xi SIZES OF FISH CAUGHT 287 



made available in certain areas by particular conditions that lead to 

 up-currents and mixing of the layers. 



With these facts in mind we can now make further inquiry into the 

 effects on the population of the sea of removal by man of large quanti- 

 ties of organic matter in the shape of fish. There are enough data to 

 give us hints of the changes that follow, and the hope that with further 

 study we may eventually understand the Great Sea Beast sufficiently 

 well to be able, if we can control ourselves, to regulate its growth to 

 our advantage. 



In a few cases it has been possible to follow the course of a fishery 

 from its early beginnings, with few vessels and simple apparatus, 

 through stages in which the fishing was progressively more intense. 

 The statistics are seldom adequate to provide us with wholly satis- 

 factory conclusions, but they suggest that (1) as fishing becomes more 

 intense, the yield per unit of fishing power declines ; that is to say, the 

 industry becomes relatively less profitable; (2) in spite of this, the 

 total yield may remain constant, or somewhat increase, even though 

 (3) the average size of the fish caught decreases. 



For example, fishing for plaice in the Barents Sea, north of Nor- 

 way, was begun in 1905, and the almost 'virgin' population in 1907 

 showed great preponderance of large mature fish; whereas in the 

 North Sea, which had already been fished for many years, the average 

 size was much smaller and few of the plaice were mature. The fish 

 taken in the Barents Sea were old and had grown slowly. Transplanta- 

 tion of fish of this stock to the North Sea, however, proved that they 

 were able to grow as rapidly as the local fish. Some measure of the 

 intensity of fishing is given by the number of days' absence of vessels 

 fishing in a given area. No doubt the measure is inexact, particularly 

 if the methods used are changed, but it provides the best estimate 

 available. The landing of plaice taken from the Barents Sea per day's 

 absence was 347 cwt in 1906, 50-4 cwt in 1909, and then fell to 46-3, 

 33-7, and 20-5 cwt in the following years. The total yield showed some 

 decline. The sequence of events is typical of what happens when a 

 stock of mature fish is first exploited. 



Graham described a good example of a similar situation in Lake 

 Victoria-Nyanza, where he found two stocks of the carp-like fish 

 Tilapia, one fished heavily and the other with primitive devices only. 

 The less heavily fished stock contained five times as many fish as the 

 other and they were much older. Yet by the intensive fishing the 

 poorer stock was being made to yield a ten times greater weight than 

 the richer stock. This desirable state of affairs for man in general is 



