Fig. 118.—The remora, or cling fish, Remoropsis brachyptera. Note sucker on top of head. (After Goode.)

Many species (cling fishes) attach themselves to the rocks by a fleshy sucking-disk. Some (Remora) (fig. [118]) cling to larger fishes by a strange sucking-disk on the head, a transformed dorsal fin, being thus shielded from the attacks of fish smaller than their protectors. Some small fishes seek the shelter of the floating jellyfishes, lurking among their poisoned tentacles. Others creep into the masses of floating gulf-weed. Some creep into the shell of clams and snails. In the open channel of a sponge, the mouth of a tunicate and in similar cavities of various animals, little fishes may be found. A few fishes (hag-fishes) are parasitic on others, boring their way into the body and devouring the muscles with their rasp-like teeth.

Some fishes are provided with peculiar modifications of the gills which enable them to breathe for a time out of water. Such fish have the pectoral fins modified for a rather poor kind of locomotion on land, thus enabling them to move from pond to pond or from stream to stream. In cold climates the fishes must either migrate to warmer latitudes in winter, as some do, or withstand variously the cold, often freezing weather. Some fish can be frozen solid, and yet thaw out and resume active living. Some lie at the bottoms of deep pools through the colder periods, while many others, such as the minnows, chubs, and other kinds common in small streams, bury themselves in the mud, and lie dormant or asleep through the whole winter. On the other hand in countries where the long intense rainless summers dry up the pools, some fishes have the habit of burying themselves in the mud, which, with slime from the body, forms about them a sort of tight cement ball in which they lie dormant until the rains come. "Thus a lung-fish (called Protopterus), found in Asia and Africa, so completely slimes a ball of mud around it that it may live for more than one season, perhaps many; it has been dug up and sent to England, still enclosed in its round mud-case, and when it was placed in warm water it awoke as well as ever."

Food-fishes and fish-hatcheries.—Most fishes are suitable for food, though not all. Some are too small to be worth catching or too bony to be worth eating. Some of the larger ones, especially the sharks, are tough and rank. A few are bitter and in the tropics a number of species feed on poisonous coelenterates about the coral reefs, becoming themselves poisonous in turn. But a fish is rarely poisonous or unwholesome unless it takes poisonous food. Where fishes of a kind specially used for food gather in great numbers at certain seasons of the year, fishing is carried on extensively and with an elaborate equipment. Such fisheries, some of which have been long known, are scattered all over the world. Along the shores of the Mediterranean Sea, and on the coasts of Norway, France, the British Isles and Japan are numerous great fishing-places. But "nowhere are there found such large fisheries as those along the northern Atlantic coasts of our own continent, extending from Massachusetts to Labrador. Especially on the banks of Newfoundland are codfish, herring, and mackerel caught." Among our fresh-water fisheries the great salmon fisheries of the Penobscot and Columbia rivers and of the Karluk and other rivers of Alaska are the best known. The whitefish of our Great Lakes is also one of the important food-fishes of the world.

In many places fishes are raised in so-called hatcheries, not usually for immediate consumption but for the purpose of stocking ponds and streams either in the neighborhood of the hatchery or in distant waters which the special species cultivated has not been able naturally to reach. The eggs of some fishes are large and non-adherent, two features which greatly favor artificial impregnation and hatching. In the hatcheries the eggs are put first into warm water, where development begins; they are then removed into cool water, which arrests development without injury, making shipment possible. The eggs of salmon and trout in particular can be sent long distances to suitable streams or ponds. The eggs of the shad have been thus carried from the East to the streams of California and trout have been distributed to many streams in our country which by themselves they could never have reached.

The salmon is a conspicuous example of those fishes which can be artificially propagated. The eggs of the salmon are large, firm, and separate from each other. If the female fish be caught when the eggs are ripe and her body be pressed over a pan of water the eggs will flow out into the water. By a similar process the milt or male sperm-cells can be procured and poured over the eggs to fertilize them. The young after hatching are kept for a few days or weeks in artificial pools, till the yolk-sacs are absorbed and they can take care of themselves. They are then turned into the stream, where they drift tail foremost with the current and pass downward to the sea. All trout may be treated in similar fashion, but there are many food-fishes which cannot be handled in this way. In some the eggs are small or soft, or viscid and adhering in bunches. In others the life-habits make artificial fertilization impossible. Such species are artificially reared only by catching the young and taking them from one stream to another. To this type belong the black bass, the sunfish, the catfish and other familiar forms.

[CHAPTER XXV]