xi REGULATION OF FISHERIES 289 



lines used, in 1930, 6,400,000 sets, but instead of the 50 million 

 pounds of fish caught in the first year there were only 23 million 

 pounds in 1930! Then in 1932 a limitation of 23 million pounds was 

 imposed on the total catch allowed. Thereafter the stocks rapidly 

 increased and it was found possible to collect the standard catch in 

 five months instead of nine, with greatly increased profit. 



Other attempts to regulate fisheries have been made; in particular 

 by the mesh regulations, designed to avoid taking small fishes. The 

 aim of regulation is nowadays not so much to save the stock from 

 undue reduction or extinction but rather to crop it in such a way as 

 shall make fishing profitable. Whatever we do it is unlikely that we 

 shall destroy all the stocks of any species, but there is reason to think 

 that the rate at which the stocks grow varies with the intensity of 

 fishing. If this is so, it may be possible to find an optimum that suits 

 both the fish and the fisherman. 



In order to regulate a fishery effectively it is necessary to express its 

 characteristic parameters in a comprehensive equation. This has been 

 done by fisheries research workers who have provided mathematical 

 models such as are so often used in operational research (Beverton 

 and Holt in Graham, 1956). These equations show the effects that are 

 likely to follow variation in such a factor as the size of the mesh of the 

 cod end of the trawl net, which is one of the means used to regulate 

 a fishery. Fishing is a form of hunting, not of agriculture, and if we 

 cannot improve the yield by cultivation we may be able to do so by 

 working out the best way to fish. 



Four primary factors are considered in the model, recruitment (R), 

 growth (W), mortality due to natural causes (M), and mortality due 

 to fishing (F). The relation of these factors to the yield by weight of 

 the fishery ( Yw) is considered and a theoretical equation for Yw is 

 derived. If the theory is correct the equation should be able to de- 

 scribe the yield of various fisheries in the past. It proves to do this 

 well and forecasts that improvements in yield could be obtained by 

 changing methods of fishing in the future. The data for testing the 

 theory come from statistics of the fishes caught, which give catches 

 per unit effort in each year class and their lengths. These are available 

 for several fisheries and we may consider the plaice in the North Sea 

 for the years 1929-38. Fish are said to be recruited when they first 

 enter the fished area of deeper water at 2-3 years old, being much less 

 than 1 per cent of the original batch of eggs. The value R for a given 

 mesh size is obtained from the number of fishes in the youngest 

 year class in the catch, with corrections to allow for the fact that 



