866 , REPOET OF THE COMMISSIOlSrER OF FISHERIES. 



found to be an excess of air, with spring waters usually an excess of 

 the nitrogen of the air alone, and the location of the g'as 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 

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

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

 fishes, and others do not develop t3"pical cases, if any, while the scup, 

 the king-fish {31enticirrJiuii) , the tautog, the cunner, the sea bass, and 

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

 scup {Stenotomus c/iri/sops) shows it most readily and in extreme 

 degree (Plate III). With a certain degree of excess not exactl}'^ 

 known, but probaljly 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 s3^mptom, which may be 

 taken to indicate a moderate excess of air. The e3'eball is sometimes 

 pushed almost completel}'^ out of the head (Gorham 1899, Plate 1'2). 

 Without much displacement of the ball the conjunctiva may be raised 

 and inflated into a balloon of gas projecting far out be^^ond the eye- 

 ball (Plates I and II of this paper). 



Among fresh-water fishes salmonoids chiefly have been seen to be 

 affected. The black sncker {Catostomus nigrlccms) shovf^ed a typical 

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

 Hyhogn'athus) 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 j^ears, 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 injurj^, and it is sometimes bitten 

 off b}" other fishes, or drops or sloughs awa^^, leaving the socket empty. 



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

 after death if thej' 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 daj's. Evidently there is a physiological 

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

 pathologic exudate which occurs in trout fr}' suffering from anemia 

 and this exudate ma}^ 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 ej^es still be 

 normal at death, more readily than health}^ ivj develop in water the 

 post-mortem exophthahnia which in this case seems to be p^i-tly physio- 



