Here is an short excerpt from the Ministry on the subject.Hoover wrote: and when your bringing these fish up from such deep depths the chance that they survive is like less than 10% they usually float back up around the boat. You might as well toss it in your cooler now instead of scooping the floater up a few minutes later.
Releasing Fish Caught From Deep Water
Releasing fish caught from deep water (more than 10 metres or 30 feet) can present special problems. Cold water species such as lake trout can rapidly equalize changes in water pressure. They can usually be released successfully regardless of the depth they came from, summer or winter. Warmwater species don't have this pressure-equalizing ability. Fish such as pike and walleye therefore should be brought to the surface slowly (to allow them to adjust to the change in pressure) and then be released immediately, if possible. Fish caught from deep water can be released by allowing them to slip out of your wetted hands head first into the water, while you are in a standing position. This not only heads the fish in the right direction, but also sends a blast of oxygen through its gills.