Uhtil now, the decline of the fish stock has been blamed totally 

 on the increased fishing intensity. This is true, provided that 

 other environmental factors do not become unfavorable. The question 

 remains - Did environmental factors in the North Sea change considerably 

 during the past 40 years* Sufficient data of the two most important 

 factors, temperature and salinity, show but little change o 



Little is known about changes in the other influential fkctors. 

 An important one is the amount of phosphate present, which is often the 

 limiting factor for the amount of food a sea may contain and conse- 

 quently affects the productivity* The phosphate content was determined 

 regularly at the western entrance of the Channel during the past 20 

 years. It appeared to be fairly high during the period 1924-29, 

 declined considerably until 1935, and increased again between 1935 

 and 1939, A decline in the number of fish larvae and a general reduction 

 of the fauna in the observation area was parallel with the decline in 

 phosphate content (Harvey, 1946). It is not known if a similar decline 

 appeared in the North Sea, but if that were so, it undoubtedly must have 

 led to a decreased productivity and would be partially responsible for the 

 decline in the catches. However, it is very doubtful that the reduction 

 in phosphate content was the only reason for the decline of the fish stock, 

 since the overfishing became apparent before the appearance of the lower 

 phosphate content. It is more likely that the disappearance of great 

 numbers of fish (containing phosphate) was responsible for the loss of 

 phosphate in the sea. Nevertheless, the phosphate increase after 1935 

 shows that this content is also controlled by another factor independent 

 of the fishery. 



If a decline in the productivity was a responsible factor, 

 together with the increased fishing intensity, it still would not 

 change the conclusion that during the period 1919-39 the fishing 

 intensity became too great for the productive power of the North Sea. 



ee. Non-marketable Fish 



Only the marketable fish appear in the data given heretofore. 

 The undersized fish, partly landed as trash for meal and partly 

 thrown overboard as dead — but which in any case are removed from 

 the fish stock — do not appear. What we finally need to know is 

 the total amount of fish that died as a consequence of the fishery. 

 Therefore, we have to estimate the amount that is not included in 

 the statistics. 



The data necessary for such an estimation are extremely scarce. 

 Reliable data can be obtained only by observations made aboard ship, 

 and these observations have to be made on a large scale since the 

 percentage of undersized fish may vary from place to place and time 

 to time. The English probably have the best data since they have 

 fish measurers serving on board the fishing vessels. Not enough of 



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