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Problems caused by Cormorants | FF0032 |
Ian Temby, Flora and Fauna Branch
June, 2003 |  |
Problem
Predation upon fish and yabbies at fish farms, including farm dams stocked for private purposes. Competing with sport and commercial fishermen and reducing fish numbers in creeks, rivers and estuaries. Faecal contamination of water stored for human consumption.
Background
The Little Black, Little Pied, Pied and Great Cormorants are common and widespread in coastal and inland waters, where they feed mainly on fish. As these species associate and feed in flocks, they have the potential to harvest considerable numbers of fish or yabbies from a feeding area. Losses from stocks of fish are increased by injuries caused to fish which are captured but not retained by cormorants. Cormorants generally roost over or near water, either in trees, on rocks or on water control structures. When they roost above water storages required for human consumption, coliform bacterial contamination from their faeces can be raised to unacceptable levels. Many deterrent techniques used commonly against cormorants are of little value after an initial "honeymoon" period. Devices designed to exclude the birds may be required to prevent excessive loss of fish.
Several studies in Australia have concluded that the impact of cormorants on fisheries is insignificant or, at worst, localised and short-term. These studies refer to open waters, rather than fish farms. Many of the fish taken are not commercial species, although the Great Cormorant readily takes Bream and Mullet in estuarine waters.
Solution
Exclude cormorants by enclosing the pondage with netting, where intensive aquaculture is practised. Netting needs to be pulled tight to minimise the risk of entangling birds. The cost-effectiveness of netting will obviously depend upon the size of the operation and the scale of losses expected in the absence of exclusion.
Optimum netting sizes are yet to be determined-some netting entangles birds.
Provide shelter for fish by encouraging aquatic vegetation or by placing logs, pipes or tyres in the ponds.
Scare cormorants using Bird Frite cartridges. To be successful, this requires vigilance and a knowledge of when the birds are likely to visit, so that scaring can be employed as soon as the birds arrive. Note that the use of gas guns and Bird-Frite cartridges may not be acceptable in built-up areas. Prior clearance from local Police and municipal authorities should be obtained before these devices are used in such situations.
Prevent roosting on trees adjacent to, or in water storages by pruning or removing trees, where this is compatible with conservation objectives.
Modify water control structures by the strategic placement of fine wires above roosting sites to prevent roosting by cormorants.
Destroy a specified number of birds under the conditions of an Authority to Control Wildlife issued by DSE, to enhance the effect of other scaring strategies, as part of a short-term remedy. Contact the DSE Customer Service Centre on 136 186 for an application form. Destruction of birds should not be seen as a substitute for long-term controls such as exclusion.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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