Although birds and mammals cease to grow after becoming fully mature, fish continue to grow until they die, provided food is abundant. Growth is fastest during the first few years of life and continues at a decreasing rate. It accelerates during warm-weather months when food is abundant. During the cold months, fish do not feed much; their metabolism slows down, and growth is retarded.
Proper determination of age and growth in fish is important in order to regulate the harvest. In both sport and commercial fish, the age and the growth rate must be known in order to reap the crop wisely. Fisheries are
controlled by rules and regulations, based on facts in the life history of fish.
Generally, fish in warm climates reach sexual maturity and grow faster than do their cousins farther north, because the growing seasons are longer and the food supply is not shortened by cold weather. For example, a large-mouth bass in Florida may spawn after 1 year; in Wisconsin the same species does not spawn until the third year; and in Canada the largemouth bass may not reach maturity until the fourth or fifth year. Under average conditions, the largemouth bass may attain 3 inches in length in the first 5 months, 5 to 6 inches in 1 year, and 8 to 10 inches in 2 years. By the third year, bass may be 12 or more inches.
A fish's growth rate is also influenced by its environment. A pond or a lake can support only a limited poundage of fish, just as a piece of farmland produces only a limited harvest of vegetables or other crops. In some bodies of water, fish never attain natural size because there are too many of them for the available food supply. Yellow perch are found stunted because only a few, if any, predators, such as bass and pickerel, are present to feed on them. Stunting may also take place because not enough fish are caught, or perhaps because anglers return all the small ones to the water unharmed.
The age of fish that live in temperate climates can be determined fairly accurately from various bony portions of their anatomy, because definite changes in seasons cause annual marks to appear in the bone. These year zones of growth are produced by the slowing down of metabolism in the winter and its rapid increase in the spring. In some species the annual ridges, called annuli, are especially pronounced and easy to read in the scales and cheekbones. In fish with tiny scales, these annuli are difficult to see, even under a microscope. Spines, vertebrae, jawbones, and earbones have to be studied to determine the fish's age. In cross-section, these various bones may show annual rings that appear similar to the rings in the cross-section of a tree trunk.
In tropical areas with seasonal rainfall, the age of freshwater fish can be denoted by seasonal growth marks caused by dry and wet seasons. In uniformly warm waters, such as the equatorial currents, fish demonstrate little, if any, seasonal fluctuation in growth, and age determination is difficult.
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