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On an Experiment in the Keeping of Salmon (Salmo 

 salar) at the Plymouth Laboratory. 



By 

 L. R. Crawshay, M.A., 



Assistaiit Director. 

 With Plate XXIV. 



FoK the past two and a half years (1906-8) some salmon, which 

 were reared at the Endsleigh Fishery, have been kept imder observa- 

 tion in the aquarium of the Plymouth Laboratory. 



In sending these fish as smolts to the Laboratory, the Duke of 

 Bedford wished more particularly to obtain information on two 

 questions : firstly, the character of the food of the salmon during its 

 sojourn in the sea, and secondly, the period of that sojourn interven- 

 ing between the smolt and grilse stages. These and other points 

 of interest that have arisen will be considered in order in giving a 

 general account of the experiment. 



The smolts were brought from Endsleigh at two years old, and 

 introduced into the aquarium in two lots (of twenty and thirty) on 

 February 6th and March 1st, 1906, respectively. The actual weight 

 and measurement were not taken at the time, but Mr. E. C. Bundle 

 informs me he has ascertained that the average weight may be placed 

 at 4-5 oz. and the average length at 8-10 inches. The fish have been 

 largely under the charge of Mr. A. J. Smith, and it is upon his detailed 

 notes that the present account is based. 



For the accommodation of the smolts, one of the aquarium tanks 

 was emptied and brought into communication with the fresh-water 

 supply, the water being led into the tank by means of a rubber hose- 

 pipe, and kept running. 



The first twenty smolts were put into this tank on February 6th, 

 1906, and allowed to remain in the fresh water for two days. Trans- 

 ference to sea-water was then effected very gradually, at a rate of 

 inflow increasing from day to day, as follows : — 



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