Pontus,
There are many different opinions on stuff like this. For instance, I was recently in Switzerland. They are technically not allowed to C&R on public land. If you catch a fish, you must keep it, you are not allowed to release it. This is an animal rights/moral thing to them. It's seen as ok to to fish or hunt for food, I mean, that's nature's way. But one should not torment the animals merely for the sake of sport! How evil. That would be like a hunter taking only leg shots, so that they could see blood and claim success but their quarry would likely live for another hunter to maim next year.
No one has ever said damn they really damaged the fishery by making it catch and release.
I have. There are cases where the stream didn't need such regs, there was no issue to begin with. But by putting them on, it makes it special regs and attracts large crowds.
There's also been discussions about overpopulation and stunted growth, right here on this forum. Many people think, for instance, Spring Creek could use a bit of a culling.
And keep in mind, with hunting, for instance, we choose when the deer season is precisely based on the time when it will damage the deer population the most. Right after the does become pregnant. Every doe you kill is the equivalent, on average, of killing fully 3 deer. It is closely regulated with management units, doe license allocations, etc. And hunters are the only group who advocates killing LESS deer. All of the other pressures come from insurance companies, timber companies, AAA, and the like.