2i6 THE POPULAR SCIENCE MONTHLY. 



the spring and fall, ,wben 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 Nor- 

 way 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 them- 

 selves 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 con- 

 ditions connected with these objects which make necessaiy such exten- 

 sive 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 neces- 

 sary for the eggs to be laid in well-ventilated waters. This is impos- 

 sible 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 shal- 

 lows 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 differ- 

 ent 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 beino- 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 



