The carrying capacity of a dam (i.e. the maximum weight of fish it can support) depends on the amount of food produced, size of the dam, water quality, species of fish and other factors.
Carrying capacity is generally independent of water depth, as food is usually produced in the water near the surface. The stocking rate is therefore calculated from surface area alone, ignoring depth and volume.
Table 1 gives recommended numbers of each species to stock in farm dams of different sizes. To estimate the size, count the number of long paces around the dam; this is approximately equal to the perimeter in metres.
If several species are to be stocked together, the quantity of each should be calculated as a fraction of the total.
Typically, NSW farm dams have a carrying capacity of 200–500 kg/ha. This means that a 0.1 ha dam with a carrying capacity of about 500 kg/ha would support about 50 fish averaging 1 kg, or 25 fish of 2 kg each.
At higher densities, growth slows. Maximum production can be maintained by removing small numbers of fish as required. temperature or water chemistry. If possible, liberate the fish near cover.
Survival and growth:
Native fishes are usually 3–5 cm long when sold and trout 7–10 cm. Provided the dam is free of predatory fish and birds, there is little advantage in stocking larger fish. Survival is usually around 70%, but far less if predators are present. Growth rates vary a lot and cannot be predicted without a good understanding of conditions in the dam. At the recommended stocking rates, fingerlings commonly reach 100–500 g in the first year.
Food:
The natural food for fish in farm dams is plankton, insects and yabbies. Animals that are not usually found in farm dams, such as freshwater shrimp and small fishes, can be added; they will breed and increase the food supply. Shrimp caught with scoop nets from permanent streams and lakes can breed to large numbers in dams with aquatic plants. Small, locally sourced native fishes such as gudgeons and smelt, or even bony bream, can supplement natural food.
Inorganic fertilisers are not usually needed, as nutrients are often plentiful in farm dams. The food chain, including plant growth, usually depends more on light penetration and algae production. Supplementary feeding with artificial food is not recommended. Uneaten food can decay and pollute the water, resulting in poor water quality, disease, and excessive plant and alga growth.
Fish stocking, survival, growth and feeding:
Releasing fish:
When releasing fish, sit the bag in the dam water for 10 minutes to let the ambient temperature adjust to prevent thermal shock. Then open the bag and let in small amounts of dam water over 20 minutes, so that there is no sudden change in temperature or water chemistry. If possible, liberate the fish near cover.
Potential problems:
Predatory birds:
Cormorants are common throughout NSW, even in dry inland areas. Their visits to dams are infrequent and unpredictable, but once a dam containing fish is located, the birds work it until they have taken most of the fish. Even if prevented from landing on the water, cormorants will land on the bank and walk in. They are the most prodigious predators of fish in dams and soon learn to ignore scare devices.
Shooting them is prohibited, as they are protected in NSW.
There is no simple single method of control, and a combination of methods may be the best. Provide refuges, such as fallen timber, to give fish some cover and protection from birds. (Where dams are regularly de-silted, such refuges must be portable.) Provide plenty of alternative food for the cormorants.
Large numbers of yabbies, shrimp and small fish in a dam will lessen the predator pressure on the stocked fish, as the birds will prey on the most abundant food source.
Other birds (pelicans, and wading birds, such as herons and egrets) do not pose much of a problem, except in shallow dams. These birds are also protected. Ducks are essentially herbivorous and therefore harmless.
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