MOVEMENTS OF YOUNG SHAD. 603 



Shad. The run of Alewives indicated by the diagram in the early part of the season at low 

 temperature was undoubtedly G. vernalis, or the Branch Herring, which makes its run on a tem- 

 perature several degrees lower than suitable to the Shad or the Glut Herring. The fact that the 

 Shad commence running into the Potomac when the temperature of the river is 56° F. or less, 

 does not antagonize the theory here stated, that the hydro-isothermal area which they prefer to 

 occupy is that having the temperature of about 60° P. If, as is probably the case, there is ocean- 

 wards a limiting wall of low temperature for the Shad occupying the Chesapeake area,' then 

 at all seasons of the year the Shad must be found at some point within that area, be the tempera- 

 ture exactly what they prefer or not. In other words, the Shad in their migrations travel on 

 temperature paths, the direction being always towards 60° F. 



Shad ready to deposit their spawn seem to prefer waters of a warmer temperature than 60° F. 

 Therefore, when the mature Shad, intent on reproduction, leave the hydrothermal area of 60° 

 P. and ascend the rivers into waters of 65° F, to 70° F. and upwards, they are unaccompanied by 

 the half-grown Shad, the latter ceasing to ascend as soon as they encounter a temperature of more 

 than 60° F. In 1882, however, when the temperature of the water was below 60° F. for the greater 

 portion of the season, the spawning had to take place in water colder than the fish would have 

 preferred, and therefore mature and young Shad were found together on the spawning grounds. 



Observations made during that season show that large numbers of young Shad were taken, 

 which would not have been the case had the temperature of the river waters risen above 60° F. 

 Inasmuch as the fishing operations are conducted with a view to obtain mature fish, and in most 

 years the young do not accompany the full-grown fish up to the fishing grounds, it would certainly 

 appear as though this was a special provision of nature to secure the continuance of the species, 

 providing against the capture of the young Shad during the fishing season. 



Observations on the movements of young Shad in the Potomac. — The young Shad 

 which are hatched out during May and June remain in their native streams until the temperature 

 of the water falls below 60° P. They then move down the rivers as the temperature falls, passing 

 into the salt water as soon as the cooler weather has reduced the river temperature below the 

 degree congenial to them, and, as a rule, return no more to the fresh waters until they are full- 

 grown fish. This statement is borne out by observations made in 1881 by Mr. W. E. Stuart and 

 Mr. Gwynn Harris, inspectors of marine products. These gentlemen, who have been largely 

 engaged for many years in the Potomac fisheries, whose interest is always manifested in connec- 

 tion with all matters relating to the fisheries, undertook, at the instance of Professor Baird, United 

 States Fish Commissioner, to observe the movements of the young Shad in the Potomac in front 

 of the city of Washington. Their observations show that on November 16, when the thermometer 

 showed the temperature of the water to be 68° P., young Shad were present in the Potomac at 

 Washington in large numbers. From this date the temperature of the water gradually fell, co- 

 incident with which the numbers of young Shad decreased until on November 23 the fish had 

 entirely disappeared, the thermometer then showing 58° P. The disappearance of these fish can 

 be referred only to the fact that the water had fallen below 60° P., for as long as that temperature 

 was preserved the fish remained in the portion of the river under observation. The lowering of 

 the temperature of the water seems to present the only variable factor in the conditions which 

 surrounded them, and to this we may reasonably refer their disappearance. 



The pekiod of eivek life. — The deposit of the eggs of the mature Shad in fresh waters 

 eeems to be a necessary condition for their development. The idea has prevailed to some 



'The Cliesapeake area includes the Potomac Eiver, the Chesapeake Bay and its tributary streams, and the 

 ocean between Cape Henry and Cape Charles. 



