By Jonah Yick
The Common Carp (Cyprinus carpio) is regarded as one of the most invasive species in the world (Lowe et al. 2000), not only because of its destructive feeding habits, but its resilience in a variety of environments, and highly fecund reproductive nature. In addition to competing with native fish species for food and space (Brumley 1991, Diggle et al. 2012, Fletcher et al. 1985, Koehn et al. 2000), carp are also responsible for habitat degradation and the increasing turbidity in the waters they inhabit (Koehn et al. 2000). The first records of carp introductions in Australia occurred in 1859 and 1865, where carp were released into ponds in Victoria and New South Wales, respectively (Koehn et al. 2000). However, these fish were isolated to these water bodies, and it wasn’t till the 1900s when carp were released into the wild (Koehn et al. 2000). The spread of carp remained fairly limited until carp were introduced into the Murray River in Victoria in 1964, where they proceeded to disperse throughout the Murray-Darling Basin (Koehn et al. 2000). The spread was further assisted by widespread flooding in the early 1970s, as well as the translocation of fish to new localities (Koehn et al. 2000). Carp are now established in every state in Australia bar the Northern Territory, including isolated populations in Western Australia and Tasmania (Koehn et al. 2000). It is the population in Tasmania which continues to push the biological boundaries of this species, where carp continue to survive despite the relentless efforts of the Tasmanian Carp Management Program, and the extreme climates associated with their location.
Picture 1. A group of carp huddling close together for protection, this photo also features a close relative the goldfish, which is often the culprit of numerous carp sighting reports to the Inland Fisheries Service each year.
The biology of the common carp is what makes them so destructive, and once established in a particular water, makes the task of eradication difficult and in many cases impossible. The biology is summarised clearly by Koehn et al. (2000), which states that carp have broad environmental tolerances and thrive in habitats that have been disturbed by human activities. Although carp can grow to weigh 60 kg and 1200 mm in length (Brumley 1996), fish in the 50 gram to 5 kg range are more commonly seen (Koehn et al. 2000). Female carp mature between 2-4 years of age and can produce over a million eggs each year (Koehn et al. 2000). This estimate in egg production is largely dependent on the body size/maturity of the fish. They may also spawn several times in a year if conditions are adequate. The carp’s ability to grow quickly to a large size, and feed at low levels of the food chain suggests that they may prevent the transfer of energy and nutrients to populations of other large fish (Koehn et al. 2000). In particular, Carp feed by filter feeding small particles from either the water column or sediment, and it is this behavior that results in the stirring up of fine sediments and increasing turbidity (Koehn et al. 2000). When you take all these factors into account, you can see why the carp is regarded as one of the most invasive species in the world!
Picture 2. Spawning carp in the warm shallows
The majority of carp research in Australia has been focused on the Murray-Darling Basin, however, the Tasmanian model is unique in that the carp are now isolated to a single lake located in the central highlands, where the chance of complete carp eradication for the State is still a reality. Carp were discovered in Tasmania in the interconnected lakes Crescent and Sorell in 1995 (Diggle et al.2004, Donkers et al.2012). This occurred on the 28th January 1995, when an angler found the remains of a fish that was being eaten by a sea eagle (Diggle et al. 2012). After confirming that the fish was a carp, the IFS undertook back-pack electrofishing surveys on the 1st February, which confirmed that carp were present in Lake Crescent (Diggle et al. 2012). The outflow from Lake Crescent was closed and downstream surveys began immediately. Lake Crescent was closed to the public on the 18th February (Diggle et al. 2012). The outlet structure at Lake Crescent was fitted with an internal 1 mm meshed screen and the outflow was reopened on the 24th February to supply water for downstream domestic and stock use (Diggle et al. 2012).
Picture 3. Legislation enabled Lake Crescent to be closed to the public soon after carp were discovered
The discovery of carp raised immediate alarm and concern in the Tasmanian community at large, including state agencies, anglers, environmentalists, and farmers (Diggle et al. 2012). Initially a carp task force was formed, which later evolved into a working group with expert representatives (Diggle et al. 2012). The task force identified the following objectives:
1. Contain carp to the lakes Sorell/Crescent catchment
2. Develop a water management plan that provides for and protects the water supplies for Bothwell, Hamilton and irrigators to achieve objective 1 and assist with 3 and 4 below
3. Reduce the existing carp population
4. Eradication of carp
5. Prevent introduction to new water bodies and the reintroduction to cleared waters from both inter and intra state sources
6. Undertake legislative and communication strategies to minimise damage to tourism, while facilitating the above objectives.
Physical removal was deemed the best option and involved using an integrated fish down approach. This included the use of electrofishing (back-pack and boat), net fishing (gill and seine nets), traps (steel and fyke nets), and tracker fish (carp surgically implanted with radio transmitters) (Macdonald and Wisniewski 2011, Walker and Donkers 2011). Further recruitment was also prevented by deploying a combination of wire mesh and purpose built polyethylene barrier nets to block carp from their preferred spawning habitats (Diggle et al. 2012, Taylor et al. 2012). After maintaining vigilant fishing effort for many years, the last wild carp was removed from Lake Crescent in December 2007 (Donkers et al. 2012).
Picture 4. A haul of carp captured from Lake Crescent using a seine net
Since then sampling surveys have been undertaken each year to confirm the eradication of carp from Lake Crescent, and for the last 8 years there has not been any presence of carp. The eradication of carp from Lake Crescent has now allowed increased effort and resources to be focussed on Lake Sorell, using the same techniques proven in Lake Crescent but modifying them when required in order to improve efficiency.
Carp eradication in Lake Sorell posed a lot more issues than Lake Crescent due to its larger size and diverse habitat (rocky shores, deep reef structures, and large expanses of marshes). Carp numbers were relatively low in Lake Sorell until a large recruitment event in 2009, where juvenile carp were detected at various marshes around the lake (Inland Fisheries Service 2010). Rotenone was used to kill over 14 000 fish in the 6 weeks after the discovery, however it was estimated that approximately 50 000 fish were recruited in this event (Inland Fisheries Service 2010, 2012). Over the next 6 years the proven techniques and strategies used in Lake Crescent were used to remove as many carp as possible from Lake Sorell. Some of these strategies evolved but the principles still remained the same; to prevent spawning and to continue to catch as many carp as possible each year.
Picture 5. Juvenile carp aggregating in the warm shallows made targeting them with gill nets very efficient
The focus of the majority of the fishing effort remained on the carp from the 2009 cohort, however, small numbers of larger individuals caught from previous cohorts were still popping up each year (Inland Fisheries Service 2011, 2012, 2015). A mark-recapture population study was implemented in January 2012 in order to try and get a better grip on how many carp were actually left in the lake (Inland Fisheries Service 2012). This method was based on a similar study undertaken in Lake Crescent over the 1998-99 season, which yielded very accurate results (Donkers et al. 2012, Inland Fisheries Service 2012). A total of 803 juvenile carp from the 2009 cohort were captured, tagged, and released back into the lake (Inland Fisheries Service 2012).
Picture 6. One of the carp tagged as part of the mark-recapture population estimate.
Following the release of the carp, running estimates of the remaining carp population were able to be calculated based on the recapture ratios of tagged to un-tagged carp. Not only was this handy for the IFS to know when reporting back to stakeholder groups, but it was also beneficial towards the morale and drive of the Carp Management Program. The estimated number of fish remaining demonstrated that a real impact on the population was being made, and the goal of eradication was getting closer.
Although the peak carp season is defined as the months between October and February (peak water temperatures in conjunction with rising lake levels), fishing effort continues in the cooler months. Koehn et al. (2000) states that carp can adapt to water temperatures as low as 2oC and as high as 35oC. The carp’s ability to survive in such a broad range of temperatures is nothing short of amazing, and to witness carp being caught in such low temperatures is testament to their resilience. The Central Highlands of Tasmania is known for its inclement wild weather, and although it limits their growth, it fails to stop the mighty carp. Although the carp become significantly less active in colder weather, they still continue to move around the lake when conditions are suitable. Historically, transmitter fish were seen to congregatein just two marginally deeper locations of Lake Sorell in winter (Inland Fisheries Service 2014, Taylor et al. 2012). It is likely that the deeper water in these areas has a stable and warmer overall temperature, as it is less influenced by external environmental factors, thus the carp move to these areas to seek warmth. Although some carp have been caught as a result of these winter aggregations, the last few winters have proven difficult, as the fish have been widely dispersed around the lake due to high lake levels.
Picture 7. A cold, still morning at the IFS Lake Crescent Field Station.
Picture 8 (a & b). There aren’t many places around Australia where carp management activities occur in this sort of weather!
Picture 9. It makes it hard to catch carp when shards of ice are tangled up in the gillnet!
Picture 10. Despite the water temperature dropping to 2.4oC with the surface of the water frozen solid, these carp were still kicking around when removed from this holding pen.
Picture 11 (a & b). A catch of spring carp in cold, snowy conditions
Is it possible to eradicate carp from Lake Sorell, you may ask? The last 2014/15 financial year resulted in 1254 carp caught, which brings the total number of carp removed from Lake Sorell to 40 135 since their discovery in 1995 (Inland Fisheries Service 2015).Locating and targeting fish is becoming increasingly difficult. Consistent and high levels of gill net effort have been identified as the fundamental technique for not only fishing the population down, but also reducing the risk of spawning. As many as 13 gill nets, measuring anywhere from 100 to 750m each, were set and retrieved each day during the peak fishing period from October to February (Inland Fisheries Service 2015). The majority of these nets were set over night. Despite drastically increasing the fishing pressure by 30 times in the last two seasons, the total carp catch and catch per unit effort (CPUE) continued to decline (Inland Fisheries Service 2015).
Therefore it was recently decided to review the population estimate generated from the mark recapture survey, given the low catch rates despite increased fishing effort. The new estimate accounted for both natural mortality, and various scenarios of tag-induced mortality (Inland Fisheries Service 2015). By taking these factors into account, it was estimated that the remaining population size of carp in Lake Sorell as of July 2015 could range from 2078 to 3603 fish. This is strongly influenced by the percentage of tagging mortality considered (Inland Fisheries Service 2015). Active fishing continues to remove the larger fish from the population limiting the number of fish reaching maturity. It is envisaged that by the end of the 2017/18 season, the Carp Management Program will eradicate carp from Lake Sorell…Then, whats next? Redfin Perch? Tench? Goldfish? foxes..?!
Picture 12. Electrofishing a small aggregation of carp in the shallows over the 2014/15 summer
For more information on the trials and tribulations of the Tasmanian Carp Management Program, please contact Jonah Yick at email@example.com.
The Inland Fisheries Service Website includes links to many of the publications cited in this article:
Brumley AR (1991) Cyprinids of Australasia. Pp. 264–83 in: JS Nelson and IJ Winfield (eds) Biology of Cyprinid Fishes. Chapman and Hall, London.
Brumley AR (1996) Cyprinids. Pp. 99–106 in: R. McDowall (ed.) Freshwater Fishes of South-Eastern Australia. 2nd edn. Reed Books, Sydney.
Diggle J, Day J, and Bax N (2004) Eradicating European carp from Tasmania and implications for national European carp eradication. Fisheries Research and Development Corporation Final Project Report (Project No. 2000/182), Canberra.
Diggle J, Patil J, and Wisniewski C. (2012) A manual for carp control: The Tasmanian model. PestSmart Toolkit publication, Invasive Animals Cooperative Research Centre, Canberra, Australia, 28.
Donkers P, Patil JG, Wisniewski C, and Diggle JE (2012) Validation of mark–recapture population estimates for invasive common carp, Cyprinus carpio, in Lake Crescent, Tasmania. Journal of Applied Ichthyology. 28: 7–14.
Fletcher AR, Morison AK and Hume DJ (1985) Effects of carp (Cyprinus carpio L.) on aquatic vegetation and turbidity of waterbodies in the lower Goulburn River Basin. Australian Journal of Marine and Freshwater Research. 36: 311–327.
Inland Fisheries Service (IFS) (2010) Carp Management Program Annual Report 2009/2010. Inland Fisheries Service, Hobart.
Inland Fisheries Service (IFS) (2011) Carp Management Program Annual Report 2010/2011. Inland Fisheries Service, Hobart.
Inland Fisheries Service (IFS) (2012) Carp Management Program Annual Report 2011/2012. Inland Fisheries Service, Hobart.
Inland Fisheries Service (IFS) (2014) Carp Management Program Annual Report 2013/2014. Inland Fisheries Service, Hobart.
Inland Fisheries Service (IFS) (2015) Carp Management Program Annual Report 2014/2015. Inland Fisheries Service, Hobart.
Koehn JD, Brumley A and Gehrke P (2000) Managing the impacts of carp. Bureau of Rural Sciences. Department of Agriculture, Fisheries and Forestry, Canberra.
Lowe S, Browne M, Boudjelas S and De Poorter M (2000) 100 of the World’s Worst Invasive Alien Species A selection from the Global Invasive Species Database. The Invasive Species Specialist Group (ISSG). International Union for Conservation of Nature.
Macdonald A and Wisniewski C (2011) The use of biotelemtry in controlling the common carp (Cyprinus carpio) in lakes Crescent and Sorell. Technical Report No. 1. Inland Fisheries Service, Hobart.
Taylor AH, Tracey SR, Hartmann K, and Patil JG (2012) Exploiting seasonal habitat use of the common carp, Cyprinus carpio, in a lacustrine system for management and eradication. Marine and Freshwater Research. 63(7): 587-597.
Walker R and Donkers P (2011) An examination of the selectivity of fishing equipment in relation to controlling the common carp (Cyprinus carpio) in Lakes Crescent and Sorell. Technical Report No. 2. Inland Fisheries Service, Hobart.
Editor’s note: This is a thoroughly prepared article based on an impressive applied research and management commitment that is supported by outstanding photographs. A massive thank you to Jonah for contributing this article.