DYNAMICS OF CERTAIN NORTH SEA FISH POPULATIONS 251 



are concentrated towards the centre of an oval-shaped area extending about 

 100 miles north-eastwards from the Straits of Dover and about thirty miles 

 across at its widest point. This zone coincides with certain clearly defined 

 environmental conditions, notably with the main area of relatively clear and 

 saline Atlantic water that flows into the North Sea through the Straits of 

 Dover, and eventually enables the young metamorphosed fish to reach the 

 inshore nursery grounds along the Dutch and German coasts. It is not 

 unreasonable to suppose that if conditions of high adult population density 

 resulted in a spread of the spawning area, the chances of survival of those 

 larvae outside the normal area might be quite sharply diminished. In the 



22-28 JAN 1947 



1 



Fig. 5. — Distribution of plaice eggs at peak spawning in the Southern North Sea, in two 

 contrasting years of total egg-production. 



extreme case, if the area of spawning increased in proportion to the total 

 number of eggs deposited, and the survival outside the normal spawning 

 area was nil, complete compensation would result with any increase in adult 

 population above a certain limiting size. 



The plaice egg surveys conducted in the Southern Bight by Buchanan- 

 WoUaston and later by Simpson (see Simpson, 1959, for a general account) 

 enable this possibility to be tested. Fig. 5 shows the contours of egg density 

 at peak spawning in two contrasting years, 1937 and 1947, when the total 

 egg deposition was very different. It is seen at once that the total area within 

 which eggs were found was very similar in both years, and that the greater 

 egg deposition in 1947 resulted only in an increased density of eggs, with 

 the centre of concentration at much the same place. The data for all available 

 years are combined in Figs. 6a-c to show the same conclusion in graphical 

 form. In Fig. 6a, only a slight tendency is seen for the area in which eggs 



