The War on Carp

Last spring’s carp spawning event and subsequent recruitment of thousands of juvenile fish in Lake Sorell marked the first real set-back for the Inland Fisheries Service carp team since the program began in 1995.

In the years after carp were first discovered and the invasion contained to Lakes Sorell and Crescent, the Service embarked on the lofty goal of eradicating the pest fish – something that no other State has attempted.

Prior to this season, the Service had made excellent progress towards this goal with the removal of over 10,000 carp from both lakes, annihilating the population in Lake Crescent and leaving less than 50 adult carp in Lake Sorell. However, a single female carp can lay over a million eggs in one spawning and the potential increase in Lake Sorell’s carp population as a result of successful breeding was still a major threat. Add to this, ideal conditions for spawning this season – unprecedented in the history of the carp program – and the carp team did well to maintain control of the population by capturing adult breeders and reducing the number of successful spawning events over the period. Despite the current battle caused by one or two female carp getting past the guards, the team is still determined and capable of winning the war against carp.

From the start of the carp program, the Service has been on a steep learning curve, firstly to understand the behaviour and ecology of the pest fish in Tasmanian lakes and then to develop the most effective ways of minimising the impact of its invasion. The foremost strategy, which has been successfully implemented under the carp program, is ‘containment’; preventing the spread of carp beyond the two interconnected lakes situated at the head of the Clyde River. Second to containment is population control; preventing a population explosion that would threaten the lakes’ ecology.

Through the process, the Service has gathered a wealth of intelligence and experience, and pioneered a range of innovative techniques to monitor, capture and prevent the spread of carp. It has developed a level of technical expertise in the area of pest fish control that is sought after by other national and international fisheries management bodies. The program has received ongoing support from both State and Federal Governments, including recent project funding from the Invasive Animals Cooperative Research Centre.

Aside from the removal of carp, the carp team has invested significant time and resources in anticipating and preventing carp breeding. This dedication has paid off with what appears to be the successful eradication of carp in Lake Crescent. The same result has been more difficult to achieve in Lake Sorell, due to the lake’s larger size, the extent of marshes offering ideal spawning habitat and the lack of control over environmental conditions such as inflow.  

The situation was worsened this year with higher than normal water levels which inundated the marshes in both lakes. Spawning carp are known to jump barriers in flowing waters but this behaviour has not been observed in Tasmania until this year, and is the likely method used by fish to access premium spawning habitat in Lake Sorell. Thus, despite the precautions undertaken in the preceding winter months with the installation of over 8 km of purpose built barrier nets to protect the marshes, several carp spawned successfully in Lake Sorell.

Although the ideal spawning conditions benefited carp recruitment, the carp team were able to counter-attack by decimating the adult carp population and targeting juvenile fish in the spawning grounds prior to their dispersal in Lake Sorell. To date, over 15,000 juvenile fish have been removed but it is estimated that thousands more still remain to be captured. With adult carp numbers now very low, however, there is a window of opportunity for the team to capture the remaining adults, while targeting as many of the juveniles over the next three to five years to prevent a large number reaching maturity.

Apart from this blitz on juvenile fish in Lake Sorell, the team plans to investigate ways of preventing future spawning within its current resource capacity. This is likely to include an expansion in the use of barrier nets and traps to protect marginal spawning areas and improvements to the fence design to cope with extreme lake level fluctuations. The team is also investigating the use of ‘pheromones’ as a chemical attractant to lure adult carp into traps at various locations around the lake. Meanwhile, monitoring for the presence of carp in Lake Crescent will continue.

The work undertaken by the carp team over the years, including these latest pheromone trials and habitat exclusion strategies, is innovative and unique but the daily grind can be arduous and set-backs, disheartening. Ridding the State of this pest fish is an immensely difficult job, and yet the carp team are dedicated, passionate and remain full of hope that the Service can ultimately reach this goal.