Moving Through Water: Water is dense, so it resists anything that moves through it. The more streamlined the object the more easily it slips through. Fish that are active swimmers are streamlined for forward movement, but they have relatively broad flanks (sides) that resist the water. By flexing their bodies side to side in a series of waves, these fish slip through the water much like a snake writhes through grass. Each wave travels from head to tail, pushing against the resistant water and driving the fish forward.

Fins: The large tail powers the average fish through the water and enhances the effect of the fishes flexing body. As it sweeps back and forth, the fin pushes against the water to thrust the fish forward. In some fish, such as the tuna, its action is more complex. The body fins of the tuna are merely stabilizers. When they are not needed the tuna folds them to its body to increase it streamlining. For other fish, like sharks and rays, the body fins perform a more important function. Most fish row themselves along with their pectorals while they feed, but they use the power of their bodies to escape from predators. Some fish, such as the sea horse, rely on their body fins alone and have no other means of propulsion.

Buoyancy: A fishes body is slightly heavier than water, so it has a tendency to sink. This is a problem for fish that do not live on the sea bottom. Sharks keep swimming slightly upward to counteract the fact that they are heavier than water. A shark is lighter in the water than many fish. It has a lightweight skeleton and a large oily liver that is buoyant. Still, most sharks will sink if they stop swimming. A bony fish such as a pike has a different system. Its inflatable gas bladder acts as a float. The fish controls the amount of gas in the bladder, enabling it to stay motionless in the water.

Fish Swim in Different Ways:
Blue Fin Tuna: On each sweep of the tunas powerful tail fin, the side facing forward generates lift, which pulls the fish along, while the side facing backward generates thrust.

Eagle Ray: Rays push themselves through the water by flapping their enlarged pectoral fins like wings.

Blue Shark: The sharks long pectoral fins generate upward lift that keeps the fish buoyant as it swims through the water.

Common Eel: The eel moves through the water with a writhing motion. This movement forces it forward, following the line of least resistance.

Sea Horse: A sea horse drives itself through the water by rippling its small dorsal fin.

Action File:
Fish power: Muscles for Cruising and Sprinting: Mid water fish such as mackerel and many sharks have to keep swimming to maintain their level in the water and to keep their gills working efficiently. They swim continuously throughout their lives. How do they do it? The answer lies in the red muscle, a band of dark tissue, rich in blood, that lies along each flank just beneath the skin. The blood carries fats and oxygen, which together fuel the muscle. As long as the fish keeps eating and breathing it can swim at a steady speed without tiring.

Large blocks of white muscle lie beneath the dark tissue and operate in a different way. White muscle has a sparser blood supply and is powered by a carbohydrate called glycogen. The muscle needs no oxygen to operate and delivers a massive amount of power – but only in short bursts. As the glycogen breaks down it produces lactic acid, which builds up and eventually clogs the system, causing muscle fatigue. It can only be cleared by a lot of oxygen. Fish use red muscle for cruising steadily at slow speed and white muscle for sprinting to catch their prey or escape from their enemies. Sprinting speed cannot be sustained for long.

A Fishes Muscle Structure:
White muscle: When a fish needs a sudden burst of energy it relies on the white muscle for power. A chemical called glycogen provides this powerful surge. If the fish sustains this level of activity for long it quickly becomes exhausted.

Red Muscle: Lying just below the skin, this muscle is rich in blood. The blood carries oxygen and fat that provide a constant flow of energy, allowing the fish to swim continuously.

Did You Know:
Swordfish can swim over 60 miles per hour. Occasionally they collide with ships at full speed, puncturing wooden boats.
The fins of most fish are adapted to work in water, but the pectoral fins of flying fish have become elongated into wings, enabling them to glide in the air for short distances.
An African catfish swims upside down so that it can eat food from the waters surface with its under slung mouth.
Some flatfish can move by jet propulsion, gulping water into their mouths and ejecting it forcefully from their gills.