70 



takes place from about the middle of January to May, and recruit- 

 ing, as evidenced by the presence of young plaice near the shore, 

 plaice of about 1 cm. in length, from March to August. The 

 spread of the larva} in the currents results in an inshore and an 

 offshore distribution. The former cpiickly become demersal, 

 some even before hatching, and undergo their changes more rapidly 

 than the latter, and all stages between these extremes may be 

 presumed. During the drift the larvae are orientated head to 

 current, but this factor may be neglected for the direction is, 

 reversed four times a day. The larvae dining the denatation 

 are attracted to shore water, and the young after their transforma- 

 tion are still liable to continue the denatation by becoming pelagic. 

 The result is a gathering in a known area of the young of the year. 

 The gathering is particularly dense in one part of the area near 

 the proximal end. From this region the population of recruits 

 diminishes rapidly next the spawning ground, and tails off more 

 gradually distally. The normal conditions are liable to be changed 

 by winds and the modifications they produce in the currents at 

 the surface, and this factor must be regarded as an important 

 one in determining the result of the annual spawning. But even 

 under normal conditions it is evident that the larvae spread over 

 deeper water are carried into regions beyond those of the school, 

 and that such are absorbed into a school or schools on the denatant 

 side of that to which the products of the spawning ground are as 

 a whole relegated. 



It is important to realise the dynamic forces at work if we 

 are to understand the distribution. Granting that in all cases, 

 such as that of the plaice, the floating egg arid the helpless larva 

 are carried from the spawning ground to the recruiting ground, 

 that the spawning ground and the recruiting ground are related 

 in a down current direction, as is proved by the constancy with 

 which the products are distributed, it follows that the distribution 

 of plaice and of other species of similar habit is produced by 

 the aperiodic currents. A succession of spawning grounds occur 

 along the path of the current, and therefore the plaice of the 

 northern current of the North Sea arc distinct from the plaice 

 of the southern North Sea. Not exclusively, for where the 

 currents merge opportunities for mingling will be presented. 

 Both the northern and the southern currents are derived from 



