30 president's address. 



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 fishermen'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 dis- 

 tributed. This method gives us what has been called ' the fishing 

 coefficient,' and this has been estimated for the North Sea to have a 

 probable value of about 0'33 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 trawling (' 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 metliod 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 on more accessible ground with more sedentary animals, 

 and Dr. C. G. Job. Petersen, of the Danish Biological Station, has 

 for some years been pursuing the subject in a series of interesting 

 Reports on the ' Evaluation of the Sea.'^' He uses a bottom-sampler, 

 or grab, which can be lowered down open and then closed on the 

 bottom so as to bring up a sample square foot or square metre (or in 

 deep water one-tenth of a square metre) of the sand or mud and its 

 inhabitants. With this apparatus, modified in size and weight for 

 different depths and bottoms, Petersen and his fellow-workers have 

 made a very thorough examination of the Danish waters, and especially 

 of the Kattegat and the Limfjord, have described a series of ' animal 

 communities ' characteristic of different zones and regions of shallow 

 water, and have arrived at certain numerical results as to the quantity 

 of animals in the Kattegat expressed in tons — such as 5,000 tons of 

 plaice requiring as food 50,000 tons of ' useful animals ' (mollusca and 

 polychaet worms), and 25,000 tons of starfish using up 200,000 tons 

 of useful animals which might otherwise serve as food for fishes, and 

 the dependence of all these animals directly or indirectly upon the 

 great beds of Zostera, which make up 24,000,000 tons in the Kattegat. 

 Such estima;tes are obviously of great biological interest, and even if 

 only rough approximations are a valuable contribution to our under- 



" F. Heincke, Cons. Per. Internat. Explor. de la Mer, ' Investigations on 

 the Plaice,' Copenhagen, 1913. 



=" See Reports of the Danish Biological Station, and especially the Report 

 for 1918 ' The Sea Bottom and its Production of Fish Food.' 



