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fish occasioned by the trawl had not already had a perceptible and perhaps 

 a serious effect. The enormous extent to which this method of fishing has 

 developed of late years furnished sufficient grounds for the fear that such 

 might be the case. 



Although other fish, the haddock for instance, play a very important 

 part in the trawl fishery, the investigations in connection with the solution of 

 this second problem were, for practical reasons, concentrated mainly upon the 

 Pleuronectidse or flat fish. And while our knowledge of the natural history (in 

 its broadest sense) of the various cod species, or gadoids, has been extra- 

 ordinarily increased through the investigations in connection with the migra- 

 tion question, we may also confidently assert that our acquaintance with the 

 Pleuronectidse has made enormous progress during the ten years of the 

 International Investigations. These fish are now beyond doubt among the 

 best known fish of nothern seas. It would scarcely be possible, in this review, 

 to state the results of these investigations. We must be content briefly to 

 enumerate the various points in the natural history of this fish with which 

 the investigations have especially been concerned. With regard to the various 

 species of Pleuronectidse found in the northern seas, the questions which it was 

 necessary to elucidate may be summed up as follows: What is their general habi- 

 tat; in what places, at what depths, and under what physical conditions they are 

 found, and how they change their grounds with growth; their size at a certain 

 age, and how this inereases with the years of life; the method of determining 

 age by length, and how it is possible also to determine (frequently much better), 

 the age from the structure of the scales or of certain bones; at what age and 

 size these fish first propagate, the sexual differences characteristic of each 

 species, the time of spawning, and the spawning grounds or spawning regions 

 of the individual species; the nature of the eggs, their size, and the method 

 of distinguishing them by pigmentation, or by the presence or absence of a 

 globule of oil, etc., and how they develope; the appearance of the larvse, how 

 and where they live, and the place and manner of further development of the 

 post-larval stages, the young fish, etc., and when and how they gradually move 

 to the places where the older fish are found. These questions it is now possible 

 to answer in the case of nearly every flat fish of the North Sea or the Baltic. 

 Moreover, in order to obtain more exact information regarding growth and 

 distribution, migrations, etc., extensive marking experiments have been made; 

 and that international co-operation has proved of the greatest advantage in 

 the case of these experiments also need hardly be said. Nor is this less true 

 with regard to the comprehensive statistical investigations which proved neces- 

 sary in connection with the question of overfishing. 



