As before, the horizontal lines represent different ages, in this case 
from 5 to 21 years, and their lengths correspond to the relative 
abundance of the fish of those ages. It will be seen that each line 
is one-seventh shorter than the one immediately below. 
Let us now suppose that fishing begins and that after a seventh 
of each age have been removed by natural means, one-quarter of 
the remaining marketable fish are captured by the fishermen. The 
removal of such a large proportion of the stock is not at all unlikely, 
for by two methods of calculation it was found that that proportion 
of the total stock in the North Sea was being removed each year by 
the fishermen. The effect on the stock of this intensity of fishing 
for one year is shown in the second part of figure 15. The numbers 
Age in years. 
Original condition. After] year's fishing. 
Fic. 15.—The effect of fishing on the stock. 
After Syears’ fishin. After 15 years’ fishing. 
of each age with the exception of those that have just attained the 
age of 5 years, have been decreased one-fourth. The conditions 
after fishing for 5 and 15 years respectively are also shown in the 
figure. The progressive decrease in numbers affects chiefly the 
older fish, but it entirely ceases after 17 years, by which time a new 
stationary condition has been reached, one characterized by very 
few old fish compared with the younger ones. Also we find that the 
average size of the fish captured becomes smaller year by year until 
the new stationary condition is reached. This effect is the more 
marked (1) the more intense the fishing, (2) the greater difference 
in size between the years, and (3) the greater the number of years 
represented among the marketable fish. Those who have fished 
the lobster during the last half-century can testify how definite this 
effect is. It is unavoidable and not due to any failure in the supply 
of young fish. The relative numbers of the various ages would not 
be changed no matter how large a number of young fish were pro- 
duced, although the total quantity of fish obtained would be 
31 
