51 



degree of evidence of a rotation along the coast, whatever its begin- 

 ning and its end may be. And if such be the case it will not be 

 necessary to look for morphological features to distinguish the 

 schools of plaice. 



In this connexion it would be desirable to know more about 

 the migratory habits of the plaice of the west coast. The few, 

 mostly large plaice, marked at Broad Bay m connexion with the 

 " Goldseeker " experiments do not throw much hght on the 

 question. 



Migrations of the Dab. — It may be concluded from the 

 above considerations that the dab entered the North Sea by the 

 Channel, along with or about the same time as the southern race 

 of plaice. This was probably, as has been suggested, durmg the 

 period of submergence. It has been said that it was at this period 

 that the plaice were hkely compelled to leave the middle of the North 

 Sea and to approach the shore. And this change would allow of 

 the dabs pressing into the deeper regions. They are thus later 

 arrivals than the plaice, but the North Sea area has favoured the 

 species, for apart from the shore they occupy a prominent position. 



We have now to consider the fact that they have spread 

 independently of current far north into the North Sea, and particu- 

 larly the annual migrations of the species in a reverse direction to 

 that of the plaice, one manifestation of which we have been able 

 to trace on the coast of Northumberland. The schools in this 

 case cannot be said to arise from a spawnmg ground in the direction 

 of which the winter migration takes place. In spite of the later 

 period of the spawning presumably the small dabs of the North- 

 umberland coast proceed from larvae carried to the region from the 

 north. Yet these form themselves mto schools which migrate 

 southwards for winter. If the larvae have thus a tendency to be 

 carried south it will probably be found that the older stages will 

 migrate beyond the hmits of the school in a northerly direction. 



In the case of dabs therefore we have strong presumptive 

 evidence of distribution by active migration, and doubtless the dab 

 is not singular in this respect. And unless it can be shown by 

 further hydrographical work that currents at the period of spawning 

 favour a north-east distribution of the larvae we must conclude 

 that schools may migrate without reference to the spawning grounds. 

 The dabs of the Channel and the southern part of the North Sea are 



