The others are right on about how to gut a trout.
I am a firm believer that most of our streams are overpopulated. Keeping a few helps thin them out, allowing the remaining fish to have more food.
I rarely keep trout, simply because I am not much of a fish eater. I usually keep a few when backpacking, just because I feel like it is part of the whole experience.
I have kept 3 rainbows in the past three years that were not caught on backpacking trips, all 3 swallowed the fly and were in pretty bad shape, bleeding badly and not really able to stay upright in the water. I was confident that were going to die, so I dispatched them and brought them home to eat.
As far as fish having predators, they do and always have had predators, birds and such. Humans keeping and eating fish has been going on for thousands, possibly millions of years. The fish are still here. With that said, logging has certainly had an impact on the populations, especially the brook trout. If the biologists felt creeling a few was going to lead to their extinction, I don't think they would allow us to keep any.
Most anglers in the backcountry release their fish, or keep a few here and there. Oldman and I kept 2 trout last weekend. The waters we fished probably hold 30000 - 50000 trout, now they hold 29998 - 49998 trout. I don't think we hurt a thing. The hole that I caught the 8.5 inch brook (one of the ones we kept) from, I caught 2 or 3 smaller brooks from that same hole. I could be wrong, but I bet the one we kept out competed those smaller ones for food. Maybe now they will grown larger without his competition for food.
I don't think the limit of 5 fish is set because few anglers can catch 5 keepers. I would say there are few anglers who could not achieve this goal on just about any given day, barring freak weather, or super low water, or just a really crappy day of fishing. I am sure we all have had a day like that here and there.