7 



Fishing and Natural Mortality 



POPULATION ESTIMATION 



The direct counting of the fishes in a lake population requires drastic 

 methods such as draining or poisoning. However, in large lakes the 

 performing of these operations is impossible or impractical. 



The disadvantages involved in direct counting have stimulated mathe- 

 maticians to develop techniques for indirect enumeration. As a result, 

 the number of animals in a specific habitat can now be calculated on the 

 basis of recaptures of previously-marked individuals. 



In the case of fishes, marking can be accomplished by the removal of 

 all or part of a fin or by the application of a tag. The fishes must be taken 

 by one or a combination of methods that reduce selection for certain kinds 

 or sizes of individuals. 



The principle of indirect enumeration is as follows: In a random sample 

 of individuals, each one is marked, and released alive, and within a short 

 period of time another random sample is taken. In this second sample 

 appear some marked individuals from the first one. The proportion of 

 recaptures to the total number of fishes taken in the second sample, should 

 be the same as the proportion initially marked to the total population.-^ 



total marked X total caught when recapturing 



Total Population = 7 



^ recaptures 



If the calculation is to approach a reasonable degree of accuracy, no 

 individuals must be any more likely than others to appear in the catches 

 on the day of recapture. From what is known of the selectivity of fishing 

 gear and the nonrandom distribution of fishes, it becomes obvious that this 

 poses a real problem in fish population estimation, unless the method is 

 used in a very restricted sense, as fish behavior (and catchability ) is modi- 

 fied by species, size, sex, season, environment, and many other factors. 



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