106 BULLETIN OF THE BUREAU OF FISHERIES. 



seems likely that the manner of death is different. In the case of F. heteroclitus it 

 was found in most instances that the endosmotic flow of water had ceased, and that 

 a decrease in weight had ensued within one or two daj's after transfer to fresh water. 

 On the other hand, it will be remembered that fishes of this species commonly did 

 not die for a considerable number of days, while many survived for a week and some 

 even for several weeks. Again, it will be recalled that the fatal effects of fresh 

 water upon this and some other species were nullified by the admixture of a very 

 small percentage of salt water. Analyses showed that in this latter case there was 

 little or no decrease in the salt content of the body. A rough approximation was 

 pointed out (p. 96) between the percentage of salts in this faintl}' saline water and 

 that in the fishes themselves. All of these facts point to the conclusion that one 

 factor in the death of salt-water fishes in fresh water is the extraction from their 

 tissues of an amount of salts sufficient to reduce the percentage below a certain 

 necessarj' minimum. 



If the question ))e asked, Why are not fresh-water fishes thus affected in their 

 own medium? it is replied that their membranes have been adapted to resisting such 

 an extraction of salts. It is perhaps also true that the irreducible minimum of salts 

 in these species is lower than in the case of salt-water ones. In any case the per- 

 centage actually present is, on the average, less. 



Whether or not salt water ever has a toxic effect, in the narrower sense, upon 

 fresh-water fishes can not be stated definitelj'. Bert denied that such was the case, 

 but, as already stated, he is not entirely consistent in his position. In view of the 

 fatal effects upon salt-water fishes of some of the individual components of .sea salt, 

 when taken separately (Loeb, 1900; Siedlecki, 1903), it seems quite possible that sea 

 water itself may act as a poison to fresh-water organisms, independentlv of any 

 osmotic effects. Indeed, both of these writers have shown that it is the chemical 

 nature of the solutions used rather than their osmotic pressures which determines, 

 in many cases, whether they sliall prove fatal. 



SUPPLEMENTARY NOTE. 



It gives me great pleasure to find in a paper by Prof. C. W. Greene on the 

 physiology of the chinook salmon, just published, that certain of the results obtained 

 by him lend strong support to the views upheld in the preceding pages. Greene 

 finds a very considerable difference between the osmotic pressui-e of blood from 

 salmon taken in sea water and of that from fishes taken at the spawning grounds 

 far upstream. The mean values for A given by Greene are —0.762^ and — <i.<)2,s , 

 respectively, showing a decrease of 17.6 per cent in the fresh-water individuals. 

 Greene is not convinced, however, that osmotic changes have been responsible for 

 this decrease, but is of the opinion that "the absence of food and the important 

 metabolisms occurring during the eight to twelve weeks' sojourn in fresh water are 

 to be considered in this connection, and possibly are sufficient to account for the 

 change " (p. 455). 



Another of Greene's results is of great interest to me, namely, that he found 

 but a slight reduction (3.3 per cent) in the osmotic pressure of the blood of salmon 



