Canned Tuna is usually lower in mercury than tuna steaks. This is because the canning removes some mercury from the fish. Canned Tuna is also more affordable than tuna steaks. However, it is essential to note that canned Tuna is often packed in oil, which can be unhealthy for dogs. If you give your dog canned tuna, make sure to choose a variety that is packed in water instead of oil.