200 THE PRODUCTIVITY OF THE SEA [PART II 



ascertained that in each square metre of sea in this area during 

 the months of January to April, there were produced 370 eggs of 

 plaice and cod. 



The fishery statistics of this neighbourhood also shewed that a 

 certain number of mature cod and plaice were annually caught. 

 Then taking the average annual catch of these fishes (deduced from 

 a nine-yearly average) Hensen shewed that about 23,400 million 

 cod eggs, and 73,895 million plaice eggs would have been spawned 

 by these captured fishes if they had been allowed to remain in the 

 sea. When these totals were divided by the total number of 

 square metres in the area fished over, it was found that the fishes 

 caught would have produced 110 eggs per square metre if they 

 had been left in the sea. That is the total productivity of the 

 fish left in the sea and those caught on the area was 480 eggs per 

 square metre. 



But the yearly average catch was 110 eggs per square metre, 

 that is about one fourth of the total number theoretically producible. 

 So Hensen concluded (and there does not appear to be any defect 

 in the argument apart from the errors of observation involved in 

 the method) that the fishermen of the West Baltic captured 

 annually about a quarter of the total number of the adult cod 

 and plaice present in the area fished over during the annual 

 spawning season. 



Thus we cannot come to any other conclusion than that 

 fishing operations, as at present carried out, do cause a very 

 appreciable diminution of the stock of fish on the sea bottom. 

 Probably some species of fishes, like the herring, mackerel, and 

 haddock, are not appreciably lessened in number by the fishermen, 

 but all that we know shews that this is not the case with other 

 species like the plaice and sole. In the exploitation of a fishing 

 area it is the larger and older fishes of a species which first suffer 

 diminution, and it is the common experience of fishermen that the 

 average size of the species which suffer depletion becomes lowered 

 as the result of intense fishing. We have to determine then what 

 degree of impoverishment such an area as the North Sea will 

 sufifer ; that is, what is the quantity of fish (like plaice, soles, &c.) 

 that can be annually drawn from it to the greatest advantage. 

 If we take more than this quantity then the stock will gradually 



