THE EFFECTS OF EXPOSURE ON THE GILL 

 FILAMENTS OF FISHES 



By Raymond C. Osburn 



What I have to present this afternoon will be rather in 

 the way of a preliminary study of this question, since I have 

 not been able to carry it to fulfilment in every respect. 



I presume that many of the fish culturists present have 

 noted that in rearing trout and salmon numbers of the 

 fry show one or more aborted gill covers, thus exposing 

 the gill filaments on one or both sides. I do not know how 

 general this is, but I have spoken with several fish culturists, 

 engaged both in Government and private work, and know 

 that this condition is frequently presented by these fishes. 



Sometime ago I examined a tank of 486 yearling silver 

 salmon in the New York Aquarium and noted a large 

 number with the abnormal gill covers. With the right 

 opercle abnormal there were 44, with the left opercle ab- 

 normal 27, with both gill covers abnormal 18. This 

 makes a total of 89 abnormal out of the 486, or 18.31 

 per cent. 



I began this investigation with the intention of trying to 

 discover whether those fishes which had abnormal gill covers 

 were more readily eliminated than those which were normal, 

 but the care of our aquarist has been so good that only a 

 few fishes have died since my observations began a couple 

 of months ago, and I am not able to draw any conclusions 

 in regard to that point. I do want to show you, however, 

 what happens to the gills, and, perhaps, I can do that best 

 by a few rather crude charts which I have drawn up hur- 

 riedly and under the strain of business. (Charts exhibited. ) 



This chart illustrates one of the gill covers sectioned for 

 microscopic study, and shows how it is turned under. The 

 normal gill cover extends straight out over the gill chamber. 

 The extent of the abnormality varies greatly ; in some cases 



