EFFECTS OF GASES ON TROUT. 357 



eyes altogether. The disease has been examined to some extent by 

 Marsh and Goreham^, more especially under some special 

 conditions of sea water, and they ascribe both No. 1, death of fish, 

 and (2) pop-eye, to excess of dissolved gas, chiefly nitrogen, in the 

 water. I may here say that both the fatal effect of the water and 

 its pop-eye producing properties diminish and finally cease with 

 perfect aeration. 



(3) As is natural of trout eggs placed in the hatching trays, a 

 certain number die. It was alleged, however, by Mr. Rides, the 

 curator of the fish hatchery here, that more eggs died in trays near 

 the outflow than in those exposed to more aerated water. Ex- 

 periments confirming this will be quoted. 



(4) Blue swelling is a disease of the yolk sac, and is, according 

 to Mr. Rides, more common in Christchurch than in any other 

 hatchery of which he has any information. A light bluish ap- 

 pendage forms on the yolk sac that is filled with some fluid, and 

 which, as the young fish is unable to absorb it, is almost always 

 fatal. It has been cured by pinching the bluish appendage and so 

 letting the fluid pass out, but this is an impracticable method where 

 many thousand young and active trout have to be examined. 



We have since shown that all these four separate effects possess 

 this in common, viz., they all tend to diminish with greater aeration. 

 They must therefore be due to the presence of some deleterious gas, 

 or to the absence of oxygen. Experiments were therefore made to 

 ascertain the manner in which the gas content altered with rippling 

 over obstacles and at the same time noting the variation in the 

 various effects under observation. 



For the investigation of effects to (1) and (2), death and pop-eye, 

 we first confined eight fish in the sand box of the Museum well and 

 ten fish in a concrete box into which the water runs after flowing 

 out of the sand box. In six da-js six out of eight had died in the 

 sand box, and four out of ten in the concrete boxes, in which we 

 endeavoured rather unsuccessfully to confine the trout. These 

 fish were very lively when first put in, and escaped surprisingly. 

 Thus the results are not altogether satisfactory, except that we can 

 certainly say that the death rate diminished with recession from the 

 well, though it occurred to an abnormal extent even in box four, in 

 which three fish out of ten, which we attempted to secure, died in 

 fifteen days. 



Dr. Chilton made a slight examination of these dead fish, and in 

 some cases found gas emboli in the gill filament, though he did not 

 find it in all. 



In fish which survived a week in either of the two boxes nearest 

 the well, pop-eye almost invariably made its appearance, though 

 more markedly in the sand box than in the others. 



We have recorded one case of pop-eye in the third box from 

 the well, though none in the fourth. 



1 Report Bureau of Fisheries, 1904, Washington. 



