Tuna are among the most beautiful of the sea’s creatures. These fish, with their delicate colors and wonderfully streamlined bodies, have evolved to exhibit highly sophisticated hydrodynamic refinement. When tuna swim rapidly, their fins retract into grooves, and even their eyes form a smooth surface with the rest of the head.
Tuna live in the open sea rather than near the shore, and stay for the most part in the upper layer of water, called the mixed layers. The mixed layer is warmed by the sun and air, stirred by the wind and waves, and is a rich environment for tuna to live in. Where the oceans are warm, tuna are there. The skipjack tuna, for example, is confined for the most part, in water at 25 degrees centigrade (78 degrees Fahrenheit) or warmer.
Tuna travel a lot. They are nomadic fish that have been tracked across entire oceans. By scientific tagging, Albacore tuna have been tracked 5,300 miles from California to Japan at the rate of 16 miles per day.
TUNA never rest; they must always be moving. Their demand for oxygen requires moving one body length per second in order to get enough oxygen. TUNA get their oxygen from water, not from the air. They swim with their mouth open, which shoots a jet of water over their gills with which they extract oxygen from the water. Because of this system, they must remain in a constant state of action. This physical exertion creates an enormous demand for energy, and a need to eat large quantities of food. A typical TUNA may eat one-quarter his own weight in food in one day.
TUNA are not cold-blooded as most fish. Instead, warm-blooded tuna maintain their temperature a few degrees warmer than the water in which they travel. As warm blood returns to the gills to get more oxygen, it passes close to cooler blood coming from the gills. As they pass, the warmth is transferred to the cooler oxygen-rich blood.
As all creatures do, TUNA camouflage with their environment to protect themselves against their natural predators. From a deep water view, the light-colored belly of TUNA blends with the surface light and makes the fish hard to discern. From an aerial view, the blue back of the tuna blends with the color of the sea beneath her. TUNA have color only when alive and at home in the sea; the color disappears when the fish is removed from the water and dies.
TUNA begin life with a female who lays her eggs (in fish, this is called spawning) in the open water. (While TUNA and tuna-like fishes are at home almost anywhere in the world, it is interesting that they return to the same general locality for spawning.) The eggs are fertilized by males who release their sperm in the water where the eggs have been laid. Very small larvae hatch from the eggs usually within 24 hours. One large female may lay as many as 6,000,000 eggs in a single spawning. The number of eggs laid is related to size of the fish; a smaller female may yield only a million eggs. The eggs can be considered tiny: they measure one millimeter (0.04 inches) in diameter at spawning, including the droplet of oil surrounding the egg which makes them buoyant. From hatching to full growth, some of these species increase their size by one billion times.
If these fish lay so many eggs, why are the oceans not clogged with them? Actually, out of the millions of eggs spawned by TUNA only about two, on average, grow to maturity from a single female. What happened to all those eggs and larvae and juveniles and young fish? They were eaten by bigger fish, even by their own (cannibalistic) kind. They also become food for many other kinds of swimmers in the sea as well as sea birds. Those who escape the perils of being eaten, and grow into adults, will find themselves at the top of the food chain, with few other swimmers to threaten them.
The daily menu for TUNA includes fishes, crustaceans and mollusks. The Skipjack TUNA is not picky about its diet, and neither are its many TUNA cousins. They will eat whatever is available including their own relatives.
The more modern marine biologists have learned about tuna, the greater the mysteries surrounding these remarkable fish. Some of the unresolved questions are: How do the tuna navigate over thousands of miles in the trackless wilderness of the open ocean? To what extent are they homoeothermic – that is, able to maintain a roughly-constant temperature above that of the water in which they live? Why do the female tuna lay such enormous quantities of eggs? How do adult tuna, living at times in the desert waters of the high seas, obtain the quantities of food they need to swim continuously? As time continues forward we can only hope researches will be able to find the answers about these nomads, the tuna.
