Introduction 



ihe first condition for a right understanding of the habits and habitats of the food- 

 fishes of the sea, and in general, of the production of the sea as regards useful fishes, is 

 an exact knowledge of the occurrence and distribution of these food-fishes at all the 

 various stages of their life, 'from the egg on to the adult mature form. The plaice is the 

 most important, ground-dwelling food-fish of the North Sea. In the interests of the sea- 

 fisheries we wish to know, under what conditions the plaice lives in the North Sea, how 

 it nourishes itself and reproduces its kind, whether an overfishing of the stock is going on, 

 if it is a stationary or migratory fish in the North Sea, and whether, according to the answer 

 to this question, an influx from other seas is occurring to compensate for the fish removed 

 or not, and whether finally, fixed protective legislation at certain regions of the North Sea 

 has a possible prospect of good results or not. In order to settle these and many other 

 points, we must of necessity first investigate and determine above all possibility of objec- 

 tion, the following facts. Where and when does the plaice spawn? Where and in what 

 relative amounts are the free-swimming eggs and the planktonic larvse hatched from 

 them? When and at what size is the metamorphosis of the plaice-larvse complete 

 and does their life on the bottom begin? Where are these youngest bottom-stages of the 

 plaice to be found, how fast do they grow there, and what size do these young plaice 

 reach at the end of their first year? Where do they live and how large are they in the 

 second, third and following years of their life? At what size and in what year of their 

 life, do the plaice become usable for a rational fishery, at which age are they mature for 

 the first time, and where do they frequent at that time and in the following years of their 

 maturity? How old can a plaice in the North Sea become in general and — in connection 

 therewith — through how many years can they, by production of eggs, contribute to the 

 maintenance of the stock? Do the plaice make regular migrations in the course of the 

 year from one part of the sea to another, how far do such migrations extend and do the 

 older plaice travel further than the younger? and so on. 



What we must know regarding the plaice first of all, holds also for the other 

 important food-fishes, especially for the cod and haddock, and then also for the turbot, 

 sole, dab, whiting etc. 



Tbe Biological Station of Heligoland, as a cooperator in the international study of the 

 sea, has endeavoured as far as possible to determine these points concerning the natural 



1* 



