296 KNUT DAHL. 



It is indeed a phenomenon known in any river, that 

 strong floods in spring and autumn generally convey not 

 inconsiderable quantities of fish from the upper to the lower 

 portions of the river. Especially obvious is this fact in rivers, 

 the lower parts of which are poor in fish, while the upper parts 

 are exceedingly rich in fish, or head from lakes where fish 

 abound. From my own experience I very well remember a 

 river in Southern Norw^ay, where the downward migration of 

 fish" during the floods was obvious. Thus the common lake char, 

 which only lived in a single remote lake at the head of one of 

 the tributaries of the river, might, after heavy floods, be caught 

 in the river. 



It is then perfectly clear, that these fish, as they gradually 

 get transported by floods down the river, must finally reach the 

 sea. Whether these, the trout forms of the fresh waters, can 

 live in the salt water is a question which I have endeavoured 

 to solve by experiment. 



In order to secure as unfavourable conditions as possible for 

 my experiments, I chose for experimental purposes the trout form, 

 which undoubtedly is most different in habits of life from the 

 trout of the sea, viz. the brook-trout. My material was pro- 

 cured from a small mountain brook which runs into the sea at 

 the Trolla iron works just outside the biological station at 

 Trondhjem. 



Sea-trout cannot on account of insurmountable obstacles 

 reach the upper parts of the brook, and it is here inhabited 

 solely by „fingerhng" trout, which even at a length of 14 — 16 cm. 

 attain sexual maturity. They do not grow larger in the brook, 

 and are thus typical brook-trout. Of these I caught by angling 

 a small number, 14 specimens. 



Arrived at my station I put the smallest and the largest one 

 into an aquarium containing water of a salinity of ca. 33 °/oo. 

 The smallest one died after 12 hours, the largest one after 

 24 hours. The others were kept for some time in a small fresh- 



