I decided that there was NO legitimate reason for someone to have left that fish like that; either the line had broken, or they were trying to fish for something bigger in absentia. Either way, I decided that the next time I went by that way, I would set the fish free.
Except that it wasn't there. I spun the canoe around, and located the float about a hundred yards away in deep water. I went to it, caught it, hauled the fish out of water, and set about trying to free it. It wasn't easy; the hook had set straight up through the bony plate in the roof of the fish's mouth (which made me suspect it had been done deliberately) and I had trouble working the barb back through the hole. I bit through the line and tried to work the hook through that way, and still had no luck. I tried working the barb out one more time, and this time it went. The whole time the fish never struggled; it just goggled at me and gasped. Once the hook was out, I put the fish back in the water, and it swam energetically straight down.
Intellectually, I know that a fish is not really much smarter than a rock, but that didn't stop me from talking to it in a soothing voice as I worked on it, and it didn't stop me from feeling that I had done something worthwhile, however insignificant, once the fish swam away.