I think sport fishing has many more positives than the alternatives of commercial netting, or fish farming, regardless of whether it's C&R only or a personal harvest situation. People that fish are more likely to care about thier rivers and water ways, it's good for you physically and mentally, and sport caught fish are going to be cleaner and fresher than any other method of obtaining fish (unless you don't handle your catch with care). It's still possible to abuse the resource by wasting your catch, harrassing spawning fish, or disturbing redds etc, but the abusers are more liekly to be the same jackasses who litter and pollute regardless of what activity they are doing and the same people who figure driving 16 million cars in canada has no effect on the enviroment.
Overall sport fishing is going to have less of an impact on the resource than the other methods that currently harvest from it. Some people like to call sportfishing a zero rent activity since there is no direct rev from selling the fish, but that only applies if the social and heath benfits are ignored and given no value.
We're the top of the food chain, and until we live in a star trek world where we only eat synthetic proteins or a truely vegitarian diet, isn't it better to be more in touch with the food we eat, and how it is harvested than blindly picking it up in the super market? I know the majority of what I eat consists of locally grown produce, wild/hatchery fish, with locally raised beef. Still a lot of "bad" or non-eco friendly food in the diet, but it's still more similar to what my grandparents ate, than the typical processed and industrial crap of my parents generation.
Put it this way, any of the fish you see in the market was taken in a way that's out of touch with the enviroment, probably killed/wasted a significant number of other fish in bycatch, or in the case of anything farmed used 3 times as many pounds of wild fish for every pound of farmed fish produced. Heck look at the marketing of some products, the "dolphin safe" moniker is used to boast that they no longer kill flipper when harvesting that canned tuna sandwich.
So long as these other presures are on the resources I have no personal moral objections to sports fishing them, provided the fish I catch and keep or C&R are not endangered but are sustainable.