A STUDY ON TROUT AND YOUNG SALMON. 297 



water tank, into which was allowed to run a small stream of sea- 

 water of 33 *^/oo salinity. In the course of a couple of days the 

 supply of saltwater was increased, until the aquarium contained 

 almost only seawater. The fishes then commenced to die. On this 

 account the supply of saltwater was reduced to a very small 

 volume and by and by in the course af 3^4 weeks, again gra- 

 dually increased. It was then evident that a salinity of 10 ^/oo 

 even 15 ^/oo did not in the least affect the fishes. Only when the 

 salinity was suddenly increased to 20 ^/oo did a few die. 



The 2 remaining fishes were again subjected to a reduced 

 salinity and 2 recently caught brook trout added to them. 

 The salinity was again gradually increased during about 1 week. 

 All passed 15 ^/oo without inconvenience. When 20 °/oo was 

 passed the 2 recently added died, while the 2 veterans lived and 

 even spawned in the aquarium. As the seawater-supply of the 

 station at this time was stopped, the experiment was broken off. 



By these experiments I consider the fact established that even 

 brook trout, which undoubtedly must be considered as the fresh- 

 water form, whose properties are most highly speciahsed, can 

 live without difficulty in water of a salinity of 15 °/oo and may 

 even endure a salinity of 20 °/oo. In most of our rivermouths, 

 at any rate the larger ones, the salinity however is considerably 

 lower during spring and summer, at least in the upper water- 

 layers, and the distribution of this low salinity is very often 

 wide. Thus the emigrating freshwater-trout here evidently en- 

 counter more favourable conditions than in my aquaria, especially 

 if we consider, that, in nature, the changes from fresh to salt- 

 water are much more gradual and afford the fish more choice 

 than the relatively rough experiments in my aquaria. 



It is also my opinion, that this emigration must not neces- 

 sarily be effected by one single individual; but that it may be 

 performed in one or more generations. 



As far as I can see the main thing, which is important to 

 the economy of trout in the sea, is the fact, that trout from 



