Because the body being no longer balanced by the fins of the belly, the broad muscular back preponderates by its own gravity, and turns the belly uppermost, as lighter, from its being a cavity, and because it contains the swimming bladders, which continue to render it buoyant. - White's Natural History of Sel-borne.
Because they have not lungs.
Although fishes possess no voice by which they can communicate their sensations to others, some species utter sounds when raised above the water, by expelling the air through the gill-opening when the flap is nearly closed: while others, even under water, as the salmon, utter certain sounds while depositing their spawn; but for what purpose these sounds are uttered, or by what organs they are produced, we are still ignorant. - Fleming.
A writer in the Magazine of Natural History tells us, that some tench which he caught in ponds, made a croaking like a frog for full half an hour, whilst in the basket at his shoulder.
Because these organs may be distinguished from others to a certain degree analogous in young frogs, etc. The gills are filled with innumerable very delicate vessels, and are mostly divided on each side into four layers, which somewhat resemble the beard of a quill, and which are attached at their basis to a corresponding number of little bones. - Blumenbach.
Because they consist of jointed rays, covered by the common integuments: these rays serve to support the fishes, and approach or separate like the sticks of a fan, and move upon some more solid body as a fulcrum. Thus, in sharks, the rays of the fins behind the gill are connected by a cartilage to the spine.
The motions of fish are indeed performed by means of their fins. The caudal, or tail fin, is the principal organ of progressive motion; by means of its various flexures and extensions, it strikes the water in different directions, but all having a tendency to push the fish forward; the action resembling, in its manner and effects, the well known operation of the sailor termed skulling. The ventral and pectoral fins assist the fish in correcting the errors of its progressive motions, and in maintaining the body steady in its position. Bo-relli cut off, with a pair of scissars, both the pectoral and ventral fins of fishes, and found, in consequence, that all the motions were unsteady, and that they reeled from right to left, and up and down, in a very irregular manner.
Because the characters furnished by their position, are employed as the basis of his classification. Thus, by Linnaeus and others, the ventral or belly fins, are considered as analogous to the feet of quadrupeds, etc.