50 . 



fore some evidence to show that mature plaice may return south- 

 wards, but not enough to show that they actually return to their 

 respective schools, and such plainly it would be very difficult to get. 



The other question is, do the migrations of the mature plaice 

 so far to the north bear any reference to migrations at a previous 

 stage to the south ? We have before us on the one hand the fact 

 of the succession of schools, and the reasonable assumption that 

 these have their origin in the j)roducts of a spawning ground m each 

 case, and on the other a migration of mature which is apparently 

 neither related to the school nor to any particular spawning ground. 

 As has been seen, mature plaice may still leave the North Sea for 

 the Atlantic by way of the Pentland Firth. The Atlantic water 

 may possibly still carry larvae from a spawning ground off the 

 north-west of Scotland into the North Sea, and from the successive 

 spawning grounds on the east coast to positions further south than 

 the limits of the school. But the most Hberal allowance of distance 

 for the passive migration of the larvae would not account altogether 

 for the migrations of the mature. 



The passive migration of larvae may be stated in general terms. 

 The southerly tidal current off the north-east coast may be said to 

 be from IJ to 2 miles stronger than the northerly one. It will be 

 intensified by northerly winds and reduced or neutraUsed by 

 southerly wmds. Leavmg this latter factor and taking the lesser 

 figure of the former factor, it may be said that the eggs and larvae 

 will each day, after a double tidal pulsation, be removed about 

 3 miles to the south. In 28 days therefore the passive migration 

 would amount to 84 miles, more or less. The effect of this would 

 be to determine the larvae from a particular spawning ground along 

 a strip of coast, related thereto, a general average distance to the 

 south. 



In other words, the mature plaice may migrate further than 

 the migrations of the larvae appear to demand. It ^dll very Hkely 

 be found to be the fact therefore that in addition to the larval 

 migration there is a migration to the south of the older plaice which 

 form the schools. Of this there is akeady some evidence from the 

 " Goldseeker " experiments. 



It is more than probable, as has been suggested above, that 

 such an active migration took place in addition to the planktonic 

 one in the early history of the North Sea, and may still be the means 

 of determining the plaice to the south. There is therefore some 



