294 
had ml 
Tt is worthy of note, however, that the decline in the 
annual quantity of Plaice taken from the North Sea may 
be regarded as the reduction of an ‘‘ accumulated stock” * 
and not entirely as the diminution of the number of fish 
annually coming to marketable size. At one time the 
North Sea was practically a virgin ground, and the large 
and numerous fish taken from it were a stock which had 
accumulated for many years and which were rapidly 
fished out. 
Obviously the estimation of the fluctuations of the 
fish population of a great fishing ground is a task of the 
utmost difficulty. We may mention the quantitative 
method of plankton investigation as one of the most pro- 
mising means of dealing with this problem. This method 
has been greatly developed during recent years by Hensen 
and the German Fisheries investigators, and it is now 
possible to make a rough estimation of the number of 
pelagic fish eggs of any species such as the Plaice, in an 
extended fishing ground. Only a rough approximation of 
the eggs present can, of course, be made since many diffi- 
culties and sources of error are obviously to be considered. 
But the Hensen method is as yet the only serious attempt 
at a “ census of the sea’’ which has been attempted, and 
it is possible that its refinement and thorough application 
may go a long way towards solving the question of fisheries 
impoverishment. We have stated that Hensen estimates 
the number of Plaice eggs floating in the North Sea 
during 1895 as about 31 billions. Now it is known that a 
mature female Plaice produces annually about 300,000 
eggs and it was then easy to calculate that about 103 
millions of mature female Plaice were present in the 
North Sea in that year. From the known ratio of the 
*See Hjort and Dahl, Rep. on Norwegian Fish. and Mar. Investigations, 
yol,1., no, 1, p. Ibi; 1900; 
_ 
