In many instances, that release mortality figure is used in determining fishing regulations and is set by using a statistical process based on randomly sampling anglers to estimate what they keep and what they let go. The number released is multiplied by the percentage of fish assumed to die. It's a complicated system with lots of room for error, but one thing is clear: the fewer fish that die, the more fish there are still swimming around to reproduce and be caught again by us sport fishermen.
The striped bass fishery illustrates why release mortality is a concern. During the test season, U.S. anglers legally kept an estimated 2.4 million stripers, and caught and released 15 million more. Scientists determined that about eight percent or 1.2 million fish don't survive release, a number greater than the entire commercial quota for that year. Release mortality is even higher for some deep-water species, like walleye or salmon, where pressure changes occur when reeling them to the surface, much like the "bends" divers experience if they surface too quickly. For these species, release mortality can be double or triple that of striped bass. So the question becomes, what can we do to assure a greater percentage of fish survive? Knowledge and a few simple, inexpensive tools can make a big difference in survivability.
For example, simply handling a fish can strip some of the protective slime coat off its body, leaving it vulnerable to infections and parasites. Not handling a fish at all is best, but if you do, you should wet your hands or wear wet soft cloth gloves to minimize slime loss.
However, the major cause of release mortality comes from where a fish is hooked and how the hook is removed, according to a study shared by Yamaha Marine. Hooking a fish deep inside the mouth can puncture the gills, gullet and even the stomach lining, and rough hook removal can compound the damage. Some studies have shown that simply cutting the line is not always the best solution because hooks left behind can impede feeding and cause infection.
One of the best ways to reduce deep hooking in the first place is by using circle hooks when fishing with bait. Traditional J-type hooks impale a fish wherever the point touches it when the angler pulls on the line to set the hook. Circle hooks are designed to wrap around a fish's outer jaw structure, an area composed of bone and membranes with few blood vessels. When a fish inhales a baited circle hook and turns to run, the angler reels the line tight-not setting the hook with a yank-and the hook slides back out of the throat and catches around the edge of the jaw. Surprisingly, studies have shown that the hook-up ratio with circle hooks can be higher than with J-hooks when used for such popular species as striped bass, catfish, and several saltwater gamefish. Upon release, the circular hooks are easier to remove because the hook eye and line are already outside the fish's mouth, and no vital organs are damaged.
The latest recommendation from some biologists is to do what you can to remove all hooks, even those deep inside a fish, with the absolute minimum of handling. I realize that goes against common angling lore that advised leaving intact hooks that were imbedded deep inside a fish gullet. The thinking is that the metal hook would eventually rust away, or the fish's enzymes would eat away at the metal or that tissue would grow around the hook and render it a non-issue.
While the jury remains out on the subject, the fact is that the faster you get a fish you intend to release off the hook, with the least contact and damage to the fish, the better.