Carp reduction program continues

Paul Donkers (Technical Officer-Carp Management IFS) outlines the latest progress.
European carp were introduced to mainland Australia in 1872. Their adaptability and fecundity have ensured their present position as the predominant fish species in the Murray-Darling basin and many other waterways on the mainland.

Carp have been implicated in macrophyte destruction, turbidity increases, invertebrate reduction and competition with native and other desirable fish species. In the mid 1970's carp were discovered in farm dams in Northwest Tasmania. By 1980 they had been eradicated by the Inland Fisheries Commission using rotenone (a fish poison).

In February 1995 IFC staff captured several carp in Lake Crescent after following up an angler's report. They were subsequently found in Lake Sorell. The confirmation of carp in two of our premier trout waters was of major concern. A carp taskforce was set up by the state Government in response to the threat to Tasmania's environment and economy. Strategies were developed to minimise carp impacts. These included measures to contain carp within lakes Sorell and Crescent and to reduce their numbers. Containment was initiated by installing fine-mesh screens at the outflow of Lake Crescent into the Clyder River.

Total containment had to eliminate any chance of carp migration via uncontrolled spillage because of floods. The Hydro-Electric Commission was enlisted to model the Sorell/Crescent system using available historical data and to formulate a water level management plan. The risks to containment posed by public access were assessed as significant because of previous human vector carp introductions o the mainland and in Tasmania. Lake Crescent was thus closed to the public. Lake Sorell having very low carp numbers, was deemed a low risk and was kept open.

Annual surveys in the Clyde system and investigations in other waterways have attested to the success of the containment process with no carp being found in other areas to date. A study was commissioned in 1998 to look at the possibility of draining the lakes and poisoning the carps using rotenone. This approach was found to be feasible, however, the associated risks or environmental damage (including destruction of the endemic golden galaxias) couples with the possibility of some carp inadvertently surviving the treatment ruled against the poisoning option.

The key to reducing numbers is to minimize recruitment. To achieve this water management was used to keep water out of the Lake Crescent marshes during the spawning seasons. This prevented mature carp from using prime spawning habitat. It also reduced the extent of suitable nursery areas for juveniles, thus exposing them to predation by trout and galaxias. Spawning triggers for carp include rising the levels and rising water temperatures in spring.   

At this time of the year they are especially attracted to warm inflows of marshy water.  This behavior has been used to advantage by artificially triggering controlled spawning aggregations through water releases from Lake Sorell into Lake Crescent at opportune times. With manpower and equipment at the ready these aggregations are efficiently destroyed. Reducing carp numbers by fishing down and destroying spawning aggregations was carried out using limited resources in 1995 and 1996.

A major breakthrough came with the introduction of radio-tracking technology in 1997.  Six male carp were implanted with radio-transmitters and released into Lake Crescent. They quickly proved invaluable for locating aggregations of carp. The implanted fish (trackers) can be picked up by receivers coupled with directional antennae from distances up to a kilometer. Batteries last for 12 months after which new males need to be implanted. Three or more trackers together often indicates an aggregation which can be surrounded by nets. Currently between 10 and 12 trackers are maintained in each lake. These fish are tracked daily from boats in both lakes during the spawning season (October to December) and at least once a week for the rest of the year.

Fishing techniques have been refined over the years and the range of equipment has increased to include gillnets, seine nets, fyke nets, traps, backpack electrofishers and an electroboat. The electofishing gear is very effective for driving carp into surrounding gillnets.
A typical day in the field for carp management staff will begin by obtaining rainfall, evaporation and lake level data. Next outlet screens need to be checked cleaned and flow adjustments made.

A boat in launched into Crescent or Sorell to thoroughly check the lake and to check fish traps. The location of each transmitter fish is marked on a map and compared to its previous position to access carp movement. If several trackers are found in close proximity near the shore it may be necessary to investigate further from land using a portable antennae  to pinpoint their exact position. If the fish are "in tight" (i.e. very close together) it will be decided to make a "hit'.

Circumstances will decide which fishing method is employed. On a sandy shore it may be a seine net. On an offshore reef a combination of 500m gillnet and  electroboat could be the preferred option and on rocky shores backpack-electronic fishing inside 100m gillnets is very effective.
Paramount to these processes is the need for stealth so as not to spook the fish. A gillnet is quietly deployed to enclose the fish. Gillnets are size selective so a thorough knowledge of carp population structure is essential for success. In Lake Sorell, for instance, three different sized nets are needed for a hit at present. First a 2.5" net is set. This forms a containment barrier to the large carp and also entangles the smallest cohort. Inside this neat are successively set 5" and 6" nets - the 5" for the three year old cohort and the 6" for the older fish. Fish growth and recruitment are different in each lake so that the type of nets employed varies according to location, time and circumstances.

Once gillnets are in place electrofishing may be used to muster the carp into the nets and to stun stragglers or a seine shot may be used to pull them to shore. Females and juvenile carp are killed. Mature male carp are tagged, weighed, measured and released into Crescent. The proportion of tagged to untagged males in subsequent captures indicates the population size and therefore our success in removing carp from the lake. Recaptured tagged fish are also used for growth estimates. A supply of tagged males is also necessary to ensure a continuing source of trackers.

Back at shore the killed carp are weighed and measured. Gonads are removed to be weighed and assessed for development stage. Otoliths (ear bones) are taken from the head to be dried and stored with identification tags, which can be later used to age the carp. Carp remains are then taken to a designated disposal site. At the end of the day boats are refueled and data sheets are completed. All data collected is entered into computers at head office where analysis is carried out to determine population size and structure, growth rates, net selectivity and many other useful parameters.

Monthly fyke-net surveys and juvenile backpacking are used through the spring and summer to check for recruitment.
No evidence of recruitment has been found in Lake Crescent for the summers of 1997/98, 1998/99 or 1999/2000. The 2000/2001 and 2001/2002 seasons are currently being assessed. The cohort from the last major recruitment in 1996/97 is now mature and able to be targeted by our radio tracking and fishdown operations. Approximately 7600 carp have been removed from Crescent in the past 6 years and last estimates indicate less than 20 adult females remaining.

In Lake Sorell, based on the radio of transmitter-implanted fish to other fish, population of adults is thought to be very small. Carp from spawnings in 1995/96 and 1997/98 are now reaching maturity and indications from catches this season reveal that both these spawning events were relatively small. In January 2001 a juvenile from a new cohort was discovered. The cohort is presumed to be the result of a 1999/2000 spawning event, though this is yet to be confirmed through otolith aging techniques.

The distribution of these juveniles was quickly assessed using sampling methods and areas of highest density were subjected to intense fishdown resulting in over 900 carp being killed. The impact of the reduction will be assessed using effort-based population estimation. More than ten aggregations were dealt with in the last three months of 2001. The largest of these occurred on October 23 in Crescent when 225 carp were caught and more than half the remaining females were removed from the lake. The biggest of these females weighed 3.25kg and carried 1.1 kg of eggs. The CSIRO in conjunction with the Inland Fisheries Service is currently modeling the fishdown procedure in order to asses the probability of removing the last female from Lake Crescent.

The CSIRO is also examining the possibility of developing a lethal gene which could be introduced into the carp population but it may be many years before this becomes an eradication option. With favorable rainfall, more effective tracking surveillance in recent years should allow marsh areas to re-establish without compromising carp reduction capabilities. Low carp numbers and high lake levels should see the re-opening of Lake Crescent in coming seasons. 

Paul Donkers

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