Even a well-fed dog that has no reason to hunt for his own food may enjoy the thrill of tracking down and capturing a meal. Not every dog follows this instinct, but the ones who do usually eat most or all of the prey they capture -- even if that prey is a fellow pet.
Dogs and Koi
Many dogs will eat a koi fish if given an easy opportunity -- for example, if the fish is flopping around helplessly in the grass. Not all dogs, however, will make the effort to catch koi from a pond, and many of those who try are unsuccessful. A dog with good hunting skills and fast reaction times can snatch and eat koi from a pond. Other dogs will merely sit at the edge of the pond and watch the koi, while others may wade into the water, but move too slowly and cautiously to catch a fish. Observe your dog to determine whether he is seriously hunting a koi, or if he's just entertained by the color and movement.
The best way to deter a dog from eating or hunting koi depends on the particular dog. A special mesh netting placed across the pond can protect fish from a variety of predators, but some dogs may become tangled in the net. Motion sensors that emit sound or streams of water are effective on many dogs, or sprinklers around the pond can be set to come on at regular intervals. Protective cages or rock caves can also be placed under water as safe places for koi to hide when the dog enters the pond.
While your dog may enjoy eating koi, and the fish does contain protein and oils that are good for him, eating whole koi can be dangerous. The small bones can get stuck in his throat and abrade his stomach and intestinal tract. If the fish is carrying any parasites, they may be passed on to the dog. Eating raw fish can also cause diarrhea and vomiting. If you choose to intentionally feed your dog a koi fish, cook it thoroughly and remove all of the bones.
If your dog is not hunting and eating koi fish, or you can successfully deter him from doing so, he may serve as protection against other predators that eat koi. Herons and raccoons often target koi in small ponds because they're easy prey. A dog that barks or chases birds and animals that try to eat koi can offer excellent protection for your fish.
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