Report on Carp removal at Brodie Road wetlands

Posted on August 25, 2015 by

Report prepared by Linton Dabinet – Horticultural Services Manager South. UrbanVirons Group

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Brodie Road Wetland (main pond)

European Carp, Cyprinus carpio Linnaeus, 1758

The European Carp is a native of Asia, but extensive introductions have helped to make it the world’s most widely distributed freshwater fish. Three strains of European Carp have been introduced to Australia, an ornamental strain near Sydney (1850-60), a Singaporean strain in the Murrumbidgee (1876), and a hybrid “Boolara” strain in Victoria (1961). The latter two strains have interbred and this species is now a major pest.

The European Carp can be recognised by its small eyes, thick lips with two barbels at each corner of the mouth, large scales and strongly serrated spines in the dorsal and anal fins. The colour is variable, but often olive green to silvery grey dorsally, fading to silvery yellow on the belly. Small European Carp could be confused with Goldfish, Carassius auratus. The latter however has no barbels on the corners of the mouth.

Carp are reported to grow to over one metre in length, and 60 kg in weight. In Australia, this species reaches 10 kg, but 4-5 kg is more usual.

In Australia, Carp occur in the Murray-Darling River system of Queensland, New South Wales, Victoria and South Australia. The species also occurs in many freshwater streams in coastal New South Wales and Victoria, as well as the brackish lower reaches of some streams and coastal lakes.

Carp are considered a pest species because they degrade the water quality of ponds and limit the opportunity for native fish populations to become established. They are a prolific breeder which reduces the survival chances for native fish.

Carp feed from the bottom of the pond, sucking in substrate (or material at the bottom of the pond) and water in search for aquatic organisms.

This decreases water quality by increasing turbidity, consequently preventing light penetration into the water. Less light penetration affects plant growth, which is a valuable food source for native fish, and contributes to erosion of the banks of the ponds.

Carp are often considered a harbinger of ecological disaster to many Australian freshwater systems. Increased incidence of disease in native fish, declining native fish populations, cross-breeding, increased water turbidity, loss of aquatic vegetation and the erosion of banks have all been attributed to European Carp populations. Unfortunately the environment in which Carp live is usually unseen to humans, and consequently the extent of damage caused is not readily understood.

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Scope of works

On Tuesday 4th August at 8am, staff from UrbanVirons Group undertook netting of carp at the Brodie Road wetland at Morphett Vale on behalf of the City of Onkaparinga. Removal of the fish was necessary to decrease turbidity in the water body.

A 75 metre long seine net with 19mm mesh size was used in conjunction with an aluminium flat bottom dinghy as a support vessel. The net was stretched across the waterway from North to South and dragged through the water body in a West to East and then East to West direction. This process was repeated twice. All of the catch was removed from the net as it made landfall at the ends of the water body. Non-native fish were euthanized in accordance with the permit conditions and indigenous fauna were removed and placed in a sterilised holding tank.

At the conclusion of the netting activity at approximately 3pm, 163 Carp of varying sizes had been removed from the waterbody, the biomass of which equated to several hundred kilograms. All fish were committed to landfill.

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Conclusion

The bulk of the Carp were removed from the waterway on the first pass of the net (approx 115 individuals of varying sizes). Including extremely large, mature fish which were heavily laden with eggs, and juveniles only a few inches long, indicating that the species has been able to successfully reproduce in this water body. Subsequent drags through the water body yielded a total of 163 fish. There was no evidence to suggest that the water body currently supports a population of indigenous fish species as none were encountered. Crustaceans such as yabbies and freshwater shrimp were present in small numbers and these likely provide a food source for the carp. 4 short necked tortoises were also netted at this location and returned unharmed to the water at the conclusion of the netting activity. Carapace lengths were 230mm, 220mm, 240mm, 220mm.

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Recommendations

It was noted that several large fish escaped the net on the final drag due to the net encountering underwater ‘snags’ and it is likely that these fish will reproduce. It is highly likely that Carp are being introduced to this water body by local persons for the purpose of recreational fishing. This was confirmed by a local resident who regularly walks his dog through the area (verbal communication).

  1. Signage should be erected to warn persons of the penalties for introducing Carp into the wetland environment.
  2. The water body should be netted twice annually (Winter , Summer) or when turbidity increases in order to reduce pest pressure.