I stumbled across this article on a device to drag fish down to the same depth you caught them at. It is the Black Tip catch and release tool. (
http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&storeId=11151&langId=-1&partNumber=12118527)
It was designed for salt water but it should work in fresh water. It makes more sense than fizzing whitch can cause infections and doesnt Gaurantee the fish will survive. I have seen a similar device (home made) that had a two pound weight attached to blunt hook that fit in the fishes gill plate. You attached your snap to a small eye on this and lowered the fish quickly back to where it originated.
The black tip seems like a better design because it secures the fish until you pull up on the line releasing it.
I dont fish deep water much anymore but this device might have the potential to save a few fish.
