232 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



The Productivity of the Fisheries. 



By " productivity " we mean the total quantity of plaice 

 which grow up, in a definite region, to a certain size, in a 

 certain period of time. It is necessary to specify the size 

 because it is only when the fish become so large that they 

 become commercially valuable, and so become commodities. 

 That means that the idea of productivity must necessarily 

 include the idea of commercial profit. 



Suppose that plaice only become commercially valuable 

 when they have attained the size of about 20 cms., and the 

 age of about three years. To convert a hundredweight of 

 13 cm. plaice into 2 cwts. of 20 cm. plaice will require a certain 

 quantity of production in the sea and this will be much the 

 same as the production necessary to convert 1 cwt. of 20 cm. 

 plaice into 2 cwts. of 25 cm. plaice. Yet the latter production 

 will have more commercial significance than the former quantity 

 because 25 cm. plaice have more value, as commodities, than 

 20 cm. plaice have. Production, in the scientific sense, means 

 the origin in the sea of plaice substance but, in the commercial 

 sense, it means the origin of plaice of the range of size that 

 sells in the markets. What the industry want is plaice of this 

 range of sizes and if such fish decrease in numbers the 

 " productivity " of a fishery region decreases. 



The Rate of Exploitation. 



The total quantity of plaice that are landed annually 

 depends not only on the productivity of the region in question, 

 but on the degree to which it is fished. If there is an increase 

 in the catching power there will generally be a corresponding 

 increase in the quantity of fish landed. To find whether or 

 not there is any change in a fishing region we require to know 

 whether there has been any change in the catching power 

 employed, and this is always a very difficult question. Plaice 

 are caught by steam vessels, motor boats, smacks and half- 



