44 8 AMERICAN FISHES. 



Merrimac in May and June, in the Penobscot most abundantly in June 

 and July, though some come as early as April, and in the Miramichi, from 

 the middle of June to October. I can only account for this seeming 

 paradox by the theory that, while Salmon are not harmed by extreme 

 variation of temperature, they may be averse to sudden changes, and 

 though strongly impelled to seek the spawning grounds are prevented by 

 the cold. I have ascertained that the cod possesses very little animal 

 warmth. The temperature of the blood of a number of individuals caught 

 in twenty-five fathoms of water was 47 , F., precisely that of the water 

 at the bottom whence they were lifted. Mackerel swimming at the sur- 

 face registered 59 or 6o°, while the temperature of the water was 58°, 

 thus indicating that they possess a trifling amount of animal heat. The 

 Salmon unquestionably changes its temperature with that of the surround- 

 ing water in much the same way, and if, as is probable, rivers rising in the 

 mountains are colder in early spring than the ocean strata frequented by 

 the Salmon, here is a possible solution of the problem. It is stated that 

 in the English rivers, which are always open, there are no regular seasons 

 of ascent, the fish constantly passing in and out ; indeed, Mr. Atkins 

 thinks it pretty certain that large Salmon in prime condition are running 

 into the Penobscot from the sea every month in the year. It is likely, 

 also, that the warmth of the rivers is an important factor in accelerating 

 the vegetative growth of the eggs in the ovaries of the mother fish. 



The movements of the Salmon are not so intimately related to the tem- 

 perature of the water as those of many other species. They are not 

 sensitive to sudden changes, and are capable of enduring a range of at 

 least forty-five degrees. In this they resemble less the migratory fishes 

 than the permanent residents of our fresh waters ; indeed, it is quite 

 allowable to speak of them as resident, for a large proportion of the 

 whole colony belonging in one river may be found in it at any season. 

 This proportion cannot fall much below two-thirds, if we consider that 

 the fish less than a year old would make up at least half its number, and 

 that the breeding fish are in the rivers six or seven months after the breed- 

 ing. The breeding fish remain during the season of greatest heat and 

 greatest cold, though their stay after they have deposited their eggs is no 

 doubt chiefly because their vitality is diminished and their circulation 

 retarded by the falling temperature, depriving them alike of the craving 

 for food and the power to seek it. Those which spawn early are believed 



