THE NEED FOR SPEED
Most anglers and seafood lovers are no strangers to tuna, one of the most highly evolved and fascinating fish in the sea. Those lucky enough to experience first hand the power, and beauty of these creatures will never forget the impression from their first encounter. Even a small juvenile can drop the jaw of a wildlife lover or experienced angler.
The most notable characteristic associated with tunas is speed. In fact, the name tuna comes from the Greek word meaning "to rush". Bluefin tuna (Thunnus thynnus) are the largest of these, living up to thirty years, growing over ten feet long, and weighing in excess of 1,000 pounds! The all tackle record for Atlantic Bluefin is 1,496 pounds caught off Nova Scotia in 1979. This species is capable of reaching speeds near sixty miles per hour! With such extreme size and speed, they must have extraordinary physical characteristics.
In order to reach incredible speeds, tuna must expend large amounts of energy. Therefore, it is important for their bodies to be very energy efficient. Oxygen is needed in high quantities to keep the muscles going at such a fast pace, and special gills are required to provide it. Most fish use their jaws and gill plates to pump water across blood vessels, which absorb oxygen and carry it to the rest of the body. Since tuna cannot afford to waste valuable energy "gulping" water this way, they simply allow it to pass freely through the mouth and gills constantly while swimming. This method of breathing is called "ram ventilation", as the fish continually rams water through its gills. The disadvantage of this process is that the tuna must constantly move forward. If it stops swimming, it will drown.
The structure of the gill is specialized. Unlike most fish, blood flows through the gill in the opposite direction water does. This process, known as "counter-current circulation", allows the maximum amount of blood to absorb oxygen from the moving water during the short time it takes to pass through the gill. Hemoglobin is the component in the blood that carries oxygen, and tuna have a much higher concentration of it than other fish. Also, gills of tuna contain more surface area, increasing the rate of oxygen transfer.
A tuna's circulation system is designed to prevent the loss of heat, making them homeothermic, or warm-blooded. They are unable to keep their temperature constant like mammals, but maintaining a higher body temperature than the surrounding environment allows tuna to venture into colder waters in search of food. It also means their physiological processes; such as oxygen transport and food digestion, occur at a faster rate than cold-blooded fish. This helps them produce energy rapidly enough to keep up with their physical needs. Such adaptations give tuna a distinct advantage in a highly competitive environment.
There are many exciting creatures swimming the oceans around us, sharks with teeth, whales measured in tons not pounds, octopuses that change color to match their surroundings. However, with lots of muscle, a streamline shape, and highly evolved physiological systems, tunas will continue to impress anyone who is lucky enough to encounter their power and speed first hand.
Keep on plugg'n!
Capt. George Howell