Knowing your enemy is important to any defence strategy, including the management of invasive fish species. Analysis of the biology of the species is therefore an essential part of an integrated pest management plan.
A lot is known about the biology and population structure of carp in Australia and certain stages in its life cycle are regarded as weaknesses that present opportunities for management. This factsheet explains these weaknesses and the implications for managing carp.
The biology of carp in Australia
Carp spawn (lay eggs) when water temperatures reach about 17oC, which is usually in spring. They prefer shallow, vegetated areas usually less than 1 m deep. The eggs are sticky and are usually attached to vegetation. They hatch within 3–8 days, after which the juvenile (larval) carp move passively downstream with high water levels. Early growth usually takes place in stream backwaters or in lake marshes.
Research by the Invasive Animals CRC has shown that most successful carp spawning in the Murray–Darling Basin (MDB) takes place in only a few localised areas. It is likely that more than 80% of carp in the basin come from a small number of key nursery areas. These include major wetland areas such as the Macquarie Marshes, the Namoi and Gwydir wetlands and the Barmah–Millewa Forest.
Spawning migrations and aggregations: Mass movements of any species usually mean that they will be concentrated at some point at a particular time. The fact that there are only a few preferred spawning sites of carp means that the carp will make mass migrations to these sites. In Australia, carp are known to:
- make predictable, annual migrations between river and wetland habitats for spawning, starting in early August
- be attracted to flowing water, especially at spawning times, and move upstream towards the source of the flow
- jump and push past migration barriers
- release pheromones when spawning, to attract other carp.
Once they arrive at their preferred spawning location, large numbers of carp usually spawn at the same time.
These spawning aggregations are often quite vigorous and visible. Adult carp are also known to form winter aggregations in specific areas in lakes and probably also in large rivers. The locations of these aggregations are quite predictable and are probably related to slight variations in water temperature.
Juvenile carp will also form large aggregations. These tend to happen below stream barriers in autumn, and are probably related to their dispersal from the juvenile growth habitat.
Carp appear to move very little in streams for most of the year, suggesting that their feeding habitat is located nearby. By contrast, carp living in lakes are known to move considerable distances when feeding. Some patterns are evident in this movement, with large, easily visible aggregations of carp feeding in shallow areas.
Population genetics: Even though some of the carp strains were introduced to Australia more than 100 years ago there is still evidence of genetic isolation in the population structure of carp within Australia.
Carp are usually only available as prey for Australian native fishes and birds in the first 12 months of their life, as they grow quite rapidly. For the early part of this growth, their preferred habitat is vegetated marsh areas in lakes or river backwaters.
Implications for management
Most control strategies are more effective if applied at particular stages in the life cycle of the target pest species, or at specific times of the year, or at certain places where the pest is found.
Specific carp vulnerabilities have been analysed by the IA CRC6. The implications for carp management are that control strategies need not be implemented across thousands of kilometres of river basin. They also do not need to be carried out all year round.
Control strategies related to spawning migrations and spawning sites could focus on targeting:
- adult carp with traps as they enter and/or leave spawning areas where there is a controllable access point
- carp that could enter these preferred spawning habitats
- water flows to help capture migrating adult carp
- water levels in wetlands to prevent access to preferred spawning sites
- water levels in wetlands to assist capture of carp
- the spawning aggregations of carp in wetland areas
- areas where carp eggs have been deposited — treating by desiccation (drying) or with spot poisoning
- juvenile carp with traps as they move out of wetlands
- spawning sites as release points for biological control agents such as koi herpes virus
- movement and aggregation of carp at these sites using attractants (such as pheromones) or repellents.
Control strategies related to other aggregations of carp could focus on:
- identifying winter aggregation sites in lakes and rivers, and targeting these using appropriate methods such as electrofishing, netting, poisoning or explosives
- identifying feeding aggregation sites in lakes, and targeting these using appropriate methods
- identifying juvenile aggregation sites and targeting these using appropriate methods.
Knowledge of the genetic structure of carp populations in Australia can be used to:
- identify small units within the overall MDB population where physical removal methods are likely to be effective due to limited carp movement
- help plan release strategies for biological control options, if they become available in the future.
Knowledge of the predators of carp, and their capabilities to reduce carp populations could also potentially be used, where suitable.
Not all of these strategies will work at any one site, and their use will depend on the nature of the particular site and how it is used by carp. A combination of strategies is likely to offer the best chance of success.
|Author||Invasive Animals CRC|
|Publisher||Invasive Animals CRC|
|ISBN/ISSN||PestSmart code: CPFS10|
|Control method||Integrated Pest Management|
|Region||Australia - national|
|Documents||CPFS10 PestSmart factsheet: Carp weaknesses & vulnerabilities [370 kb PDF]|