08 KULLETIN OF THE BUREAU OF FISHERIES. 



lens; increased secretion of niucu.s; loss of weight. 'I'lie muscles responded to stiiiuili, 

 ai}d the heart continued beating after all external signs of life had disa])peared. 

 Bert found that his tishes would endure abrupt transfer to mixtures of fresh and salt 

 water, proviiled that not moi-e than one part of sea water to two parts of distilled 

 water were used. He states that the fatal etfects commence at the point where the 

 water is capable of exercising an exosmotic action on the fish, though he gives nn 

 evidence for this view. By a gradual process of acclimatization, however, he accus 

 tomed fresh-water tishes to live in water having one-half the salinity of that of the 

 sea. The specific gravity of such individuals was thereby raised, for. if icturned to 

 fresh water, they were found to sink to the l)ottom. 



Conversely, Bert states that in the case of certain unspecitied sea animals ■•accli 

 matization takes place easily up to a diminution of about a third in the salinity ol' 

 the sea water, and that be3'ond that death comes very readily." That this latter 

 conclusion is unjustitied in the case of the great majority of marine tishes is show u 

 in the next section. 



Bert's explanation of the "mechanism of death." in the case of fresh-water tishes 

 transferred to salt water, will be discussed later. 



CHANGES IX WATER DENSITY AT THE NEW YORK AQUARIUM." 



The present salt-water supply of this aquarium is derived from New York Bay, 

 whicli. as is well known, receives the waters of the Hudson River and a number of 

 lesser streams, and contains in consequence a much-diluted sea water. Its specific 

 gravity rarely attains and perhaps never exceeds l.Olfi, while during the spring 

 months it falls at times to LUUtJ, and occasionally lower. Considering the density of 

 water in the open sea as 1.027 (the mean for the North Atlantic), the water supply 

 of the New York Aquarium never reaches two-thirds, and occasional!}' falls to one- 

 tifth of this. Nevertheless, marine tishes of 173 species*, belonging to 72 different 

 families, have been kejjt in this water, some of them surviving for a considerable 

 nunilier of years. Except in the case of tishes from the innnediate vicinity, where 

 the water is also of low density, the new arrivals undergo an abrupt change from 

 full-strength sea water to this much-diluted water of the aquarium. This is true, for 

 instance, of more than 50 species of tishes from Bermuda, many of which have been 

 kept very successfully under these unnatural conditions, some individuals having sur- 

 vived as nmcli as 7 _vears. It must be added, however, that these semitropical tishes 

 are received only during the suumier months and that the water containing them is 

 warmed during the winter. 



EFFECT OF ALTERNATION OF FRESH AND SALT WATER. 



Expv^'iiiient 37. 



Woods Hole, August and September. 1904. Twentj-tive jp. lu-tt'i'dd'dus. 

 from sea water, put into fresh and salt water alteruatel}', one change being 

 made daily. During the 13 days of alternating density' only 1 fish died. 

 Compare with experiment 9, in which more than half of the fish (taken from 

 the same stock but kept in fresh water) had died in this period. 



" Tliese facts are furnished through the courtesy of the director, Jlr. C. H, Townsend. 

 '• Based upon an incomplete list. 



