54 REPORT OF COMMISSIONER OF FISH AND FISHERIES. - 



to be ascertained. It is probable that they change their locations but 

 seldom, living as they do at great depth, where the prevailing low tem- 

 perature (30° to perhaps 45°) is thought to vary but little. 



Until within a, few years little has been known of this group, the re- 

 searches of the Challenger having been principally instrumental in 

 showing its extent, variety, and the remarkable peculiarities of its dif- 

 ferent members. Many species have also been revealed to us by the con- 

 tributions of the Gloucester fishermen to the U. S. Fish Commission. 



Probably the only important factor in influencing the change of sit- 

 uation in this group of fishes is the search for food or the pursuit by 

 fellow fish, cephalopods, &c. 



In addition to the regular, periodical, or occasional movements offish 

 just referred to, there are cases in which the change of location is not so 

 easily explained. Among these may be mentioned the selection of a 

 fresh- water abode by species which are generally exclusively marine, and 

 vice versa. Of course, the change in anadromous fishes is intelligible; 

 but why such fishes as the sawfish, shark (Pristis), the sting-ray, and 

 quite a number of other kinds should live and apparently thrive in 

 fresh w r ater, is not so easily understood. Other species are found up 

 rivers to a considerable distance from their mouths beyond the brack- 

 ish portion. 



Hibernation. — Another subject which may be considered in connec- 

 tion with that of migration and movements is that of hibernation. 



Many fresh-water fishes, such as carp and others, are known to bury 

 themselves in the mud, either partially or entirely, during the cold 

 weather, and to remain there until the warm season of the year. This 

 is also the case to a greater or less extent with the eels, both in fresh 

 water and on the coast. To what extent other kinds of strictly marine 

 fish exhibit the same habit is at present difficult to determine. The 

 disappearance from our coast during the winter season of the mackerel, 

 menhaden, and some other species has given rise to the belief by some 

 that they bury themselves in the mud at suitable places off the coast. 

 Indeed, there are not wanting statements to the effect that mackerel 

 have been speared in the mud by persons who were attempting to 

 capture eels in this well-known method. Some of these instances ap- 

 pear to be fairly well substantiated; but whether they represent any- 

 thing like a permanent condition it is now difficult to say. Those who 

 believe in the hibernation of mackerel point to the existence of a film 

 over the eye on the first appearance of this fish in the spring, which the'y 

 suppose to be the result of the long exclusion of light or of contact with 

 the mud, this film going away in the course of the summer. 



The sturgeon is believed to be a hibernating fish to some extent. 



Having thus considered the better marked movements of fishes under 

 their different heads, I now propose briefly to consider the causes of 

 such movements so far as we can understand them. 



Physical causes. — The more regular changes of position with the 



