Forest Preserve District of Cook County, Illinois
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The Growth of Fishes
Nature Bulletin No. 272-A   June 3, 1967
Forest Preserve District of Cook County
Richard B. Ogilvie, President
Roland F, Eisenbeis, Supt. of Conservation

Baby fish, by the millions, are hatching now every day in our lakes, streams and ponds. Some kinds come from eggs sown broadcast among water plants; others from eggs laid in clusters or nests; some from masses of eggs hidden in underwater holes; while the eggs of many little fish, such as minnows and darters, are attached in neat patches to the underside of rocks or sunken logs. For some time before hatching, the young fish can be seen wriggling inside the eggs.

Newly hatched baby fish -- or fry, as they should be called -- look much alike, regardless of the size or appearance of their parents. Each is almost transparent except for the large dark eyes and a bulging stomach which encloses yolk from the egg. Under a magnifying glass, the pumping red heart can be seen and the mouth gulping water. The tiny fins are beginning to form, a few dots of dark pigment may show in the skin, but there is little or no sign of scales. They vary from an eighth to a half inch or more in length, depending upon the species and the size of the egg.

Within a week or so, the yolk has been absorbed, the little fish has become longer and more streamlined, and it begins to feed on the microscopic life in the water. Like other young animals, their heads seem too large for their bodies but they soon begin to look like small editions of their parents.

The growth of fish is quite different from that of man and other warm- blooded animals. They do not get their growth or reach an adult size at a certain age and then stop, Instead, most fish continue to grow in length and weight until they die. Further, the growth rate may be very fast or very slow, depending on the amount of food and other conditions. For example, in lakes where food is scarce and competition with other fish is keen, the fry of largemouth black bass may not exceed two inches in length at the end of their first year. In other lakes, such new impoundments where fish are few and food is plentiful, these bass may reach a length of 12 inches in their first year -- 6 times the length and 300 times the weight of the year-old fry in overcrowded water deficient in food.

Unlike most other animals, each fish carries in its body a record of its age and growth that can be read accurately by experts. Their scales show rings that can be counted, one for each year, like the annual growth rings in a tree. Some of the spaces between these rings may be wide and others narrow, depending on how much the fish grew in those years. By measuring these rings we can trace the growth of a fish in each year of its life. In fish without scales, like catfish, their age can be learned from growth rings in their bones.

Among our larger kinds of freshwater game fish, only a few individuals reach an age of 10 years -- very, very rarely 15 years. Ages between 3 and 6 years are most common for good-sized bluegills, crappies, perch, bullheads, and other panfish. At a little over a year, some minnows and other small species have reached a ripe old age. At the other extreme is "Old Oscar", a rock sturgeon, who was estimated to be 15 or 20 years old when taken from the Mississippi River and has been exhibited at the Iowa State Fair for 25 years. Rock Sturgeon or Lake Sturgeon, Alligator Gar, Paddlefish or Spoonbill Cat, Blue Catfish, Flathead Catfish, Muskellunge and Lake Trout are kinds of fish in the Middle West which, taken by nets or by hooks during the past 75 years, are reported to have exceeded 100 pounds in weight. Only the first three of these have been known to reach 200 pounds.

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