598 NATURAL HISTORY OF AQUATIC ANIMALS. 



well established by recent observations. For instance, it is well established that the runs of Shad 

 into the Susquehanna and Potomac Rivers are characterized by alternations of abundance; that 

 is to say, an excessively large yield for any given season in the one involves a corresponding dimi- 

 nution in the yield for the same season in the other, thus precluding the possibility of each 

 individual returning annually to its native stream. Again, it was confidently expected that all 

 the young Atlantic Shad which were transferred to and planted in the Sacramento River would, 

 on their return from the Pacific Ocean as mature fish, find their way back to this stream. This 

 was not, however, the case, for, to the utter astonishment of many fish-culturists, a considerable 

 number of these now mature fish made their appearance in many streams of the Pacific lying far 

 north of the Sacramento River streams to which Shad had never been indigenous and in which 

 none had ever been planted. 



These facts go a long way to disprove the theory of instinct of locality, and indicate that the 

 river movements of the Shad are regulated by involuntary and extraneous influences. The migra- 

 tion and colonization of this fish northward along the Pacific coast has been so general that at the 

 present day new generations of a single plant are found in every stream on the Pacific from the 

 Sacramento River to Pujjet Sound. 



THE "FEEL" OF THE KIVERS. Some writers, notably Mr. Charles G-. Atkins, have suggested 

 the idea that the upward river migration of the auadromous fishes is directed by an instinct which 

 impels them to swim against the current. It is supposed by him that in their coastwise movement 

 the Shad, when opposite the mouths of the rivers, feel the outflowing current and, responding to 

 the invitation, immediately turn to and stem it and are thus led into and up the stream. The 

 conclusive reply to this supposition is that in the wide estuaries of our North Atlantic streams there 

 is no sensible current, excepting that produced by the tidal ebb and flow, which is far too indeter- 

 minate to be the directing cause of the migrations of those vast schools of Shad, Alewives, and other 

 species which annually enter our rivers. Even if the fish were attracted up stream by the gratifi- 

 cation of that presumed impulse or desire to swim against the current, how can we account for 

 their migration down stream, at the appropriate season, this movement being as regular and as 

 universal as the upward migration f 



CHANGE OF SALINITY. It has been suggested that Shad may be sensible of the decreasing 

 salinity of the water as they enter and ascend the rivers, and that they may be led into continental 

 waters in order to enjoy a more congenial habitat; but in this event it is necessary to explain 

 why they do not remain in the rivers altogether. 



WATER TEMPERATURES. Prior to the last decade, very little attention was paid to the water 

 temperatures in connection with the migrations of fish. We have on record but few series of 

 observations of water temperature during the season of our river fisheries. Since the inauguration 

 of the United States Fish Commission, however, and the establishment of hatching stations on the 

 rivers, it haa become possible to make a closer study of this subject. It will, however, require a 

 connected series of such observations, made during several seasons and at many stations, in order 

 to obtain sufficient data for a satisfactory discussion of "the relation of the movements of fish to 

 the water temperatures." Up to the present time the drift of investigation goes to prove that the 

 movements of fish, auadromous and otherwise, are controlled largely, if not entirely, by the tem- 

 perature of the medium in which they live. 



In the case of "bottom -feeders," their movements are dependent, no doubt, principally upon the 



