POPULATION ESTIMATES 



In order to assess the importance of the shovelnose sturgeon spawning 

 run into the Tongue River, it was necessary to determine the strength of that 

 run and variations in the number of fish using the river each year. The 

 validity of population estimates based on mark-and-recapture techniques is 

 based on certain assumptions: 



1) marked fish do not lose their identifying marks and are recognizable 

 upon recapture; 



2) either marked fish are randomly redistributed throughout the 

 population or the sampling effort is proportional to the density 

 of the population; 



3) both marked and unmarked fish are susceptible to the same degree 

 of capture; 



4) numbers of fish are not increased as a result of recruitment from 

 growth or immigration; and 



5) losses from death or emigration are equal for both marked and 

 unmarked fish. 



Since the sturgeon population is migrant, assumptions 4 and 5 are not 

 met, with continual immigration and emigration. However, Ricker (1958) felt 

 that the Schnabel estimator can still be useful even if not all conditions 

 are met completely. 



The population of shovelnose sturgeon utilizing the lower Tongue River 

 was estimated using three formulas to compare and evaluate population strengths. 

 As found in Ricker (1975), the formulas are: 



Schnabel N = £ 



.Ml 



t ^t 

 Schumacher-Eschmeyer N = 2^ ^ h 



R^. + 1 



Chapman or Modified Schnabel N = 2^- 



t 



where: N = population estimate 

 C^ = number of fish caught 

 M|. = number of fish marked 

 R^ = number of fish recaptured 



All estimators are multiple censuses, involving the addition of 

 marked fish in the population. Negative bias enters into the estimates if 

 the combination of number of fish marked and number examined falls too low. 

 Ricker (1975) suggested that this bias can be ignored whenever the number 

 of recaptures is four or more. 



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