Migration is the mass movement of fish (or any other animals) along a route from one area to another at about the same time annually. This group travel is induced basically by factors of food and spawning. At times, mass movement may take place for other reasons, but such travel should not be confused with migration. Sudden adverse conditions, such as pollution, excessive sedimentation, or water discoloration caused by unusually severe storms, may force large groups of fish to leave the affected area.
Some fish, called "tide-runners," move with the tide to shore and then out again while searching for food. This movement is simply a daily feeding habit and is not considered migration. Some fish remain in deep water during the day and move to shore at night for feeding. In some lakes the entire population of a certain species may move at times from warming shallows to deeper, cooler waters to survive. The lake trout and the walleye are good examples of sportfish that make seasonal movements of this sort.
The bluefin tuna, one of the largest of the oceanic fish, migrates about the same time each year between the coasts of southern Florida and the Bahamas, where it spawns, to waters off Nova Scotia, Prince Edward Island, and Newfoundland. On reaching these far northern waters, the bluefin will find and follow huge schools of herring, sardines, mackerel, or squid in the same localities, year after year. If the temperature rises higher than usual, or other water changes take place, the bait schools will depart from their customary haunts, and the bluefin will follow.
Inshore fish such as the shad and the striped bass may travel varying distances along the coast before arriving in freshwater rivers or brackish stretches that meet the requirements for their spawning activities. Some species do not travel along a coast or migrate north and south; instead, they move offshore into deeper water in cold weather and inshore during warm weather. Others combine a north-south movement with an inshore-offshore migration.
The California grunion, a small, silvery fish, is an example of a unique and precisely timed migration. It spawns at the turn of high tide and as far up the beach as the largest waves travel. This action takes place during that period when the water reaches farthest up shore. The grunion deposits eggs and sperm in pockets in the wet sand. Two weeks or a month later, at the time of the next highest tide, when the water reaches the nests and stirs up the sand, the young are hatched and scramble out to sea before the tide recedes and prevents them from escaping.
Members of the salmon family participate in what may be termed classical migration. All have the same general life pattern. The eggs are hatched in shallow streams; the young spend their early lives in freshwater, grow to maturity in the ocean, and then return to the stream of their birth to spawn. The length of time spent in freshwater and saltwater habitats varies among the species and among populations of the same species. All five species of northwestern Pacific salmon die after their first spawning. The Atlantic salmon drops back to saltwater; those fish that survive the hazards of the sea return to spawn again. Salmon migrate various distances to reach their spawning sites. The chum and the pink salmon usually spawn a few miles from saltwater and often within reach of the tides. The chinook, largest of the salmon family, may cover thousands of miles and surmount many obstacles before reaching its ancestral spawning grounds. And after spending from 1 to about 4 years far out to sea, each individual returns to spawn in the river where it was born.
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