In the case of most river systems, it is unlikely that stocking has yet significantly affected local stocks. It is thought that, over time, local conditions will tend to select out introduced strains. But problems could be developing in some rivers. We don't know exactly.
Therefore, it is better not to take risks and to apply the precautionary principle.
There are many differences between individual populations e.g. hatching times, growth rates, migration patterns, homing behaviour, spawning times, parasitic immunity, shape, appearance.
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