I’d call the trapper first from someone else’s phone and tell them to stop. Might be enough. There should be a phone number on their truck.
Sometimes bad things are done from ignorance rather than malice.
Here’s the proper way for nuisance animal carcasses to be disposed of:
Where and how is a carcass disposed?
To dispose of carcasses, please keep these precautions in mind and follow these instructions. When handling dead birds or other wildlife, work outdoors and try to stay upwind of the carcass. When bagging the carcass, keep the open end of the bag away from your face. Do not eat, drink, or smoke while handling carcasses. Do not store carcasses in a cooler, freezer, or refrigerator used for human or animal food, and wash hands thoroughly after handling. Wash any clothing that comes into contact with the carcass with normal household detergent at normal temperatures.
Disposal of dead animals: open up two heavy plastic trash bags. Avoid touching the carcass with your bare hands. Pick up the carcass using disposable gloves or plastic bags worn on your hands. Place the carcass in the first heavy plastic trash bag. Keep the open end of the bag pointed away from your face while tying securely. Next remove gloves or plastic bags from your hands by turning them inside out. Dispose of the gloves or plastic bags in the second heavy trash bag and place the first bag (containing the carcass) in the second bag, as well. Tie the second bag securely and place it in the garbage. If there are many carcasses to be disposed of, please first speak with your local waste management facility.
So, that’s the right way to do it. There’s a right way and many, many wrong ways, including throwing them in a river. He (assuming he) might be thinking he’s feeding the fish (like bait) and doing a favor – “letting nature take its course”. But it’s the wrong way to think. when there are people whose water supplies (well) may be coming from there. Also, it’s the wrong way to go about it