New strategy implemented to combat carp in Lake SorellThe attempt to eradicate carp from Tasmania hangs in the balance. While there is a good chance that carp can be eradicated from Lake Crescent, Lake Sorell poses greater problems. Over 7700 carp have been removed from Lake Crescent since they were officially found back in February 1995. There has been a steady fish down and the last mature female captured from this lake was in November 2003. There has been no successful recruitment in this water since the year 2000. Any females that remain should be on the verge of becoming sexually mature in the coming spawning periods (October-January). Population estimates show that if there are any females the numbers they will be very low.
Lake Sorell has some distinct differences to Lake Crescent and this is where the problem lies. From 1995-99 only 46 carp were captured in Lake Sorell. A mature female had not been caught for a long period of time and then wham juvenile carp turned up during surveys in 2001. It was quite a shock but showed that despite reducing carp to almost undetectable levels successful spawning could occur. Subsequent estimates showed that less than 3000 carp had come from this spawning.
Over 2200 of the fish from this cohort have now been caught. Intense effort was then put into trying to stop any further spawning occurring. Despite this effort in the December 2003 another spawning occurred at an isolated site in Lake Sorell. During regular surveys this spawning was detected, the site was contained within a fine mesh barrier, fished extensively using a variety of techniques and then treated to kill any remaining carp. This reduced the impact of the spawning but it is now known that some of these juvenile carp had already dispersed from the spawning site and have shown up in small numbers during recent work.
Lake Sorell (5310 ha) is over twice the size of Lake Crescent (2305 ha). It has a much larger catchment and therefore responds more quickly to any rainfall. This is important to carp success. The lake also has numerous wetlands and marsh areas where carp prefer to spawn which makes it difficult to prevent spawning despite intense effort.
What next then?
From the information that we have collected it is clear that Duck Day is the "hot" spawning site in Lake Sorell. An exclusion fence was constructed and has been maintained across the front of this marsh since 2001. This has been successful in preventing any spawning at the site since this time but has pushed the spawning carp to other locations. In the past two spawning seasons considerable effort has been put into fencing and placing traps to prevent access to numerous spawning sites around the lake. This has also been successful but further initiatives are needed to try and catch the remaining carp before they spawn.
Data collected from radio tracking over the past eight years has shown that there are clear movements of carp from the eastern to western side of Lake Sorell at different times of the year. All of the radio transmitter carp do it. A topographic examination of the lake shows that for the fish to pass from either side of the lake they need to pass through the isthmus between St Georges Island and the mainland (750 metres) or between the Island and Murdochs Point (1.5 km) through the Blowfly. This area is quite defined and becomes more so as the lake level drops.
There are distinct channels through each of the areas. For some time thought has been given to how this movement could be best exploited. Discussions with a local net manufacturer have been held and a plan devised to run barrier nets across both of these areas of the lake, effectively cutting Lake Sorell into two distinct sections. It is intended to place traps in these barriers to capture the carp as they attempt to travel through these areas in a similar manner to the work that has already been undertaken to block off the marshes. The barrier net is under construction and should arrive soon. It will be laid in place over the winter months.
Thought is being put into trap design and these will also be fitted. All of the marsh exclusion fences and traps will be inspected and maintained.
The division of Lake Sorell into two separate water bodies will have an obvious impact on people boating on this water. Boats will not be able to pass through the barrier net zones but access will still be available from the boat ramps at Dago Point and Silver Plains enabling boating on each side of the lake.