One thing to keep in mind as summer comes up is the warm water on top. As the lake stratifies the warm water and cold water will be seperated, the cold water being where the fish like to be but also deeper tham the warm water. As you bring these fish up through the warm water they go through fighting you and the temperature they don't like. After you land these fish they are extremely exhausted and stand a very slim chance to go back and survive. During the spring when the water is under 65 degrees they stand a pretty decent chance to survive upon release but when that water is in the 70s or above it is best to keep each legal fish landed. Same thing goes with touching of the salmon and then released back. Do not release a salmon that was touched with your hands no matter what the temperature is, shake them off with a pair of pliars at the side of the boat as their scales will come right off leaving them exposed to bacteria and viruses.
Not only temperature but the studies by Michigan State show most lake trout released at more than a 50 degree surface temp don't survive. It's also barotrauma from being hauled up from the depths. If you regularly or even occasionally want to release some salmon or trout, get a Seaqualizer (seaqualizer.com) which will allow you to hook the fish to a downrigger weight and release it in the deep cold water boosting its chance for survival to near 100 percent.
The South End provides useful resources and discussion forums for those that fish The South End of Lake Michigan for Salmon, Steelhead, Perch, and Bass as well as elsewhere in the Region and Chicago area.