Many fishes are valuable as food and the fisheries are extensive industries, in which large sums of money are invested.
There are four great groups of fishes:
1. The sharks and rays, with cartilaginous skeletons.
2. The ganoids of which the sturgeon and garpike are examples, with heavy plates or scales.
3. The bony fishes—salmon, pickerel, mackerel, cod, halibut, etc.
4. The lung fishes, that live partly in air.
SHOVEL-NOSED STURGEON
This fish is covered with bony plates instead of scales. The roe is made into caviar. Range: Upper and middle Mississippi Valley.
There are many species of sharks. Among the more common ones in Atlantic waters are the Smooth Dogfish which have pavement-like teeth; the Sand Shark with catlike teeth; the Hammerhead Shark with its eyes on stalks. The near relatives of the sharks are the Skates. The most common example of the ganoid fish is the sturgeon, which is heavily clad with a bony armor. Most of the fishes that we find, however, belong to the third group, i. e., bony fishes. Among the salt-water species, the cod, the halibut, the mackerel, and the bluefish are especially valuable as food. Of the salt-water fishes that go up the rivers into fresh water to breed, the salmon and the shad are widely known. Of a strictly fresh-water fish, the sunfish and catfish are very common. Among the game-fish are the trout, bass, pickerel, and salmon.