I think what probably happened was the people filleting knew they had illegal fish. They probably had a cooler they were throwing the fish into. Maybe they got spooked by either a car coming or a boat coming downstream that didn't look "right" to them. They freaked and tried throwing all the fillets in really fast, missing one. I doubt someone who was taking their time filleting would leave one behind. Plus, seeing that the fish were wild (I assume there's a closure on wild fish up there and it's out of season) they were probably in a hurry and screwed up missing a fillet.