868 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



It 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 coining 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 difficult}'. 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 Dalil, Rep. on. Norwegian Fish, and Mai'. Investigations, 

 vol. L, no. 1, p. 151. 1900, 



