315 



50 100 150 200 250 300 350 400 450 

 Total catch 



1954 



Total catch 



Figure 6. — Effectiveness of gear in capturing hybrid trout and grayling in Grebe Lake during the mark-and-recapture 



study, 1953 and 1954. 



Methods of forming population estimates which 

 have been used by biologists and the prerequisites 

 necessary for the use of various methods have 

 been adequately discussed by Ricker (1948), 

 DeLury (1951), Cooper and Lagler (1956), and 

 others. It is essential, however, to indicate which 

 of these prerequisites have and have not been 

 fulfilled by this study and hence what reliability 

 can be placed on the estimated numbers of trout 

 and grayling. 



The first assumption of a mark-and-recapture 

 study is that either (1) marked fish distribute them- 

 selves randomly throughout the lake, or (2) that 



the fish are captured by a random placement of 

 the nets proportional to the population density 

 in different parts of the lake. Nets in this par- 

 ticular study were set randomly and fish captured 

 were redistributed in various parts of Grebe Lake 

 to ensure randomness. 



Secondly, mortality among the marked fish must 

 not be greater than among unmarked individuals. 

 The result of such an error would be a decrease in 

 the marked specimens and a population estimate 

 that is too large. Ricker (1949) was unable to 

 demonstrate significant mortality in a population 

 of fish from which one fin had been removed, and 



