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 aërated 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
