CAUSES INFLUEKCINft MIGRATIONS. 597 



water is suitable. It is but natural that the waters of a creek or short stream, not having its 

 source iu the mountains, should in the spring become warm long before those of a large river 

 whose headwaters are far up among the mountains; for which reason we may expect to find, in 

 the case of two rivers, the most southerly of which has a longer water-course than the other, that 

 the Shad will first enter the more northerly, yet shorter, and consequently, at a given date, warmer 

 stream. The question, therefore, appears to be rather one of temperature than of geographical 

 location. 



The greater portion of the life of the Shad being spent iu salt water, the possibility of close 

 observation as to their food, habits, or precise habitat is precluded. The young fry, hatched out 

 in the rivers iu spring and early summer, remain there until the following fall, when,. the temper- 

 ature of the waters having fallen below 60°, they leave for the ocean. Nothing more is seeu of 

 them until they return to the rivers as mature fish for the purpose of spawning. In these upward 

 migrations the schools of mature fish ascend the rivers either until obstructed by imi)8ssable falls 

 or dams, or until the volume of water becomes very inconsiderable. Before artificial impediments 

 were placed in the rivers, the limit of this movement was the natural and insurmountable falls 

 to be found at the head of almost all of our principal streams. "For example, in the Savannah 

 River the Shad used to ascend to the Falls of Tallula, at the very source of the river in the 

 northern part of Georgia. In the Potomac they ascend as high as the Great Falls. In the 

 Susquehanna River, in which there exist no natural obstructions, their migrations extended up. 

 into the State of New York, a distance of several hundred miles above the present limit. On 

 the Hudson River they ascended to Glens Falls. On the Connecticut at one time tliey went as 

 high as Bellows Falls, but recent obstructions in this river have materially reduced the extent of 

 their range. 



The present limit of the upward movement of the Shad in our rivers, the natural limit before 

 obstructions were interposed, and the extension of the natural limit which may be obtained by 

 overcoming these natural and artificial obstructions now existing, are shown in the accompanying 

 chart. It will be seen from this that the breeding area has been diminished from one-half to one- 

 fourth its original extent, involving a corresponding reduction in the productive capacity of these 

 streams.! 



Hereditary instinct of locality. — The annual migration of the Shad in the spring of 

 the year into the fresh waters of cur rivers has been explained by various theories. In regard to 

 the salmon, which has been long known and observed in European waters, the fact seems to have 

 been established that the same individual will return year after year to the same stream for the 

 purpose of spawning, and that young fish bred in a certain stream usually come back to the same 

 upon their return from the ocean as mature fish. This habit has not been conclusively established 

 in regard to any other family of anadromous fishes, but it is generally believed that all salt-water 

 species which spawn in fresh water return for this purpose to those streams in which they them- 

 selves were deposited. An examination of the literature of fish culture will make it evident that 

 this opinion has been held very generally, and, indeed, has furnished to a great extent the argument 

 for the prosecution of the work of artificial reproduction. It is a common belief, too, among fisli- 

 culturists that the mature individuals of all anadromous species, including the Shad, are led back 

 to the waters iu which they were spawued by a conscious wish on their part to return to those very 

 localities in which they spent their young life. Important exceptions to this rule are, however, 



' See Chart of tlie River Basins of the Atlantic Slope. 



