Fishin(r Mortality 183 



Factors involved in this situation are: 



A = weight of new recruits reaching catchable size, 



G = total weight of all flesh added to catchable fish during the year, 



C = weight of fish captured and removed, and 



M = weight of fish that die of natural causes. 



In any case, 



So = Si + (A + G) - {C + M) 



Thus, if the catch of fish (C) is low and many new recruits (A) are 

 added, So may so nearly approach the maximum poundage of fish sup- 

 ported by the water, that the addition of flesh (G) may be very small, 

 particularly if the fish population at the beginning of the season (Sj) 

 was already comparatively large. In contrast, if Si were very small, growth 

 ( addition of flesh, G ) might be large in spite of a low catch ( C ) and high 

 recruitment (A). 



Natural mortality (M), the other decimating factor operating with 

 catch, can be a constant or can vary considerably with the various age 

 components of a population or from season to season. For this reason 

 natural mortality is difficult to evaluate except by direct methods. 



In this chapter we are primarily interested in forces which cause losses 

 of fish, i.e., the catch, in total amount and rate, and the natural death rate 

 to the extent that it may be determined. 



FISHING MORTALITY 



The average angler does not think of himself as a mortality factor for 

 fish. What he takes he assumes is justifiable, particularly if he is operating 

 within the law. Such an attitude may be reasonable because in many 

 situations he is simply substituting himself for other mortality factors that 

 might remove the same or greater quantities of fish. 



As discussed in Chapter 5, fish have been subjected to high mortality 

 rates for almost as long as they have existed on the earth's surface, and 

 it must be assumed that high mortality rates are normal and beneficial. 



Angling Compared to Natural Predation 



Most methods of angling are not only very inefficient for taking warm- 

 water fishes, but are also highly selective for certain species and sizes. 

 Unless many natural predators are present, this inefficiency and selectivity 

 create a problem in the management of small artificial lakes because 

 pole-and-line fishing permits the survival of too many small fish."^ 



Under a system of population control through natural predation, fish 

 populations are cropped in relation to the relative abundance of the com- 



