Plants provide food and shelter for insects and shrimp, and also for stocked fish, but too much plant growth can hinder fishing.
If plants die in large quantities (due, for example, to spraying or winter die-off), the decay will cause massive oxygen depletion. Fish, and even the whole food chain, can be destroyed.
Excessive plant growth is mainly due to an overload of nutrients entering the dam.
The best way to prevent such growth is to exclude nutrients. Bypass any waste and run-off from dairies, piggeries, cattle yards and fertiliser stocks. Even run-off from heavily fertilised pasture can cause nutrient problems.
If you cannot prevent nutrients entering the dam, then plants may have to be removed mechanically or controlled with chemicals.
Mechanical control:
The simplest and least harmful method in full dams is to remove the plants with a rake or a dragline, but this is practical only in small dams. Another way is to drain the dam completely and either remove the plants or let them die. (Note. Filamentous (stringy) green algae will quickly re-colonise a dam if even a green tinge remains).
Lack of oxygen:
Low dissolved oxygen is a common problem in farm dams in summer, especially after rain, when organic matter (dead grass and animal droppings) has been washed into the dam. The first sign is usually dead fish, or dying fish which appear at the surface gasping for air.
The water must be circulated to expose the deeper water to the air. It is best pumped from the bottom and sprayed into the air.
The water body can often be ‘turned over’ simply by hosing the surface with a fine spray; the cool surface layer will eventually sink bringing the deep water to the top, where it becomes re-oxygenated.
Harvesting fish:
Angling laws must be followed, even on your own property. It is illegal to harvest a pond with a net, unless a permit is held. (Hand-hauled yabby nets are allowed.) If in doubt, contact your local Fisheries Officer.
Some fish develop off-flavours from blue-green algae. The taste can be removed by holding live fish for several days in clean aerated water or a 5g/L salt solution.
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