358 REPOKT OF THE COMMISSIONEE OF FISHERIES. 



giv^en quantity of dissolved air per unit of water, at a given tempera- 

 ture and pressure, occasions a fatal process among fishes, a suificient 

 increase in the pressure or decrease in the temperature ma}" render the 

 water perfectly harmless to fishes; but it does so by abolishing the 

 excess of air, though no change occurs in the absolute quantity of air 

 concerned. The temperature factor alone is not so easily varied, and 

 no direct experiments have been made involv^ing it, but the statement 

 above can hardly fail to be corroborated by such tests. For the pres- 

 sure factor some interesting experimental facts have been obtained. 



Scup placed in live boxes at or near the top of a reservoir storage 

 tank of the Woods Hole water which was causing gas symptoms in 

 aquaria were usually killed within twenty-four hours, the characteris- 

 tic embolism and external symptoms always present. At the bottom 

 of this tank, the depth of water being 8 or 9 feet, several days were 

 requii'ed to produce the symptoms, and death occurred only after a 

 still longer time. At half the depth the results were intermediate. 

 There was a constant flow of water through the tank and it was evi- 

 dently the hydrostatic pressure which inhibited the usual process. 

 Carr3-ing these observations further, a large glass jar was arranged to 

 hold aquarium water with a constant flow and under a pressure vary- 

 ing between 6 and 7 pounds per square inch. in addition to atmos- 

 pheric pressure. Five adult scup were placed in this jar and remained 

 alive under the pressure, without food, for twenty-nine days without 

 developing any gas symptoms. The same water which flowed through 

 the jar would at the beginning of the experiment at atmospheric pres- 

 sure produce external lesions within twenty-four hours and was fatal 

 within two or three da3'S, the time var3'ing considerablj^ After 

 removal of pressure at the end of the experiment, all the five scup 

 died within five days with free gas in the vessels of each. They were 

 fed for the first time on the foui'th day after the removal of pressure. 

 During various experiments at Woods Hole some evidence was inci- 

 dentall}' brought out indicating that starvation retarded the gas-disease 

 process. This it ma}" be conceived to do by a general lowering of 

 metabolism. 



Except under experimental conditions, no cases of gas disease caused 

 by reduction of pressure alone have been observed by the writers, and 

 it is doubtful Avhether any occur. In a former paper by one of us 

 (Gorham, 1899) it was thought that the reduction of pressure was the 

 only cause. The factor of the supersaturation of the water was not 

 recognized at that time. From experiments performed in connection 

 with that former work and new ones in connection with the present 

 study we are sure that mere reduction of hydrostatic pressure — that is, 

 the reduction incident on bringing fishes to the surface of the water — 

 is not suflicient to produce the disease in those fishes which have been 

 studied. A number of scup were kept in a live car at the surface of the 



