if this were the only occurence like this that I have seen, I may be inclined to call it a festering wound from a hook as well. this particular fish came from a small piece of water. also, I have caught other trout both rainbow and brown trout, and seen growths similar to the one pictured but in different spots that normally a hook - treble or single - would not be found. its a mystery, but this fish was caught right near a city street run-off thingy, and when its raining and I happen to be fishing this location, the water coming from opening looks like used motor oil, I wonder if this is partly to blame?