_ 21 — JOE. HJORT AND 0. G. JOH. PETEKSEN 



stages of the dab are never found on such places, but only in water ca. 18 m. deep. This 

 difference in occurrence is so characteristic, that years occurred between the first discovery of 

 the young of the plaice and flounder and that of the dab; in fact, it was possible to find them 

 at many places, only after the new young-fisli trawl had been brought into use, and always 

 in deep water. By this relation therefore, the young of the dab is at once distinguished from 

 that of both the other species; but between these two also, there is a characteristic difference 

 in distribution as one nears the Baltic; the young of both, it is true, occur together on the 

 shallow sandy shores of the Kattegat and Belts, yet certain places seem better suited for the 

 flounder, others for the plaice; the bottom-stages of the plaice, however, have not been found 

 in the Baltic further than the south of the Danish islands, whilst the flounder has been met 

 with quite to Finland, where they have been discovered in 1904 by Sandman. 



With regard to the fry of the witch, no specimen of this species of a length less than 

 10 cm. has ever been found inside the Skaw in spite of the most zealous search, though the 

 adult stages are extremely common in the deeper parts of the Kattegat; when however, the 

 international investigations from Denmark began, some quite developed bottom-stages were found 

 in the depths of the Skager Eak; but the true resort of the fry is probably to be sought 

 for still further out towards the open and deep sea along the coasts of Norway, Iceland, the 

 Pseroes and Shetland; in the greater part of the North Sea also, it will probably be quite 

 absent. 



The fry of the halibut has never been met with off Denmark. The smallest individuals 

 found by us were ca. 30 cm. long. It is curious that the fry has been able to escape notice 

 hitherto at most other places also. But in this year, this fry has been found in extraordinarily 

 deep water in the Atlantic Ocean south from Iceland: it must also occur naturally, along the 

 Norwegian coasts though it has not hitherto been discovered there. 



The facts described concerning these 5 flat-fishes show therefore, that of 5 species which 

 live in the Kattegat so close to one another that it is quite possible to take specimens of 

 all 5 species in one single haul of the trawl, only the 3, namely plaice, flounder and dab, go 

 through their entire development from the egg to the adult fish in the Kattegat; whilst the 

 fourth, the witch, appears only as individuals of relatively grown-up form, from which we must 

 conclude that they wander in at certain times from the deepest part of the Skager Eak and at 

 other times wander out again; and the specimens of the fifth species, those of the halibut, are 

 to be considered as visitors, coming from a stock whose true resort is to be found extremely 

 far distant from the coasts of Denmark. This conception of the occurrence of -these fishes in 

 the Kattegat, has only been learnt and could only be learnt by systematic investigation of the 

 special biological features of each single species in its various stages. 



c. The peculiarities of the Baltic 



In the Baltic the distribution of the age-groups of several species of fish has quite a special 

 character, which is associated with a decreasing salinity towards the east and north and also with 

 characteristic conditions of temperature. It is now generally accepted, that the eel (Auguilla 

 vulgaris), which is to be found as adult as far as St. Petersburg, never spawns in the Baltic; 

 and the latest Danish investigations have confirmed, that its larvse (Leptocephalns brevirostris) 

 occurs only in deep water (c. 1800 m.), e. g. south from the Tseroe Isles, but could not be found 



