Dart in a Duck

A few days ago, I was walking across the millstream to Goudy and watching the ducks. They were all facing upstream and kept sticking their heads underwater into the seaweed. I stopped and watched them for awhile trying to figure out what they were doing. Were they eating the seaweed or some kind of bugs in the seaweed or something else? I couldn't tell but while I was watching them I noticed a huge red dart sticking out of the back of one of the ducks. At first, I thought it was some kind of method to tag and track them, but as it swam nearer I could tell it had red feathers attached to a metal stick and was definitely some kind of dart or small arrow. I couldn't understand who would ever try to kill a duck with darts. But I also was baffled by the fact that the duck was completely oblivious to the fact it had a huge metal spike protruding from its spine.