The Productivity of Fish Food in Oneida Lake 225 



try to find out what mass of living substance is periodically 

 generated." 



In the marine quantitative studies effort has been mainly 

 directed toward ascertaining the quantity of utilizable produc- 

 tion. Thus the number of hsh in the North Sea and in other 

 fishing districts has been estimated from the size of the catch 

 landed at the fishing ports, the assumption being that a fourth 

 of the adult fishes are caught yearly. This is obviously, as 

 stated by Johnstone (pp. 180-186), too low% as it includes only 

 the marketable individuals and species. It is the total produc- 

 tion of life in a body of water (either in the sea or in the inland 

 waters) that is now sought by the modern economic investi- 

 gator. We wish to know what may be the total mass of life 

 generated in a year. Johnstone ('08, p. 187) provides a simile 

 which is very apt and which is here repeated. " At the begin- 

 ning of the year a certain mass of life is present, and the end 

 of the year much the same quantity is present. But in the 

 meantime all organisms have been reproducing and growing. 

 The mass of life at the beginning of the year is the capital ; 

 the mass generated during the year by the reproduction and 

 growth of the capital is the interest. At the end of the year 

 the capital remains the same; the interest has been eaten up, 

 or otherwise destroyed. What is the rate of interest? " 



Johnstone (p. 187) adds a table in which his idea of the 

 knowledge necessary for finding this rate of interest is indi- 

 cated. These are arranged under five heads, as follows : 



1. Rate of reproduction of each species of organism under different 



conditions (temperature, weather, etc.). 



2. Rate of growth of individuals of each species (and under different 



conditions). 



3. Average duration of Ufe of the individuals of each species. 



4. Duration of reproductive activity in the life history of each species. 



5. Amount of natural destruction due to enemies. 



If the specifications indicated in this table could be met we 

 should have a body of data which would enable us to vinder- 

 stand very thoroughly the needs of our food and game fish, 

 and this data would aid greatly in solving many of the problems 

 of fish cultttre. 



