602 NATUEAL HISTORY OF AQUATIC ANIMALS. 



Now, in all other streams on the Atlantic coast, the fish appear to wait until the temperature 

 of the river has risen above that of the salt-water area into which the river empties, before they 

 ascend in the spring. The migration of the Shad into the Saint John's Eiver is clearly not for the 

 immediate i^urpose of spawning, as that operation is not performed for months, but in order that 

 they may keep within the limits of the hydro-isothermal area appropriate to them. We must 

 supijose that the temperature of the ocean waters, on the continental plateau outside the coast 

 line, is higher than 60° F., and although uncongenial to the fish, yet they must necessarily 

 remain in that temperature until the waters of the Saint John's, cooling as winter advances, have 

 fallen below the temperature of the outside waters. As soon, therefore, as water of a lower tern 

 perature than that in which they are commingles with the ocean water, it serves as an incentive — 

 as it were the signal — for their migration into the estuary of the Saint John's. 



Obsbryations on the Potomac Eiver in 1881. — In 1881 the writer, then in charge of the 

 shad-hatching operations on the Potomac Eiver, collected full statistics of the catch of Shad and 

 Alewives from four of the seine fisheries occupying that section of the river lying between Indian 

 Head and Mount Vernon. From these statistics the fluctuations in the run of the Shad up the 

 river have been closely approximated, and at the close of this paragraph general deductions rela- 

 tive to the same will be made. Through the courtesy of the Light- House Board and the United 

 States Signal OfiBce, observations on the water temperature at Winter Quarter Shoals and at 

 Norfolk, Virginia, have been obtained. The former point is a light-house in the Atlantic, lying 

 about fifteen miles from the Virginia coast, and situated, it is believed, on the inner edge of the 

 cold arctic current that flows down the coast inside of the Gulf Stream. The observations 

 taken there represent the temperature of the water on the continental plateau between Cape 

 Charles and Cape Henry. The records taken at Norfolk serve as an index of the temperature 

 of the waters of the Chesapeake Bay, but are subject to inaccuracies, Elizabeth Eiver being hardly 

 more than a tidal estuary, and the temperature of its waters being influenced very materially by 

 local meteorological conditions. 



A graphical representation of the temperatures at these two points, as also of the corre- 

 sponding temperatures at the Potomac hatching station, is given in the accompanying diagram, 

 which serves to illustrate the influence of hydrothermals in determining the direction of the move- 

 ments of the Shad and Alewives and in limiting their range. In the diagram are also presented 

 the fluctuations of the run of these fish in the fishing season, as deduced from the records of 

 ^' catch," furnished by the four seine fisheries already alluded to.' 



By reference to the diagram it will be seen that during the first seventeen days of April (1) 

 the temperature of the water in the Potomac was occasionally lower than at Winter Quarter Shoals 

 ■during the same period of time; (2) that the water of the Chesapeake Bay was warmer than that 

 of the ocean between Cape Charles and Cape Henry, and also warmer than the water in the 

 Potomac Eiver, and that (3) during that time the temperature was in none of those waters above 

 ■60° F. As soon as with the advancing season the water in the river became warmer than in the 

 bay the Shad commenced to ascend the Potomac, and when the temperature of the river rose to 

 60° F. the upward run attained its maximum ; the main body of Shad and Herring ascended the 

 river M'hen its temperature ranged from 56° F. to 66° F.; and, further, that when the temperature 

 ■of the river passed above 66° F. the run of Shad and Herring rapidly diminished. It may be 

 seen also that in general the fluctuations in the run of the Herring closely followed that of the 



'Although the data obtained from those four shores do not by any means represent the total catch for the whole 

 river, yet, covering as they do a complete section of the river, they furnish figures from which the fluctuations in the 

 .upward migrations of the Shad and Alewife for the whole river can be approximated. 



