_ 23 — JOH. HJOKT AND G. G. JOH. PETEESEN 



quite as characteristic, however, for' the older stages (the half- or fully-grown fish), that they 

 should keep each by its own species, so that one can indeed affirm that there are not two species 

 which have quite the same distribution or exactly the same mode of life. The biology of 

 each separate important species must accordingly be studied by itself, if we 

 vrish to penetrate deeper into the comprehension of their relations and habits of life. We are 

 acquainted with fishes which pass their whole life in small enclosed fjords, and with others which 

 always wander about in the various layers of the ocean and only periodically come to the 

 coasts at certain times of the year. If we wish to attain to an understanding of the period 

 decline and increase of the fisheries, it is worth while to study exactly the habits of life of 

 each single species. 



a) We know that the viviparous blenny (Zoarces viviparus) and various smaller 

 fishes pass their whole life in the smallest fjords; if their number decreases or increases we 

 must accordingly seek for the reason, firstly, in the purely local conditions. We call such fish 

 stationary or non-migratory fishes. 



Other fishes are like the eel, which also lives in small fjords and in fresh water, but 

 has come in from the ocean during its youth and remains only until it is grown; then it 

 wanders back again to the ocean; it is thus a temporarily stationary fish. Again, 

 the mackerel and garfish are true migratory fishes, which appear only for a short 

 time in the year at the coast or in the fjords, chiefly only to spawn; then they disappear 

 again into the open sea. 



b) The plaice is one of the salt-water species which have been studied the most; its 

 habitat during the various stages of life and its migrations, are known by the marking of 

 thousands of specimens ; so closely has it been investigated that methods were devised to 

 increase its yearly productiveness in the Kattegat and North Sea, by interfering at the stage 

 of life prejudically affected by the fishery and by helping the small plaice over the difficulties 

 which nature had placed in their way (see further Appendix H). 



c) The cod has likewise been closely studied from Iceland to Norway and the North 

 Sea, and there are thousands of measurements with regard to it from the most different regions. 

 We would refer to the writings of Joh. Hjort and simply mention here, that as regards this 

 species also, the question of cultivation and increase of the quantities by artificial rearing has 

 played a great part amongst the fishing population of the various countries. Fish-rearing 

 establishments were, however, erected before any knowledge had been gained of the occur- 

 rence of the young fish in the waters where the stations were erected, and before it was 

 known whether the stock of fish was to be considered in general as a small local stock, which 

 might be influenced by a relatively few millions of small young fish just liatched, or as the 

 small part of a great whole on which not even thousands of millions of small young fish could 

 exert any influence worth mentioning. 



It is possible that we have to do with small conditions of things at some places, at others 

 however, with enormously great; with regard to this, the investigations seem already to 

 have given an answer in part. 



The investigations on the biology of the older fishes consist, partly of fishery experiments 

 with different apparatus, partly in the study of the fisheries themselves and theh* results; by 

 means of these, the occurrence of the species in the different periods of the year can be given 



