Popular Science Monthly/Volume 18/December 1880/The Migrations of Fishes
THE periodical migrations of birds, grand as is the scale on which they are performed, and fitted as they are to excite astonishment, are insignificant compared with those which are made by the fishes of the sea, A faint illustration of the stupendous character of these movements is given off the west coast of Norway at the opening of the fishing-season in the spring, when one, looking out over the sea in quiet weather, will be witness of a stirring spectacle. The surface of the water as far as the eye can reach glistens in diversified colors; the fiords and bays are alive with silvery streaks playing in constant movement. The agitation is caused by the schools of herring, which are so closely packed that a boat can not pass through them, an oar may be made to stand up among them, and they may be dipped up in buckets or caught with the hand by the thousand. The enemies of the herring also come with them—the mackerel, the sharks, and the dolphins enlivening the scene with their graceful movements, with great flocks of gulls. The sprat also appear in great multitudes on the coasts of the North Sea, and the pilchards on the coasts of France and Spain and the southwestern coasts of Great Britain in such immense schools that millions of them have been taken with a single draught of a large net.
The fish of the family of the Gadidæ regularly visit the northern seas in innumerable hosts. The codfish come between January and March to the shallow bays of the Loffoden Islands and the banks of Newfoundland, where their fishery gives employment to more than ten thousand vessels and about one hundred and fifty thousand fishermen.
Codfish and herring belong entirely to the sea. Many other fish wander from the sea into the rivers. The sturgeon and the white-fish go from the Caspian Sea to the Volga to spawn in such numbers that, before the fishery became so destructive to them as it is, the children on the shore could scoop them up with their hands. Still more remarkable are the schools of fish of the salmon family that resort to the great rivers of Siberia after the breaking up of the ice.
The resort of the fish to the same place is repeated every year with a wonderful regularity. The appearance of the herring in Norway varies at most not more than fourteen days. The energy of the movements, is remarkable. The salmon, traveling from the sea to its spawning-places, surmounts considerable difficulties, leaping up to the tops of falls several feet high, and repeating its jumps if it fails at first, till it succeeds. Eels are able to ascend waterfalls forty or fifty feet high, and it has been asserted that they have been known to climb the falls of the Rhine at Schaffhausen; and since the sluices have been put down they have been able to pass the six falls of the Trollhätta, which have together a height of a hundred feet.
Fish travel to very considerable distances in these journeys. Brehm estimates that the salmon of the Obi and Irtish travel about 7,000 kilometres (4,340 miles) a year up and down the stream; and salmon and sturgeon often go from 1,500 to 2,400 kilometres (930 to 1,500 miles) from the sea to their spawning-places, and salmon to a height of 2,000 feet above the level of the sea. Salmon may occupy six or eight months in going up the stream and accomplishing their spawning, but will return to the sea in one or two months, traveling from ten to thirty kilometres (64 to 184 miles) a day.
Fish, like birds, return from the most distant journeys to the places of their nativity. This has been ascertained by marking individuals and watching for their return. This faculty of localization bespeaks a higher degree of intelligence than we have been accustomed to ascribe to fish.
The theories that have been proposed to account for these migrations have failed to give a fully satisfactory explanation of them. The migrations as a whole may be considered under five heads, of which the first and most important comprises the journeys to the places of spawning. The most notable instances of such excursions are those of the salmon tribe, and of the sturgeon, lampreys, eels, and tunnies. The proper home of all these fish, except the eel, is the sea; and, besides the eel, all of them except the tunny make yearly considerable journeys up the rivers to find places suited to the development of their spawn. Such places are, for the sturgeons, about the middle of the course of the river, in shallow, sandy spots; for the salmon kind, among the hills near the sources, or in the fountain-streams themselves, where the water runs in a lively current over a stony or gravelly bed. The lampreys ascend about as far as the sturgeons. Their young, which are very different in appearance from the parents, may be found in great numbers in nearly all the still brooks and ditches of the middle parts of the river-courses. The eel is the only European fish which goes from fresh water to the sea to spawn. Its journeys take place some time before the fish are ready to spawn, an abode in the sea seeming to be essential to the ripening of the ova—a property which makes the study of the procreative functions of this fish more difficult. The tunny lives exclusively in the sea, but goes to the coasts of the Mediterranean, particularly to Sicily and Sardinia, to spawn. The sea graylings ascend the rivers of Spain and France in such numbers that the water seems covered with them.
The journeys of the fish in returning from their spawning-places after spawning are seldom performed in masses, but individually and in small groups. The fish, which went up fat and in fine condition and flavor, are exhausted, lean, and weak. Not much is known of the migrations of other fish than the salmon during these journeys, for the fishermen pay little attention to them and they therefore seldom come under the observation of science.
Next in order of the migrations are those of the young brood from their spawning-places. The young herring do not as a rule remain longer than four or five months where they are hatched. They then go down to the sea while the young eels go from the sea up to the rivers after about the same time. The young herring are observed with difficulty, for it requires a skilled eye to perceive their minute, transparent bodies in water that is in any degree disturbed; but in perfectly still water the schools may be seen moving to and fro like fine flecks of cloud. The salmon remain a full year in the mountain-streams, and do not go down to the sea till they have become a vigorous, greedy fish of about a finger's length.
Journeys in search of food are not periodical or regular, like the previous migrations, or are only incidentally so. The most important of them and the nearest to being periodical are the visits of the codfish to Newfoundland and the Loffoden Islands, concerning which it is as yet not certain whether they may not be partly connected with purposes of reproduction. Schools of other smaller fishes appear along with the cod, a salmonid, the herring, and a number of squids, which are all alike used by the fisherman as bait. The migrations of the predatory fish which follow the other fish in their spawning-journeys naturally partake of the periodical character of those journeys; the fish that pursue the herring follow them into the farthermost corners of the bays to which they resort.
The autumnal visits of mackerel to the Gulf of Kiel are of particular interest. They do not take place every year, and are not often marked by very great numbers, but they have attracted attention since 1624, when they were described by Schonevelde, on account of the peculiar character of the food that attracts the fish. The Gulf of Kiel is visited in August and September by great numbers of the Medusa aurita, which fill its waters, perform their reproductive duties, and perish on its shores, leaving hardly a trace of their watery tissues behind. In their maws swarm numerous individuals of a moderately large parasitic crab, and it is for the sake of these that the mackerel throng in the bay, attack the Medusæ, and consume them. Mackerel feed upon minute crustaceans, chiefly copepods, which swarm on the surface of the water and often cover it, and follow them hither and thither as they are carried about by the currents.
The number of fish which lead an irregular vagabond life is not inconsiderable. Foremost among them are the sharks, which singly or in small companies will follow a ship for days at a time in order to snap up whatever may be thrown overboard from it. They are often accompanied by the pilot-fish, which has a peculiarly strong sense for food of all kinds, and directs the shark, is protected by him, and gets a share of the spoil. Other formidable fish, unsocial in their habits, being scattered over the ocean, are less accessible to science.
When more than the usual number of fish go up to spawn, the number of fish pursuing them is likely to be also increased. It sometimes happens thus, that species of fish which have not commonly followed, the schools are attracted to them by the extraordinary abundance of food, and find their way to places where they were before unknown. Many fish are found in opposite quarters of the globe. The Trachurus trachurus, of the mackerel family, inhabits South American and Australasian as well as British waters. The sprat, common in the North European seas, has been discovered near the coasts of Tasmania, and thus lives at diametrically opposite points, while it has never been observed in the intervening seas. Inasmuch as migrations may often lead to a permanent enlargement of the domain of certain species, a knowledge of the laws and circumstances by which they are influenced has an important bearing on the study of the geographical distribution of species.
Migrations may also be performed under the influence of circumstances not connected with reproduction or the search for food. It is not certain whether fish are ever' driven from their homes by a cooling of the water. Removals from such a cause would not take place in large masses, and might easily escape observation. As a rule, fish are not sensitive to changes of temperature, and can endure the greatest diversities provided they have food enough. Certain tropical fishes have a remarkable faculty of performing journeys by land. The climbing fish and an ophiocephalus of the East Indies and the Doras costatus of South America are able, when the ponds and swamps in which they live are dried up, to travel for several hours over the land to find places affording more water. The eel has been said to travel for considerable distances from one pond to another. It is certain that eels are able to live for a considerable time out of the water, and, though the fact has not been scientifically established, there is no reason to doubt that they can travel. The stickleback is often found in pools wholly unconnected with other waters. It may be that the eggs of the fish have been carried on the feet of waterfowl, or that the wanderers have found their way to such places during the rains of the spring and fall, when the fields, the ditches, and even the wagon tracks are running with water. Fish often remove from their abodes under the influence of circumstances unfavorable to their existence. If there is an unusual abundance of their food in one year, the number of fish will be greatly increased, to die of starvation as soon as the food is consumed. They are also often driven out in consequence of the pollution of the rivers, either dying or going to other places where the waters are more favorable to them. Whole communities in Norway and Sweden have been ruined by the sudden and unaccountable disappearance from their shores of the herring, on the catch of which they depended. In such cases the fish have sometimes absented themselves from their former haunts for a hundred years or more, while fishermen and students have endeavored without success to discover the causes for the change.
The conditions of a scientific explanation of the migrations of fish are not satisfied when we say that they take place in search of food or with the purpose of reproduction. We have still to ask what are the conditions connected with these objects which make necessary such extensive journeys. The answer is easy in cases where food is the object of the journey. The fish go where they can find the food that suits them. But why does the herring go to the shallows of the coast instead of leaving its eggs in the deep sea? Why does the salmon leave the ocean and go away up to the sources of the rivers? Experience gained in the artificial propagation of fish has partly helped to answer these questions. One of the most essential requisites to a good hatch of the eggs is a plentiful supply and free circulation of air. Hence it is necessary for the eggs to be laid in well-ventilated waters. This is impossible if they are spawned in deep water, where they will sink away below the reach of atmospheric movements. They must be deposited in waters that are disturbed to the bottom. Such waters are the shallows near the shore, where the herring lay their eggs, and the living streams, which are the resorts of the salmon and sturgeon. The fish, impelled at spawning-time to go in the direction of the most air, keep on till they find it in the places best suited for breeding. Different species of fish require different amounts of oxygen, the same as different animals do. The salmon and trout need much, and for it seek those waters which have the liveliest motion—mountain-streams. The opinion that these waters are more favorable to the development of the eggs because they are fresh is based on erroneous premises. Many of the species that commonly go to fresh waters also lay their eggs in salt waters, and even salmon sometimes lay them in the sea. Salt water really appears, from the most recent researches, to contain—other conditions being the same more air than fresh. The same cause which impels the salmon to ascend to the lively, fully aerated streams of the mountains attracts other fishes from the deep seas to the shallows and rivers, and the eel from the bottoms of still-water ponds to the wind-disturbed waters of the bays. Those fresh-water fishes that do not wander away, go to the well-aired spots in their neighborhood to spawn—to the shore-waters, the wet meadows, or the junctions of rivers, or to the tributary streams of the lakes in which they live. Those salt-water fish which live at the bottom likewise go to the waters near the shores, where the flats and the meadows swarm during the spring with their young. The eggs of the cod and mackerel are buoyed upon the surface of the water, where the winds blow constantly over them. The stickleback will swim before its nest and fan it with its pectoral fins by the hour. Thus every fish illustrates in some way the law that a constant change of air is essential to the development of its eggs. Agitated and sun-lighted waters are also most favorable to the larvæ of crustaceans and mollusks, of echini and polyps, and to the microscopic creatures of which the food of the fry chiefly consists, and thus fulfill another condition of the most vigorous growth of the young fish.—Translated and abridged from Die Natur.