182 Fishing and Natural Mortality 



It is important that the date of recapture be close to the date of mark- 

 ing, so that there is a minimum of replacement of marked individuals 

 through death and recruitment; however, in some situations replacements 

 may occur at a fairly constant rate in relation to time, and adjustments 

 can be made to compensate for them. The ramifications of this population 

 estimation method are not given here; however. References 1, 14, 17, 18, 

 21, 23, 26, 29, 40, 43, 44, 45, 46, 49, 50, 51, 52, 54, 56, and 58 provide an 

 adequate introduction to the subject. 



Another method of population estimation -^' '^^ is based on changes in 

 the eflFort-catch relationship as a population is reduced by the removal 

 of individuals. Here, the precision of an estimate is primarily dependent 

 upon the capture and removal of a sizeable proportion of the population. 



FORCES ACTING UPON A FISH POPULATION 



Each year in every pond and lake containing fish thousands of their 

 eggs hatch; some of these small fishes die from predation, accidents, and 

 disease before becoming free-swimming fry; more of them expire between 

 fry and finger ling stages. It has been estimated that in one lake the loss of 

 bluegills between these two stages was about 86 per cent.^^ Fingerling- 

 sized fishes are not only subject to death by accidents and disease but 

 also they are still decimated by predation. However, a few survive and 

 grow to sexual maturity and old age, eventually succumbing to disease or 

 senile degeneration. Whether an individual fish reaches maturity and 

 produces progeny is unimportant as long as total recruitment and growth 

 in a population equal total losses from various causes. If such is the case, 

 the population will continue to be numerically healthy. A simplified 

 discussion of the interaction of these factors is given by Russell,^^ although 

 he was describing the dynamics of a population exploited through com- 

 mercial rather than sport fishing. 



If we apply Russell's reasoning to a sport fishery, all fish that reach a 

 "catchable" size in any given year are liable to capture. Thus, the total 

 stock may be divided into the catchable and the noncatchable. Of the 

 catchable stock, individuals will either ( 1 ) survive to the end of the year, 

 having grown in the interval, or ( 2 ) be caught with a growth increment 

 proportionate to the length of time they have survived or ( 3 ) die in some 

 other way by natural causes. The catchable stock will receive additions 

 through growth among noncatchable individuals which, once they are 

 of catchable size, are subject to the same forces of fishing and natural 

 mortality that affect the others. 



The weight of all stocks at the beginning of the year ( Si ) may or may 

 not equal those at the end of the year (82).^^ 



