BEAVER 



Beaver investigations conducted during the biennium were pri- 

 marily directed toward determining the status of populations. Knowl- 

 edge of population levels is essential to management for susta'ned 

 yield production. Tliese population studies included analysis of age 

 ratios, aerial colony counts and harvest analysis. 



The age-ratio of a beaver po;)ulation changes in response to varj'ing 

 intensities of trapping. A method of de'.ermining age-ratio figures 

 through iirlt-nicasi rcment analysis was developed earlier by Section 

 personnel and is now in use on a statewide basis. 



Aerial l)eaver colony counts were continued in the fall of 1954 on 

 a statewide baris and in eastern Montana only in 1955. These counts 

 consti'.ute an index to populat on density and permit year to year com- 

 parisons to show population trends. 



Comprehensive harvest analysis is conducted at the end of every 

 season. This gives the number of beaver trapped, dates of capture, 

 trapper success and ratio of private land to public land utilized for 

 every beaver trapping area. 



A study of beaver-waterfowl relations in a mountainous habitat 

 was completed in 1954 on three streams in Beaverhead County. Eight 

 segments of the flood plains of one-half mile or more in length were 

 selected as study areas. Data were obtained by observations on water- 

 fowl and live trapping, steel trapping and observations of beaver. 

 Findings of the study indicated that beaver ponds receiving the greatest 

 use had two features in common; comparatively greater size and 

 shallower water containing growths of sedge. In addition, evidence 

 was obtained to show that beaver, through their impoundments, created 

 a habitat more suitable for waterfowl on the areas studied on two 

 of the creeks. A positive correlation between the presence of beaver 

 and waterfowl usage was not found for tlic third creek. 



MINK 



Mink are second in importance among iVfontana's fur animals. 

 Analysis of catch records has shown tliat mink have been sul)ject to 

 heavy trapping pressure for a nunihrr of years. Since the size of the 

 mink take is influenced by market and weather conditions as well as 

 by population numbers, information based on biological indicators has 

 been needed to supplement annual catch records as a measure of 

 ixipuiation condition. Accordingly, a study of age and sex ratios of 

 the amiual mink harvest has been in i)rogress since July. 1953. 



Before determining age ratios of female minks it was necessary 

 to establish valid criteria for separating juveniles and adults. These 

 aging criteria, determined from a large collection of known-aged ranch 

 minks, appear to be adequate for management purposes. 



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