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Why does a marked plaice that has migrated grow faster 

 than those that can be fished on the ground of liberation ? 

 One is apt to say that growth depends mainly on the supply 

 of food, and that is true to some extent. But females grow 

 faster than males, and then, after a certain age, the males 

 seem to die. Therefore there is an intrinsic growth-factor 

 which is different in its " strength " in different individuals. 

 We must assume (in order to attempt to explain the difference 

 in rates of growth of marked and unmarked fishes) that this 

 growth-factor is associated with, or may be the same thing, 

 as the intrinsic factor that urges one plaice to migrate sooner 

 in its lifetime, and to farther distances, than another one. 

 Looking, then, at the general results of the marking experiments 

 we can distinguish between an " ordinary " plaice population, 

 growing sluggishly because of the low intrinsic growth-factor, 

 and another fraction of the general population, growing rapidly 

 and migrating sooner for the opposite reason. Therefore a 

 (probably large) fraction of the plaice population off the 

 Lancashire coast is a residual fojmlation^ the more vigorous 

 individuals migrating to outside areas. And so, it would appear, 

 " protection " of this residual population may not be of any 

 practical administrative value. 



