Keil — Sniolt Period in Salmonoids. 181 



the smolt coloration. One of the local guides who had promised 

 to send certain information, reported that fall that the larger 

 of these fish had turned silvery and was beginning to move out of 

 the streams. 



In order to determine the average age or size at which this 

 color transition takes place, in both salmon and trout of a migra- 

 tory type, five seasons were spent in gathering data for compari- 

 son. This was necessary on account of the great variation in fti- 

 cubation and growth during different seasons and at various locali- 

 ties. From observations upon thousands of specimens of known 

 age, it was definitely established that with fish of normal growth, 

 this change from parr to smolt begins during the latter part of the 

 second year. 



All writers handling the subject of the life history of the 

 Atlantic salmon, while briefly mentioning the remarkable color- 

 change of the young fish previous to migration, studiously avoid 

 commenting on the probable significance of this change. No doubt 

 it would be regarded as an adopted protective coloration, were it 

 not for the fact that it takes place before rather than after migra- 

 tion. This change of the trout-like parr to the silvery smolt, is 

 not one of color alone, for there are several anatomical changes 

 as well. During this stage, the form, fins, and especially the tail, 

 take on certain characteristics of the adult fish; while the sexual 

 organs, as if in preparation for functioning, undergo their first 

 real period of development. The silvery, metallic-like appearance 

 at this time is not caused by the formation of new scales, but by 

 the depositing of a pearly pigment over the sides and gill-covers 

 of the fish. This, in the opinion of the writer, is nature's method 

 of armor plating that portion of the body containing the thousands 

 of tiny ducts, whose functions would be impaired or destroyed 

 by the tremendous water-pressure of lake or ocean. It was with 

 this idea in mind that the following experiments were carried out, 

 which resulted in evidence so definite in character that they may be 

 regarded as having solved the entire problem. 



These experimental tests were carried out to determine what 

 effect or influence the water-pressure had on landlocked salmon 

 and steelheads in the various stages from advanced fry to 2-year- 

 olds. In order that the results might not be influenced by factors 

 other than water-pressure, the experiments were conducted at such 

 times as the water temperature and dissolved gases were uniform 



