2-48 PROF. W. A. HERDMAN ON THE ABUNDANCE 



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 re-caught 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 distributed. 



It is difficult to imagine any 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 on more accessible 

 o-round with more sedentary animals, and that is what I have been attempting 

 to do. Dr. C. G. Joh. 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"f. 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 .5000 tons of 

 plaice requiring as food 50,000 tons of "useful animals" (Mollusca and 

 Polychret 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 estimates 

 are obviously of great biological interest, and, even if only rough approxi- 

 mations, are a valuable contribution to our understanding of the metabolism 

 of the sea and of the possibility of increasing the yield of local fisheries. 



But on studying these Danish results in the light of what we know of our 

 own marine fauna, although none of our seas have been examined in the same 

 detail by the bottom-sampler method, it seems probable that the animal 

 communities as defined by Petersen are not applicable on our coasts, and 

 that the estimates of relative and absolute abundance ma}' be very different 



* Heincke, however, has attempted to estimate the adult plaice at 1500 millions, of 

 which 500 millions are caught annually. 



f See ' .Reports of the Danish Biological Station,' and especially the Report for 1918, " The 

 Sea Bottom and its production of Fish Food." Professors L. Jouhin and GuiSrin-Ganivet 

 have published a series of papers, entitled " Gisements de Mollusques comestibles des Cotes 

 de France" (Bull. Mus. Ocean. Monaco, from 1906 onwards), surveying the shell-fish beds 

 and fisheries, but giving no estimates of numbers present. 



