Why Migrate? Fish migrate to find food or breeding sites. Many feed on the seasonal growth of microscopic plants and animals, called plankton, or prey on smaller, plankton eating fish. During the breeding season some species migrate to special spawning grounds to lay their eggs. The spawning grounds are located far from the feeding grounds because the young fish have different feeding requirements from the adults. The distance also lessens the chance that the adults will eat their own young.

Adaptations of Migrating Fish: Migrating fish must adapt to moving between freshwater rivers and the much saltier oceans. In fresh water the body fluids of a fish are more salty than the water in which it swims. As a result, water seeps through the fishes skin to create a balance between its saltier body fluids and the less salty water. This process is known as osmosis. A freshwater fish maintains the concentration of its body fluids by expelling the surplus water as diluted urine, which prevents the fish from becoming waterlogged. 

Unlike fresh water, sea water contains a higher concentration of salt than the body fluids of most fish, so osmosis works in reverse. Rather than gaining water, the fish loses water through its skin. To avoid dehydration (drying out), the fish drinks sea water but it must expel the excess salt as highly concentrated urine. Any fish that travels downstream from the river to the sea makes the transition from fresh water to salt water through reverse osmosis. Trout and salmon are among the few fish that are able to survive in both types of water.

River Migration: Most migrating fish move from one ocean to another, but some travel from rivers to the sea. Brown trout, for example, swim down river each spring to feed in the sea. Fish that migrate from fresh water to salt water must adapt to the change. But the greater availability of food in the ocean allows them to grow twice as large as the freshwater fish. The brown trout return to the rivers to spawn (breed) because their eggs must be laid in shallow fresh water.

Navigating the Oceans: Salmon navigate by smell. Each fish remembers the scent of the stream where it was hatched. As it migrates back across the ocean toward the spawning river, the fish instinctively finds its way as the familiar scent grows stronger. Sea trout also use their sense of smell to navigate, but they are not as able as salmon to follow a scent. If a migrating sea trout is caught and taken beyond its home range and then released, it becomes lost.

Salmon Migration: Salmon are born in fresh water but spend nearly half their adult lives feeding in the ocean. Some salmon travel thousands of miles through the ocean to reach the spawning rivers. The upstream migration to the spawning grounds occurs only once in the lifetime of most salmon. When they reach breeding age salmon migrate thousands of miles to reach the spawning grounds. Once they arrive at the rivers mouth, they gather in the brackish (slightly salty) water and wait for high waters to carry them upstream.

The journey upstream may take several months. The fish must often leap over waterfalls and rapids to reach the shallow spawning streams. Because the salmon do not feed in fresh water, they have lost 40 percent of their body weight by the time they lay and fertilize their eggs. Most of the salmon then die.

Key Facts:

Eel Migration: For centuries little was known about the European eels breeding habits. Elvers (young eels) swam upriver where they gradually developed into adults. They fed in the fresh water for several years before they became sexually mature, at which time they began swimming down river toward the sea. When they reached the sea, people were no longer able to track them. Years of patient research determined that once the eels left the rivers they migrated to their spawning, or breeding, grounds. Their migration carried them 3,700 miles from the coasts of Europe to the Sargasso Sea, a region of calm water in the North Atlantic northeast of the islands of the West Indies.

How the eels find their way to the spawning grounds is still unknown. Since birds appear to migrate by following the earths magnetic field, some scientists believe that migrating eels do the same. Alternatively, they may be sensitive to changes in the waters temperature, pressure, or scent, which they use to help them navigate. The eggs are laid in the warm (68 degrees) waters at depths of more than 1,000 feet; the Sargasso Sea is one of the few places in the world where this is possible. 

Since the adults do not eat during the six-month migration, they die after spawning. The eggs hatch into leaf shaped larvae that drift slowly east with the currents, arriving on the coasts of Europe two years later. The migratory instinct is so strong that eels travel across land if necessary.

Tracking Migration: Little is known about fish migrations; still, it is becoming easier for scientists to track the fish as technology advances. Tracking is done by using transmitters. The type of transmitter used depends on the location and the kind of fish being followed. A common method for tracking fish is to attach an acoustic device to the fish externally or insert it in a body cavity. The device emits ultrasound pulses, which are then converted into sounds that humans can hear. 

The signals are monitored from either a boat or a riverbank, and they usually have a maximum range of half a mile. Using such tracking devices allows scientists to learn more about fish migration. Much of the tracking requires that the fish be followed closely by boat, but computers and other automated equipment are being developed to make it easier to gather information.

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