?>6Q ^ EEPOBT OF THE COMMISSIONER OF FISHERIES. 



found to bo an excess of air, with spring waters usuallj an excess of 

 the nitrogen of the air alone, and the location of the gas will be behind 

 the eyeball. Some species of fishes are not susceptible to this symp- 

 tom from supersaturated water, or at least it has not been observed in 

 them. The anatomical structure and the degree of the excess seem to 

 be the factors which control. Among marine fishes, the dog-fish 

 {Mustelus canw) and other sharks, eels, puffers, sea-robins, the flat- 

 fishes, and others do not develop typical cases, if an}", while the scup, 

 the king-fish {JlentichThus), the tautog, the cunner, the sea bass, and 

 the butter-fish may exhibit it in various degrees. Of all these the 

 scup {Stenotomus chrysojjs) shows it most readily and in extreme 

 degree (Plate III). AYith a certain degree of excess not exactlj^ 

 known, but probably above 3 c. c. of nitrogen per liter, embolism 

 becomes fatal before there is time for an accumulation of gas behind 

 the eye. An excess of not over 2 or 3 c. c, and probably less, per 

 liter is favorable to the development of the symptom, which may be 

 taken to indicate a moderate excess of air. The e^^eball is sometimes 

 pushed almost completel}'^ out of the head (Gorham 1899, Plate V2). 

 Without much displacement of the ball the conjunctiva ma}^ be raised 

 and inflated into a balloon of gas projecting far out beyond the eye- 

 ball (Plates I and II of this paper). 



Among fresh-water fishes salmonoids chiefl}' have been seen to be 

 affected. The black sucker {Catostonius nigriccms) showed a typical 

 case at Erwin, while some cyprinoids {Notropis galacturus and a 

 Hyhognathus) under the same conditions died with the eyes normal. 

 It is no doubt because not many fishes save the trout of artificial 

 propagation have been observed in supersaturated fresh water that 

 few fresh-water species are known to show the lesion. In brook and 

 rainbow trout the pop-eye is seldom so extreme as that shown in the 

 illustrations of the scup. The excess being slight, the S3'mptom may 

 grow very slowly and be present for months, or even years, impairing 

 more or less the activities of the fish. Blindness frequently results, 

 with accompanying increases of dark pigment in the skin. The expos- 

 ure of the eyeball makes it subject to injur}-, and it is sometimes bitten 

 off by other fishes, or drops or sloughs away, leaving the socket empty. 



In trout fry past the sac stage a certain exophthalmia may develop 

 after death if they remain in water, and the younger and smaller the 

 fry the more quickly it appears. In general its development requires 

 from twelve hours to three days. Evidently there is a physiological 

 post-mortem accumulation of transudate' behind the eye. There is a 

 pathologic exudate which occurs in trout fry suffering from anemia 

 and this exudate may localize, sometimes in the abdominal cavity, 

 causing ascites, sometimes behind the eye, causing exophthalmia with- 

 out gas. Fry having this form of anemia, though their eyes still be 

 normal at death, more readily than healthy fry develop in w^ater tha 

 post-mortem exophthahnia which in this case seems to be partly physio- 



