Anglers like to cite the work of Dr. James Rose, a professor of zoology and physiology at the University of Wyoming. He wrote a paper in 2002 debunking the idea that experiments such as injecting fish with bee venom and then observing their behavior leads to any reasonable conclusions about fish pain. He argues that fish don't have the neurological capacity to experience the "psychological aspect" of pain. They react to pain, but they cannot mentally process "pain" when painful things happen to them. It's the human observer who puts these mental states on the fish, who sees them lying on the ground and saying "Thanks a lot for killing me." The fish-feel-pain side counters that fish may perceive pain in ways that we cannot comprehend. Who are we to say that fish don't have a consciousness? And so on.
Whether animals feel pain, and whether humans have the right to inflict pain upon them, is a massive topic. We could all go read Peter Singer's Animal Liberation and meet back here in a week, but there is a brilliant shortcut to take. In 2003, David Foster Wallace attended the Maine Lobster Festival and dilated on whether or not a lobster being boiled alive is suffering terrible pain. I urge you to read the entire article, "Consider the Lobster," but this passage from a footnote helps us here.
Suffice to say that both the scientific and the philosophical arguments on either side of the animal-suffering issue are involved, abstruse, technical, often informed by self-interest or ideology, and in the end so totally inconclusive that as a practical matter, in the kitchen or restaurant, it all still seems to come down to individual conscience, going with (no pun) your gut.
What does my gut tell me about fish pain? Not happening. When I reel in a trout, I may be stressing the fish—making it expend precious energy—but it's not howling in agony.
That's not to say that I think fish should be treated cavalierly. Back in the day, whenever I caught a sucker fish (i.e., a carp) in my home stream, I'd pick it up and hurl it onto the railroad tracks. (The justification being that carp are taking up space in the stream that could be used by trout.) I wouldn't do that now. I don't have a good reason why. It's just a vague, gut-level notion that fish should be treated with respect, just as you shouldn't speed up in your car to run over rabbits. Fishermen admire their peers who catch a lot of fish, but these guys (and, often, gals) can leave you cold with their talk of "ripping lips" and "hauling them in." I remember an old guy who refused to wear polarized lenses (which provide the great advantage of seeing through the glare on top of the water). He told me: "I'm not a killer."
Then again, perhaps I should kill fish more often. One danger of catch-and-release fishing is that it makes fishing seem bloodless and clean, like target shooting on the water. It also turns angling into a numbers game. How many did I hook today? The overzealous angler can be a bit of rampager. Some of the fish that are caught-and-released don't live. I once landed a beautiful snook in a Florida bay, put it back in the water, and then, as the snook was recovering, watched it get chomped by a dolphin. If I were really concerned for the greatest fish good, I might do better to catch a fish, bash it over a rock, then take it home and eat it. With a side of lentils.