360 REPOET OF THE COMMISSIONER OF FISHERIES. 



coldest surface water. As for pressure conditions, then, no such deep- 

 sea fish should liberate air from its blood when brought to the surface. 

 Since, however, its habitat may be water whose nitrogen was dissolved 

 at a low temperature, and it may be brought up into comparatively 

 warm surface water, there exist theoretical conditions in which this 

 result would be possible. That it actually occurs is unlikely, but is 

 a matter for observation. The air-bladder factor has not been 

 thoroughly worked out in the present study and is an interesting field 

 for further experiment. 



AlthoLigh under these experimental conditions it is possible to pro- 

 duce the gas disease by reduction of pressure alone, yet the conditions 

 are quite dili'erent from those which obtain when fishes are brought to 

 the surface from depths. Fishes are in the habit of coming to the 

 surface for short periods under natural conditions. Thej^ can accom- 

 modate themselves for short intervals, at least, to changes in pressure 

 ranging from that at the surface to that of considerable depths, 

 though the amount of gas to be eliminated when a fish with an air 

 bladder comes to the surface is very large. When forcibly drawn up 

 from considerable depths great changes take place, for the eyes bulge 

 from the head and sometimes completely out of the sockets, the fish 

 is often "poke-blown," the stomach and other viscera pushed into the 

 cavity of the mouth, and the air bladder expanded or ruptured. The 

 removal of pressure causes the free gas alwa3^s present within the 

 body to expand, and occasions displacement of tissues and organs. 

 It is an interesting question whether such fishes have free gas within 

 the blood vessels. 



When the pressure is reduced below that of the surface, and quite 

 rapidly, we would expect that the fish's powers of accommodation 

 would be overstepped and they would not be able to take care of the 

 surplus gas so quickly produced. No opportunity for adjustment is 

 given. Still more when a fish is brought from a considerable depth 

 and confined at the surface in water which is already supersaturated 

 with gas, the gills would be unable to discharge the excess from the 

 blood and the production of the gas disease would be hastened. 



On the other hand, that the supersaturation of the water alone with- 

 out the reduction of pressure is sufincient to produce the disease, we 

 have abundant evidence. Surface fishes like Fundulus^ usually quite 

 hard}'^, succumb to the effects of the supersaturated water. Fresh- 

 water fishes, like the trout, which have never been subjected to any 

 decrease of pressure, quickly show the effects of supersaturation. 



CONCLUSIONS. 



In the light of these facts it seems to follow theoreticall}^ that no 

 matter how great the quantity of air dissolved in water no gas disease 

 can appear, provided the pressure is high enough; and conversely, no 

 matter how high the pressure the gas disease will aj^pear, provided 



