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Here's an interesting article that I found while Googling.
Leave Hooks In Fish?
Excerpt from Fall 1997 American Bass Association Bass News magazine
by Ralph Mann and Steve Quinn
For years, anglers catching gut-hooked fish have cut the line close to the hook, leaving it deeply embedded, as fishing authorites, biologists, and conservation agencies have advised. The message has been that hooks rust and fall out of the fish.
But what is this advise based on? To our knowledge, almost no studies have followed the fate of fish with hooks in their esophagus. Studies of immediate and delayed mortality of hooked fish have invariably found the highest mortality in fish hooked in the gills or esophagus.
John Foster, Recreational Fisheries Coordinator for the Fisheries Division of the Tidewater Administration of the Maryland Department of Natural Resources, studied whether fish shed hooks left in their gullets. The Chesapeake Bay striped bass fishery, which was closd in the mid-1980's due to low population levels, was reopened when fish spawned heavily and stocks rebounded. Atlantic coast states imposed various combinations of size and bag limits to protect the growing striper population. But limits that required release of fish won't work if many released fish die.
Biologists throat-hooked medium (16-to-22 inch) and large (22-to-28 inch) stripers and held them in tanks of half-strength seawater (15 parts per thousand). Stripers in Chesapeake Bay occupy water that ranges from nearly fresh to almost full-strength seawater, so the experimental treatment represented average water conditions. And of course, hooks in saltwater rust much faster than in freshwater.
Foster and his colleagues at the fisheries lab ran two experiments to test long-term hook retention. In the first, they tested hooks of stainless steel, bronzed (steel with a polyurethane coating), nickel, and tin-cadmium. They placed 1/0 or 2/0 hooks in the fish's esophagus, with the point up and left an 18-inch length of line on the hook. Thirty fish were hooked with each type of hook.
After 120 days, 78 percent of the hooks remained in the stripers, including fish that died. 80 percent of the tin-cadmium hooks remained, though mortality was about 20 percent due to cadmium poisoning. Hook companies have since stopped using cadmium in hooks. Bronzed hooks were the likeliest to fall out, though 70 percent remained after four months in brackish water.
In the second test which ran for 60 days, line was clipped at the hook eye. 81 percent of these hooks remained, with retention of hook type ranging from 100 percent for stainless steel to 56 percent for tin. Mortality was higher in the secon test, when all the line was trimmed.
Foster theorizes that the length of line hanging from the fish's mouth kept the shank flat and allowed food to pass below it. Without the line, food tended to force the hook eye and shank down, which blocked the esophagus. With the shank held flat, the hook may move to one side, allowing the fish to feed. Hooks that rusted did so in stages, leaving ever smaller portions of the bend and point in the fish.
Stripers also formed scar tissue around the hook, the body's way of isolating this foreign matter. Once tough fibrous scar tissue forms in the mouth, however, it can't be removed. Months after hooking, fish developed latent infections around hook wounds, which caused mortalities. These infections might have been caused by bacteria that became active as water temperature changed, or they may have been triggered by seasonal stress, such as spawning.
Based on these findings, Foster recommends that anglers carefully remove hooks from deeply hooked fish, using a tool like the Deep Throat DeHooker (DeHooker, Inc., 7 Cherokee Avenue, Rt. 17, Palm Coast, FL 32137, (904)446-1634. Their web site is www.dehooker4arc.com
Another article on gut hook fish study
I keep gut hooked fish as long as they are legal size. Although it really occurs when fishing live bait which I dont do anymore. I have caught several fish that have still had leaders clipped off at their mouths.
I'd like to see more data on hook deterioration in fish.
Makes me wonder if it wouldn't be better to remove a hook from salmon caught ice fishing then to leave it in.
I know the longer you have a fish out of water at below freezing temps, the more of a chance that the delicate gills will freeze. I wonder which is the more injurious to the fish.