When a fish is totally immersed in water in water it displaces an amount of water equal to its body volume. If the weight of the displaced water is greater than the weight of the fish, the fish is said to be positively buoyant and will tend to float. If the weight of the water is less than that of the fish, the fish is said to be negatively buoyant and will tend to sink. If the weight of the displaced water equals that of the fish, the fish is said to be neutrally buoyant. So, underwater, the weight of an animal is counteracted by its buoyancy (Mandy Dyson 2003).