There is nothing more heartbreaking for a gardener or grounds-person than discovering a Fish kill in lakes
. Such a sudden and mass extinction of aquatic life can take many years to recover from. Fish kills can actually occur in a number of ways, some more serious than others.
It may happen that the kill off only effects one or two species or only the smaller individuals, but it may be the case that every fish meets their end. The number of fish that are affected can be as low as a dozen or as many as thousands. Once the problem has been brought to attention it is usually too late to bring about a solution.
The only method you can adopt to remove the threat of this problem is prevention. But before you can have peace of mind that you are doing all you can to reduce the threat of a fish kill from occurring it is important that the causes are understood in full.
The number one factor which leads to fish kills is oxygen depletion. For any species of fish to be healthy and thrive requires an adequate level of oxygen in the water. The recommended amount is between four and five parts per million.
If the oxygen levels were to fall to two or three parts per million than any fish will become stressed; this can be seen by a change in behaviour of the fish, they will be more likely to swim only at the surface of the water and will occasionally gulp the air. It is the larger individuals that are usually the first to expire if there are low oxygen levels. This type of kill off can happen rapidly in the space of twenty four hours.
If the pond or water body is rife with algae then this can have a serious impact on the conditions within the water. Less sunlight can impact on the ability for fish to stay healthy. Also, if a treatment is used to kill off algae this can result in a large volume of decaying organic matter which depletes oxygen levels considerably. Never let algae cover more than twenty per cent of the pond as if you do there is every chance that some of the fish will die prematurely.
Did you know that all ponds and lakes have distinct layers of water at various depths? Usually fish tend to frequent the warmer layers close to the surface which are also oxygen rich. The bottommost layer is often a cold region devoid of aquatic life and filled with harmful gases. At certain times of the year, usually the fall and the spring, the pond's layers can become mixed which destroys the balance of oxygen and allows toxic gases to rise up resulting in a fish kill.
If you live close to agricultural land then it is possible that your pond experiences a kill off of fish due to harmful chemicals entering the water. Herbicides, nitrates, and similar farm based compounds may be important for crops but are disastrous for aquatic life.