SHAD IN THE SAINT JOHN'S KIVEE. 599 



migrations of their prey; but here again it is probable that the movements of the latter are influ- 

 enced by temperature. 



In the case of Menhaden and Shad, which species feed as they swim, the temperature of the 

 water is probably the main factor in determining their movements. It is a fact, for example, that 

 the disappearance of Menhaden from the coast of Maine 1 was, and has each year since been, co- 

 incident with a uniformly lower temperature of the water along that coast duiing the menhaden 

 season. 



The causal relations of the migrations of the sea Herring to water temperatures is a matter 

 recognized by the pisciculturists and fishermen of the North European Atlantic region ; but their 

 observations, as with our Shad, have not been sufficiently extensive to enable them to define accu- 

 rately the relations of the one to the other. 



In regard to the Shad, and presumptively to other fishes also, it is believed to be true that 

 there is a certain temperature of the water in which these fish prefer to live; in other words, 

 that they aim to occupy a hydrothermal area of certain temperature; and, further, that their 

 migrations are determined by the shifting of this area. 



To state this theory somewhat differently, it is believed that all migratory fish have a normal 

 range of temperature in which they seek to remain. As before stated, observations on this point 

 .are not as yet extensive, and therefore the limiting hydro-isothermals within which a given species 

 may at any time be found cannot yet be absolutely defined. 



So far as this matter has been examined with regard to the Shad, the following conclusion 

 has been reached, namely, that they occupy an hydro-isothermal belt, or area, limited by the tem- 

 perature of CO F. to 70 F. ; that they move with this belt, i. e., as the season advances, into and 

 up the rivers. This movement, at least in the case of the Shad and Herring, takes place at the 

 time when they have nearly matured their spawn, and just at that important crisis, by means of 

 that exact balance and adjustment which nature everywhere provides, the fish are brought by 

 influences of which they are entirely unconscious into such relations and under such conditions 

 as make reproduction possible. But, although the operation of spawning is mainly that for 

 which the fitness of relations and conditions has been ordained, the following statements will show 

 that the fish in moving up the rivers are not always actuated by the immediate desire to deposit 

 their spawn. 



OBSERVATIONS ON THE SAINT JOHN'S RIVER, FLORIDA. In the Saint John's River, Florida, 

 the Shad appear in the river several months before the spawning time, and, although this season in 

 the Upper Saint John's is not largely in advance of the same season in rivers as far north as certain 

 tributaries of the Chesapeake, yet by reason of their early presence in the Saint John's the shad 

 fisheiies, as has before been noted, are there prosecuted duiing the entire winter. The.y do not 

 enter the river at this time for the purpose of spawning. By reference to tables giving the tempera- 

 tures of the Saint John's River at. Jacksonville for twelve months beginning March 1, 1877, and 

 ending February L'S, 1878, it will be seen that in the Saint John's River the temperature of the 

 water gradually descends, reaching G0 F. at Jacksonville about the last of November. This date 

 is coincident with the first appearance of Shad in the Saint John's. 



1 This commenced in 1879, and they have not yet reappeared to any extent. 



