Part I: 



The Northern Cattegat. 



The investigations which the Biologicai Station made into the fish- 

 faiuia of the Cattegat, in the years 1897 and 98, were mainly limited to 

 the northmost part of this sea, from the Skager Rack to a line a little south 

 of Læso. Considered as a whole, the Cattegat is hy uo means a deep sea ; but 

 in the part liere described, we find depths of 50 — GO fathoms (at one particular 

 spot east of Læsø even 75 fathoms), consequently considerably greater than in 

 our other seas within the Skaw. 



It is evident that a sea whose depths vary from O — GO fathoms, must 

 present verj' different conditions of life to the animals living in it. That this 

 holds good of the lower animals in the Cattegat had already been proved by 

 the extensive dredgings undertaken by C. G. Joh. Petersen*). The trawlings 

 which the Biologicai Station, of late years, has made in the northern Cattegat, 

 show, that it holds good of the fishes also, that these by uo means are equally 

 distributed over the whole sea, but that most species of fishes live ^\^thin cer- 

 tain helts (see later on), which thus get their peculiar stock of fishes. 



This is not the place to enter more closely into the causes of this faet, 

 but it must be emphasised that there are many concurrent factors, each of 

 which being of some importance, which together produce the said result. 



We must say a few words, however, of one of these factors — the con- 

 clition of tlie hottom — as this, to a certain degi-ee, must be said to be the basis 



*) C. G. Joh. Petersen: Det videnskabelige Udbytte af Kanonbaaden >Hanch« s Togter 

 i (le danske Have indenfor Skagen i Aarene 1883 — 86. Copenhagen 1893. 



