82 THE ECOLOGY OF ANIMALS 



often a narrow margin and are calculated over a 

 long period — is always complicated by this fear of 

 destruction by pests, and by fire. Fruit orchards 

 come on the border-line between forestry and agri- 

 culture, and it is here that some of the most spec- 

 tacular pests have been studied and in certain cases 

 controlled. Summaries of the regional problems 

 connected with forestry and with fruit growing will 

 be found in Wardle (1929) and in numerous textbooks 

 of forestry. 



We come now to a more positive side of economic 

 ecology. The fisheries of the world were the first 

 problem to be tackled seriously from the ecological 

 standpoint, and it has taken some fifty to sixty 

 years of co-operative work between West European 

 countries to achieve a preUminary knowledge of the 

 problem. The chief issue is the determination of 

 policies in regard to the amounts that may be fished. 

 It is estimated that at least a third of the fish popu- 

 lation of the North Sea is taken every year. An 

 idea of the intensity of fishing may be gained from 

 the fact that most of the larger Pleistocene bone 

 fossils on the surface of the Dogger Bank have been 

 dredged up some time ago. A valuable summary of 

 fishery ecological problems is given by Russell (1932). 

 Similar problems arise in freshwater fisheries, in 

 regard to overfishing, and also to the acclimatization 

 of fishes in new countries. The salmon has special 

 problems of its own, particularly in regard to pollu- 

 tion in the estuaries of large rivers, and to the 

 periodicity in its numbers. Recently (after three 

 hundred years of uncontrolled slaughter and destruc- 

 tion) the whale fisheries have been studied by modern 

 ecological methods, during the Discovery expeditions. 

 One of the indirect advantages of the world depres- 

 sion was the reprieve of a good many whales in the 

 southern seas, owing to the fall in oil prices. The 

 conservation of mammals and birds raises problems 

 in connexion with national parks and sanctuaries and 



