Adding more fish for anglers to catch causes ecological problems:
"Stocking waters with fish carries the risk of introducing parasites and diseases that may harm the fish, and developing fish stocks that could damage the environment, either by changing habitats or competing with other species.
Loch Lomond is a good example of the problem of introducing new species of fish. In recent years, ruffe have settled in the loch. They have almost certainly been introduced there by anglers using them as live bait for pike-fishing. These bottom-feeding fish may well be serious predators on the eggs and fry of powan, a rare whitefish living in the loch."