CH. XIl] THE CIRCULATION OF NITROGEN 289 



and taken ashore. A certain fraction of the nitrogen entering the 

 sea from the land is therefore returned to it in the shape of 

 economic marine products. How great is this fraction ? 



In any attempt to answer this question we become bogged in 

 a statistical quagmire. I am conscious of our inability to give 

 very approximate figures. But one must make what use is possible 

 of existing data. In 1899 Ehrenbaum^ made the first reliable 

 estimate of the productivity of the North Sea fishing grounds, and 

 considering all the data then accessible came to the conclusion 

 that about 875 millions of kilogrammes of fish were landed 

 annually from this area. 



Now since the beginning of the International Fishery Investi- 

 gations we are in possession of rather more accurate statistical 

 information and I have already given (in Chap. IX.) an estimate 

 of the approximate value of the North European sea-fisheries. In 

 the year 1904 the total weight of fish landed from the North Sea. 

 was not less than 967 millions of kilogrammes, and that taken 

 from the fishing grounds of Northern Europe was not less than 

 1571 millions of kilogrammes. It has been estimated that one 

 kilo, of fish contains, on the average, about 119 grammes of 

 proteid, and this latter quantity (basing the calculation on Play- 

 fair's empirical formula for proteid) contains about 19 grammes of 

 nitrogen. Therefore the North Sea, in 1904, supplied to the land 

 about 18 millions of kilos, of nitrogen; and the wider fishing area 

 about 29 millions of kilos. Putting the same values in English 

 units, the North Sea supplied about 18,000 tons of nitrogen; and 

 the whole North European fishing area about 28,000 tons. 



I would repeat the caution so frequently expressed in these 

 pages, that the fishery statistics are not at all accurately collected ; 

 and that such figures as I give here can only be regarded as 

 approximations to the truth. But it is generally admitted that 

 the fishery statistics really underestimate the mass of economic 

 animals taken from the sea. Probably then the above estimates 

 are minimal ones. Probably also, a certain mass (how great we 

 we do not know) of organic matter is taken from the sea, and is 

 not even indicated in the fishery statistics. Considerable quanti- 

 ties of sea- weed are taken from the shore and put upon the land 



^ Mittheilungen deutschen seejischerei Verein, Bd. xv. 1899. 

 J. F. 19 



