322 FOUNDERS OF OCEANOGRAPHY 



in a comparatively small series of hauls of the tow-net, but 

 the data were probably quite insufficient and the conclusions 

 may be erroneous. It is an interesting speculation to which 

 we cannot attach any economic importance. His own 

 colleague, Heincke, says of it : " This method appears 

 theoretically feasible, but presents in practice so many 

 serious difficulties that no positive results of real value have 

 as yet been obtained." 



All biologists must agree that to determine even approxi- 

 mately the number of individuals of any particular species 

 living in a known area is a contribution to knowledge which 

 may be of great economic value in the case of the edible 

 fishes, but it may be doubted whether Hensen's methods, 

 even with greatly increased data, will ever give us the 

 required information. Petersen's method, of setting free 

 marked plaice and then assuming that the proportion of 

 these recaught is to the total number marked as the fisher- 

 men's catch in the same district is to the total population, 

 will only hold good in circumscribed areas where there 

 is practically no migration and where the fish are fairly 

 evenly distributed. This method gives us what has been 

 called " the fishing coefficient," applicable to the North Sea 

 for those sizes of fish which are caught by the trawl. 

 Heincke,^ from an actual examination of samples of the 

 stock on the ground obtained by experimental trawHng 

 (" the catch coefficient "), supplemented by the market 

 returns of the various countries, estimates the adult plaice 

 at about 1,500 millions, of which about 500 millions are 

 caught or destroyed by the fishermen annually. 



It is difficult to imagine any further method which will 

 enable us to estimate any such case as, say, the number of 

 plaice in the North Sea, where the individuals are so far 

 beyond our direct observation and are liable to change their 

 positions at any moment. But a beginning can be made 



1 F. Heincke, Cons. Per. Internat. Explor. de la Mer, " Investiga- 

 tions on the Plaice," Copenhagen, 1913. 



