are recorded. The population estimate is basically obtained using the formula 



*■¥ 



where P = estimated number of fish, 



M = number of initially marked individuals, 



C = number of marked and unmarked fish collected during the recapture 

 run, and 



R = number of marked fish collected during the recapture run. 

 This formula, although somewhat modified in its final form for statistical 

 reasons, is the basis of the mark-recapture technique. 



The numbers of fish were estimated by length groups. Those 0.5-inch 

 length intervals having similar or equal recapture efficiencies comprise a 

 length group. This grouping is necessary because recapture efficiencies are 

 dependent on fish size. Generally, electrof ishing is more effective for 

 capturing larger fish due to their greater surface area and higher visibility 

 when in the electrical field. Because recapture efficiencies are length- 

 related, the number of fish must be estimated by length groups, then added to 

 obtain the total estimate. Generally, at least seven recaptures are needed 

 per length group in order to obtain a statistically valid estimate. 



Pounds of fish are obtained by multiplying the average weight of the 

 fish within each length group by the estimated number, then adding to obtain 

 the total pounds. Estimates can also be obtained for different age groups of 

 fish. This mark-recapture technique, which is thoroughly discussed by Vincent 

 (1971 and 1974), has been adapted for computer analysis by the Montana 

 Department of Fish, Wildlife and Parks (MDFWP) . 



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