removing adult tilapia from the population resulted in an increase in the number of juveniles, which may reduce the risk of individuals dispersing out of the weirs due to overcrowding.

What didn’t work

• Mozambique tilapia swim erratically when electrofished, making them more difficult to capture than Australian native fish.
• A mark-recapture study was not done at the start of this program, making it more difficult to determine the tilapia population size and probability of capture, as well as the effectiveness of the program. However, given the declared pest fish status of Mozambique tilapia in Queensland and the limited resources available for their control, a mark-recapture study was not a viable option in this case.

Conclusion

In the short term, regular electrofishing appears to have halted the spread of Mozambique tilapia from the Herberton weirs into the greater Herbert River catchment. Targeted electrofishing removal has resulted in the tilapia population now being dominated by juvenile fish. Integration of control methods such as gill netting, trapping and/or native predator introduction will be needed to achieve full eradication of Mozambique tilapia in the weirs.

References

1. Russell DJ, Thuesen PA and Thomson FE (2010). Development of Management Strategies for Control of Feral Tilapia Populations in Australia. Department of Employment, Economic Development and Innovation, Brisbane, Queensland. Pp 55.
2. Cowx IG and Lamarque P (1990). Fishing with Electricity: Applications in Freshwater Fisheries Management. Blackwell Scientific Publications, Oxford

More information

Thuesen PA, Russell DJ, Thomson FE, Pearce MG, Vallance TD and Hogan AE (2011). An evaluation of electrofishing as a control measure for an invasive tilapia (Oreochromis mossambicus) population in northern Australia. Marine and Freshwater Research 62:110–118.

PestSmart Toolkit for Tilapia

Invasive Animals Ltd has taken care to validate the accuracy of the information at the date of publication [August 2013]. This information has been prepared with care but it is provided “as is”, without warranty of any kind, to the extent permitted by law.