The artificial rearing of trout has been taking place in the UK and elsewhere for much longer than salmon farming and it is much more widespread. Trout rearing not only produces fish for fishmongers and supermarkets, but also maintains or improves the fish stocks in rivers for anglers.
As with salmon farming, the eggs are stripped from a mature female, fertilized by milt from a male and then hatched in a hatchery. After spending their juvenile stages in large tanks through which clean river water circulates, the young fish are then transferred to large ponds or to cages suspended in a freshwater lake or loch.
The same environmental problems arise from trout farms as with salmon farms. Waste food and faeces are present in the outflow water from fish-rearing ponds and these can give rise to the same effects on water quality as those of a sewage discharge, namely, the dominance of pollution-tolerant organisms in the aquatic life, depletion of dissolved oxygen and the increase in organic matter and nutrients. Artificially raised trout are prone to a variety of diseases which have to be controlled by treatment chemicals. The most common diseases are caused by bacteria or by a fungus and these are treated by adding formalin or Malachite Green. Fortunately trout are not so seriously affected by parasites and are not attacked by lice, so fewer chemicals are used in trout farming than salmon farming.
The main problem associated with trout farming is the release into the surrounding water of nutrients from excess feed and waste from the fish. As mentioned earlier, the input of phosphorus in particular can activate the development of algal blooms because this nutrient is usually in short supply for algal growth in unpolluted fresh water.
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