On the average, therefore. 



25 X 2 + 42 X 6 + 55 x 25 - + tw 

 100 ^^^ 



of the total oatches of plaice will have been destroyed undersized. 

 For the period 1919-50, this percentage perhaps will have to be 

 somewhat lower, for instance 8 percent. For the years 1956-38, it 

 is somewhat higher, for instance 14 percent. For haddock, a 

 percentage of 20 percent does not seem to be too high for the period 

 1930-40, and 10 to 15 percent for the periods 1918-50. Approximately 

 5 percent of the catches of other species will have been undersized. 

 Probably this percentage increased during the last years before 

 World War II (up to 8 percent after 1955). 



With the aid of these data and the known landings of marketable 

 fish (Bulletin Statistique, 1903-58) it is possible to estimate the 

 total catches from the North Sea. This has been done in table 8 for 

 the average annual catch during the period 1905-13 and for the years 

 1919-38, 



2, Conclusion of the Review of the Progress of Fishing in the North Sea 



It is now possible to produce graphically the relation between 

 the fishing intensity and the rate of increase (or productivity) of 

 the fish stock, which has been done by Graham (1935). 



First we have to construct the S-shaped curve, which shows what 

 strength the stock would have from year to year in the hypothetical 

 case that an empty North Sea would re-populate itself and inversely 

 shows how the fish stock would dp'-''-'^'- t-- «.. steadilv increasing 

 fishing intensity. 



Here, we will represent the strength of the stock by relative 

 numbers and the average strength during the periods 1903-13 will be 

 set at 100. The fishing standard is the percentage of the fish 

 stock which is caught in the nets each year. This can be estimated 

 from the percentage of tagged fish that is recaught annually. During 

 the above period it was about 40 percent ( i ..utija, - '35)» We assume 

 that the annual catch was equal to the natural annual increase during 

 this period which, therefore, is also 40 percent. If the fishing 

 would stop, the stock would increase. With the passage of time, the 

 rate of increase would become smaller. We assume here that the rate 

 of increase changes equally with the decrease of the difference be- 

 tween the existing strength and the maximum strength of the stock. 

 The percentages of increase of the growth of the stock at each period 

 of time can then be calculated, if we know the maximxjm strength (in 

 respect to the accepted value of 100 for the period 1905-13). To 



2f 



