t 



of the Fishery Board for Scotland. 



375 



or more before in the Firth of Forth, and which must have travelled at 

 least 150 miles to the north. One, indeed, was retaken off Dunnet 

 Head, Caithness, to the west of the Orkney Isles, more than 200 miles 

 from the place where it was liberated. It must riot be supposed that 

 because these plaice moved so far north from the Firth of Forth that 

 the spawning areas of the plaice, which stand in relation to the Firth of 

 Forth, are situated there. I have shown by many experimental observa- 

 tions that the irritation caused by the presence of a small metallic disc 



Fig. 5.- 



-Showing the northward movement of marked plaice in the 

 Firth of Forth and St Andrews Bay. 



on the skin interferes with the rate of growth and with the development 

 of the reproductive organs. Had the reproductive organs of the plaice 

 mentioned developed normally they would have become ripe, and the fish 

 would have moved outwards to the offshore waters probably before 

 reaching the Moray Firth. But as development did not occur, and the 

 impulse to the offshore migration was therefore absent, they continued 

 their northward movement. The irritation of the skin referred to cannot 

 be a factor in determining the movement, because all the marked plaice 

 were got to the north, and none to the south ; and while one might 

 argue (but erroneously) that the irritation caused the migratory move- 

 ment, it is impossible that it should cause it to be invariably in a definite 

 direction. More than 10 per cent, of the marked plaice — over 1000-^ 

 have been recovered. 



