PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 263 



individuals takes place between different areas, or whether the 

 individuals of a region are the offspring of parents which lived in the 

 same region. At present it is difficult to give answers to these 

 questions. The English Channel is so extensive that we can 

 confidently conclude that the plaice found there are the offspring of 

 parents that also lived there. But we cannot be certain that the eggs 

 of plaice which spawn between Lowestoft and the Dutch coast are not 

 carried by the currents to some distant region, most probably to the 

 Heligoland Bight, where they would develop into plaice of larger size 

 at maturity. Similarly we cannot be certain that young plaice on the 

 German coast near Heligoland are the offspring of parents which 

 themselves grew up on that coast. To obtain evidence on these 

 matters we must trace with more certainty the movements of the adult 

 fish, and the course which the eggs are compelled to take by the 

 currents. Something has been done in this way by the hydrographers, 

 and by Dr. Fulton in his experiments with floating bottles, and the 

 results indicate that the plaice of the Heligoland Bight are largely 

 derived from spawn shed in the central part of the North Sea. In the 

 Western Baltic the plaice have marked characteristics, distinguishing 

 them even from those of the Cattegat, especially in the small size at 

 which they are mature. Yet according to Petersen, young plaice, in the 

 first summer after their development from the egg, are not found in the 

 Baltic east of Zealand, Moen, and Falster at all, but enter it from the 

 Cattegat when a year old. At the same time, Petersen finds indications 

 that the mature fish in the Baltic emigrate through the Great Belt and 

 spawn in the Cattegat, so that the dwarf plaice of the Baltic, with all 

 their peculiarities, might be the offspring of parents which lived in the 

 Baltic. 



