1. Gentle hold - In my experience a very gentle hold (no squeeze), results in a calmer photo subject. And hopefully a healthier one when you release it back to its watery home.
2. Get the eye in focus! - If the eye of the fish is in focus then your depth of field does not matter. In fact, having some of the fish or image blurry can be a great effect.
3. Keep the fish low and over water. - This is both for the visual and the fish. When practicing catch and release, the fish should ALWAYS be a place where if it squirms out of your hand it is good to go!
4. Backdrop matters. - This one sounds like a no brainer, but in the chaos of the moment, with a good fish in the net, it is important but not easy to think about what is behind the fish when you shoot. Showcasing the river or lake scenery as opposed to a rocky bank is always a good bet.
5. Shoot a million frames. - I am assuming you're not shooting with a throwback film camera, in which case you are taking digital images, not wasting film, so shoot, shoot, shoot! But quickly. Using photo-bursts is a great way to do this - one click and tons of photos. Remember, time out of water is harmful for trout survival so shot fast. The more you take the better your odds at THE shot you were hoping for.
-Garrison Doctor RYW Designer