REACTIONS OF FISHES TO SALTS 279 



water the reaction was again reversed and the fish became 

 negative once more. 



Acclimatization of fishes to salts must certainly be concerned 

 with internal adjustments, for Sumner ('07) has shown that 

 the specific gravity of fishes' blood is altered when they are 

 changed from fresh to sea-water, and vice versa. An alteration 

 in the density of the blood seems then to result in a reversal in 

 the reactions of the organism to density in the environment. 

 Green ('04) has shown that changes in the specific gravity of 

 the blood of the salmon occur at the time the fishes are entering 

 the fresh water; the blood gradually acquires a density that 

 averages 17.6 per cent less than that of salmon in sea- water 

 (I.e., p. 454). Jones ('87) has proved that age, exercise, sexual 

 maturity, pregnancy, food, etc., have a measurable influence 

 upon the density of the blood of man, and Sumner ('07) states 

 that there are seasonable differences displayed by fishes, in the 

 osmotic phenomena through their gills. It may be that the 

 specific gravity of the blood of anadromous fishes at different 

 stages in the life cycle, can be used as an index to the physiological 

 changes that are going on in the organism. Also the effect upon 

 the organism of a higher CO2 production within the tissues must 

 vary with the density of the blood and would probably be more 

 marked when the l)lood is less dense. 



An investigation of the changes in the density of the blood of 

 the salmon could perhaps best be begun with the fry in the 

 fresh water streams. As the fishes remain for 2 years or even 3 

 in the fresh water before leaving for the ocean, a thorough study 

 of the relative densities of the blood and the fresh water could 

 be made in this period. That the instinct which causes these 

 fishes finally to reject the fresh water for that of the sea, is backed 

 by some very strong stimulus is indicated by data given by Day 

 ('87). Day speaks of an experiment which was carried on by 

 Maitland in 1880. Eggs of salmon were hatched in fresh water, 

 and the young salmon were placed in ponds shut off from the 

 sea. These fishes ate well and grew vigorously until they were 

 about 2| years old. At this stage in the life history, the indi- 

 \'iduals are known as 'smolts' and it is at the smolt stage that they 



