492 Dynamics of the Ecosystem 



other basis, and depends among other things upon the size of the 

 standing crop. Thus one aspect of productivity affects another 

 aspect. These relationships may be iHustrated by data for the total 

 yield of haddock caught on the Icelandic fishing grounds in compari- 

 son with the catch per unit effort (as measured by the landings per 

 day's absence from port) and the total hours of fishing by British 

 trawlers (Fig. 13.10). The total yield of haddock declined slowly 



1910 



1930 



1905 1910 1915 1920 1925 1930 1935 



Fig. 13.10. (Upper) Total catch of haddock from Icelandic Fishing Grounds by 



all countries. (Lower) Catch per day's absence from port (solid Hne) and total 



hours of fishing (broken line) by British First Class Trawlers. (Modified from 



Russell, 1942, The Overfishing Problem, Cambridge Univ. Press.) 



from 1905 to 1915, and the catch per unit effort declined regularly 

 after 1911. During the war period 1914-1918 the haddock population 

 had an opportunity to recover, and, with the resumption of fishing 

 by the British trawlers in 1919, the catch per unit effort reached an 

 all-time high. The total yield reached a peak in 1920 and a some- 

 what higher peak during 1927-1929, owing in part to the increase in 

 fishing time and in part to the adoption of a new type of trawl. From 

 1924 to 1937 the hours of fishing were doubled, but the catch per unit 

 effort exhibited a downward trend to a value in 1937 less than one 

 quarter of that obtained in 1919. The explanation is found in the fact 

 that the standing crop of haddock in Icelandic waters became progres- 

 sively depleted, as a result of the intense fishing, and consequently a 



