can be made from the air in more- 

 northern areas even when the marsh 

 is covered with snow. In wind- 

 swept areas the snow is quickly 

 blown from the tops of the houses, 

 making them stand out. Under 

 more-moderate conditions, the 

 warmth generated by the muskrats 

 in an occupied house is sufficient to 

 melt most of the snow from the 

 crown of the house, while the feed- 

 ing huts remain covered. In 

 marshes that are burned early in 

 the fall, as in the Gulf States, the 

 straw-colored houses stand out 

 clearly against the blackened, 

 charred vegetation. 



Determination of the muskrat 

 population on the basis of the house 

 count requires some knowledge of 

 the number of muskrats usually 

 found in an occupied house. The 

 last litter of the season usually re- 

 mains with the parents through the 

 first winter and until the spring 

 mating season begins. Observa- 

 tions of the writer indicate that the 

 litter size varies with the race. The 

 number of animals in a dwelling 

 house will depend, therefore, on the 

 average litter size of the race con- 

 cerned and the survival rate at the 

 time the house count is made. 



The arbitrary count of five musk- 

 rats to a large house or bank den is 

 frequently used as a conversion 

 figure in estimating muskrat popu- 

 lations. This may be too low a 

 figure for good years. When 

 known, the average litter size of the 

 race concerned is preferred to con- 

 vert the number of inhabited 

 houses, as determined in the house 

 count, into population estimates. 



Maintaining a Balanced 

 Population 



Controlling muskrat numbers by 

 trapping 



An important principle of suc- 

 cessful muskrat management under 

 natural conditions is that the an- 

 nual crop be harvested at the right 

 time both for maximum economic 

 returns and for sustained yield. 

 Muskrat populations cannot be car- 

 ried over from year to year to build 

 a huge crop for a single trapping 

 season. High population densities 

 are a danger signal and removal of 

 the surplus animals is the key to 

 successful fur-animal management. 

 The majority of State and National 

 wildlife refuges are managed pri- 

 marily for waterfowl. The prob- 

 lem is to keep the muskrat in the 

 status of an asset rather than a lia- 

 bility, and trapping excess numbers 

 of this fur animal is necessary. 



The productivity, or carrying 

 capacity, of a marsh 'depends 

 largely on the type and quantity of 

 its vegetation and on the amount of 

 its brood stock. There is also a 

 direct correlation between the size 

 and weight of a muskrat and the 

 quality and abundance of food 

 available to it. Under Atlantic- 

 coast tidewater conditions, trap- 

 ping should be undertaken w^hen the 

 population density reaches that of 

 one house to the acre. A density of 

 2.5 dwelling houses an acre calls for 

 immediate, intensive trapping to 

 prevent serious eat-outs. Allow^ed 

 to increase uncontrolled, muskrats 

 will deplete the vegetation and 

 cause the marsh to revert to open 

 w^ater and mud flats. The animals 

 are then forced to migrate to sur- 



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