Forest Preserve District of Cook County, Illinois
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Bulletin No. 96   December 7, 1946
Forest Preserve District of Cook County
William N. Erickson, President
Roberts Mann, Supt. of Conservation

Fish are stream-lined. They have to be. Some kinds, like the catfish, are covered with a tough naked skin, but most kinds ere protected by flexible armor of scales. The gar, one of the living relics of the Age of Fishes, has hard heavy scales that were use by indians as arrow points. Its armor-plated skin was used by the Caribbeans for breast-plates end by some pioneers to cover their wooden plough shares.

The sturgeon, another of our most primitive fishes, has five rows Or great bony bucklers on its back and sides; while trout and salmon have only tiny transparent flakes imbedded in their skin.

Examined under a hand lens or low-powered microscope, fish scales show e delicate pattern of concentric ridges something like a fingerprints. Like the rings in the trunk of a tree, or the ridges on a clam shell, these represent years of growth. In a tree, the rings not only tell us its age, but the width of each ring tells us whether growth conditions that year were good or bad.

Fish scales, too, offer one of the most useful tools for the biologist studying fish and fish production. Their interpretation is being worked out in greater and greater detail. It has been shown that, in most common fish, these annual rings are formed in late spring or early summer; and that the growth of the scales keeps step with the growth of the fish the rings being closely spaced in years of slow growth; widely spaced when growth is rapid. Abundance of food, diseases, floods and other happenings are thus recorded on the scales.

Fish scales sifted from Indian mounds and midden heaps, hundreds of years old, show that our common fish -- with the exception of the carp - - were present then and that they grew at about the same rates as now. Fossil scales tell the stories of fish that lived millions Or years ago.

Each fish keeps a diary.

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