79 
migrate into the deeper extra-territorial waters, and even there they 
appear to have the tendency to migrate into deeper and deeper 
water as they become older and larger. This in itself would serve 
to correct to some extent the general southerly and westerly move- 
ments of the ova and larvae. Some of the mature plaice certainly 
do find their way back to the same or other inshore waters, im- 
pelled more than probably by recollections of sand eels, but such a 
return cannot be said to be more than rare. 
But we look to the international researches to show if there is a 
distinct or even a general relationship between size and depth, and 
if this may be looked upon as the only impulse which tends to 
move the plaice in a definite direction in the North Sea. 
Population. — Following a method which has been adopted 
before in the case of such experiments, a calculation may be 
offered to give an indication of the number of the resident popula- 
tion of plaice of from 2 to 4 years old. We marked 471, and we 
recovered from the catches of fishermen in the district 54. This 
is, allowing slightly for loss, one-eighth part, and may be taken 
therefore to represent approximately the catching power of the 
inshore fishermen. As near as we can gather the fishermen caught 
in the inshore waters for the year ending June 30th, 1904, 440 
cwts. of plaice. The population of plaice of the inshore waters 
from about 7 to 14-in. long may be said to be therefore 3,520 cwts. 
If the average weight of the fish be, say J-lb., the population in 
numbers is about 800,000. Considering the large proportion 
obtained from some of the districts this number will probably be 
found to err by being too large rather than too small. On the other 
hand, however, many more plaice are caught than are sent to market 
or used for crab bait ; and as a matter of fact our labelled fish were 
sometimes caught by others than professed fishermen. 
Pathology. — Several plaice were marked which were abnormal 
as to colour, and one of these. No. 818, was re-captured and for- 
warded to me. When caught it had a rectangular white patch 
with a central spot of pigment on the upper side near the ventral 
fin, and the label was attached near the edge of the spot to mark its 
position. When it was sent to me, after a free period of 288 days, 
it still presented the patch and the central spot, so far as I could 
see, unchanged either in area or appearance. It was a male, and 
its abnormal colour did not have any effect upon its rate of growth. 
