I don't believe that fish in general will only seek out and eat only corn and nothing else, so I don't think it will harm a fish in the long run. A lot of controlled studies are performed to achieve the results that they want to find by over feeding fish only one type of food source (corn). This is what may harm the fish and support their findings.
See below article I found online.............
Why the use of corn is illegal for fishing in some states
by Rex Trulove
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Created on: June 26, 2010 Last Updated: June 29, 2010
It certainly isn't illegal to use corn for fishing in all areas. However, it definitely is illegal to use corn for fishing in others. The sometimes stiff fines levied is a hard way to learn it for anyone believing that it is legal to use corn everywhere.
Many people may wonder why in the world it would be illegal to fish with corn. It doesn't take a fish biologist to be able to figure it out, however. Understand the traits and habits of different fish give a solid clue to answer the question.
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Some fish hunt by the sense of smell, while active predators tend to hunt using their eyesight. For instance, a bullhead catfish will eat nearly anything that was once living. They have poor eyesight, and hunt almost exclusively by smelling the food. Because of this, they will eat decaying flesh, vegetation, and just about any other food source they can smell.
Because of this, nature has allowed the catfish to have a digestive system that can get nutrients for this kind of food. A catfish can digest a small amount of corn, though it doesn't get a great deal of nourishment from it.
Consider sight feeders, though. Sight feeders include bass, crappie, trout, and salmon. Each of these fish is an active predator. They live life in the "fast lane", if you will. They will normally dart and dash after prey. That doesn't mean that they won't eat a lifeless worm, but it does mean that they have a much faster metabolism.
In turn, this means that their intake of protein needs to be substantially higher than with less active fish, if they are to survive. This is one of the major reasons the use of corn isn't allowed in so many places. Quite a few of these active fish will eat corn, but though it does have protein, the quantities are low. The fish doesn't get the protein it needs, and can die because of it.
According to Tim Hayden, a retired fish biologist, a study was done regarding this, in the state of Oregon, in conjunction with the Oregon State Universitybiology department. Fingerling trout were fed corn, two hours before being fed the normal protein rich food. As Tim put it:
"The fish gorged on the corn. By the time the normal food was given, they were full, and little of the good food was eaten. The result of the study was a 100% mortality rate."
This shouldn't be lost on the fisherman. Most discover, sooner or later, that after fish have been allowed to eat their fill, such as during a night with a full moon, it is exceptionally difficult to get them to go after a fly, lure, or bait. This really shouldn't be surprising to anyone. Once you are totally full, you probably aren't ready to sit down to the best meal that can be made, either.
The problem is that corn doesn't have the protein to allow many fish to survive, though they may fill their bellies on it. They die, because they don't have the food they need to survive. The same is true of almost any animal, including humans. If a person totally loves French fries, to the point that the fries are all the person eats, they are going to have problems because of lack of nourishment.
This is one of the main reasons that the use of corn is forbidden in so many places, for fishing. In some, chumming is against the law, but the use of corn is a greater threat to the fish. They can end up starving to death, with a full belly.
Sources:
Oregon Department of Fish and Wildlife
Oregon State University
Special mention: Richard Hayden, retired
Oregon State fish hatchery, Ft. Klamath, Oregon