Managing the threat posed by carp
We are particularly interested in better managing the impacts of carp and their effects on native fish populations. Historically, many catchments have sustained large numbers of native fish, which have now substantially declined and have been replaced with exotic species, such as Carp and Redfin. Today, many rivers only have relatively small, yet resilient populations of native fish including Trout cod, Murray cod, Silver perch, Macquarie perch and Two-spined blackfish.
Facts about carp in our catchment
- Carp are a very adaptive species and have successfully colonised many of the creek, river and lake systems in our catchment.
- Carp can compete with native fish for food and habitat.
- In some areas carp now represent up to 96% of fish biomass.
- Their can tolerate a wide range of conditions, from healthy through to very degraded aquatic habitats.
- Carp breed rapidly – females can lay between 300,000-500,000 eggs for every kg of body weight. In the upper Murrumbidgee catchment this can occur twice a year.
- Large numbers of offspring subsequently colonise the catchment.
- Anecdotal evidence suggests they are spreading into areas previously known to be carp-free.
- They carry and spread the parasitic copepod, Lernaea species which can affect native fish species.
- They feed on native fish eggs and larvae, which is a direct barrier to native fish recruitment.
A catchment approach
There is little doubt that carp are linked to aquatic habitat degradation, yet the exact nature of that relationship is not always entirely clear. For example, we know that carp feeding behaviour is directly linked to the reduction and up-rooting of aquatic vegetation and the undermining of banks, however their ability to inhabit severely degraded ecosystems also means that carp may not be the only cause of degradation. Instead, carp have the effect of further compounding the impacts of degradation rather than being the direct cause of it. As carp tend to stir up fine sediment, increase nutrients, further weaken rover banks and destroy aquatic plants, they add to degradation already present. The result is a decline in river health including decreased water quality, algal blooms, increased erosion and a decreased plant cover - all of which adversely affect native fish species. In order to protect and increase river health, all causes of degradation in our catchment need to addressed, and carp are one part of the problem.
Carp control strategies - Concurrent projects
Recently, the River Revival - Lachlan River Carp Cleanup Project identified that the most effective carp control strategies and tools to reduce carp biomass include:
- Identifying and targeting carp recruitment hotspots,
- Installation of traps at entry and exit areas to hotspots,
- Use of hormone implants to attract carp to traps,
- Trapping using Williams carp separation cages,
- The use of ‘Judas’ carp,
- Environmental water manipulation,
- Targeting physical carp removal to key times (e.g. when drought conditions lower water levels).
Research also shows that the most effective carp control programs target carp at breeding locations, remove carp migrating to spawning locations, exclude carp from breeding locations and/or control juveniles as they move out of spawning areas. Therefore if we are to manage carp successfully in any specific area, it is really critical that there is good information across the catchment about:
- Spawning/aggregation/nursery sites;
- Carp feeding/wintering/breeding behaviours; and
- Movement patterns of carp within rivers.
In another area, the Upper Murrumbidgee, the UMDR Carp Reduction Strategy
identifies that we are missing key pieces of information to help us better manage carp, such as knowing all locations across the catchment where carp might breed and tracking the extent and rate of carp dispersal. Gathering this knowledge via this FeraFishScan mapping facility is the first step for our catchment.