€H. VIIl] A CENSUS OF THE SEA 175 



been living in the North Sea in this year. But only 600 millions 

 have been captured. Is this what one would expect ? I think it is, 

 for our 9000 millions might easily include one-third or one-half of 

 that number of small unmarketable fishes which would either not 

 be caught, or, if caught, would be returned to the sea. If it in- 

 cludes one-half, then the fishermen captured only about 13 per cent, 

 of the fishes actually present. 



There are no means of ascertaining exactly what proportion of 

 the fish which frequent a fishing ground during a whole year the 

 fishermen do catch. It is pretty certain that when a trawl-net is 

 dragged through a shoal of fish on the sea bottom it only catches 

 ■Si few of those which lie in its way. Heincke and Henking (in the 

 paper cited) marked about 600 living plaice and then liberated 

 these on a restricted area of the Xorth Sea, and trawled with 

 the object of recapturing their marked fishes. But the results 

 w^ere inconclusive. Marking experiments of this nature have been 

 carried out on a rather extensive scale, both in England and on the 

 continent of Europe, and the results seem to indicate that from 10 

 to 25 per cent, of the marketable plaice present on a sea bottom 

 throughout an entire year are caught by the fishermen. But so 

 many uncontrollable factors affect experiments of this kind that 

 their results are problematical ones. 



Density of population on the sea bottom. If we assume 

 again that our estimate of 9000 millions of cod, haddock, plaice, 

 flounders, dabs and long rough dabs represents roughly the popu- 

 lation (with regard to those species) of the bottom of the North 

 Sea, then we may calculate the density of distribution. The area 

 of the whole sea is 547,623 millions of square metres, so that if we 

 divide 547,623 by 9000 we obtain the average area inhabited by 

 each fish. This is about 60 square metres, or each fish inhabits a 

 square of sea bottom which measures about 8 yards along each side. 

 Here and there the density will be much greater, and at other 

 places it will be much less. Those species form the bulk of the 

 bottom-living fish population, and even when we remember that 

 other edible and inedible fishes and invertebrates are present also 

 on the sea bottom, the estimate of the density of life on the sea 

 bottom does not seem to me to be an improbable one. But we 



