these muskrat 'Svipe-outs" have 

 been attributed to the paratyphoid 

 organism, Salmmiella typhim/u- 

 riimu by various workers, but it is 

 now generally agreed that an uni- 

 dentified virus is responsible for 

 most of the loss. The hemorrhaged 

 character of the lungs and intes- 

 tines is the best indicator of the dis- 

 ease. The Peyer's patches of the 

 small intestines are often conspicu- 

 ously enlarged, and look like a mass 

 of frog's eggs through the intestinal 

 wall. Numerous gray necrotic 

 spots are usually present on the 

 liver. 



Estimating muskrat populations 



As the density of a muskrat pop- 

 ulation is constantly changing, an 

 accurate estimate of the numbers 

 present on a marsh is required each 

 year for intelligent management. 

 Practical estimates can be made 

 fairly easily and accurately in late 

 fall by counting the conspicuous, 

 haycock-shaped houses, or beds, 

 scattered over the marsh. 



The house count is recognized as 

 an excellent management tool for 

 estimating anually the number of 

 muskrats on an area, determining 

 population trends, assigning areas 

 to trappers and fixing their limits, 

 and regulating trapping quotas. 

 State conservation officials are in- 

 terested primarily in drastic i-educ- 

 tions or increases in the general 

 population as a basis for shortening 

 or lengthening the legal trapping 

 season. The marsh manager needs 

 such information to make trapper 

 assignments and set trapping quo- 

 tas. These counts also offer an ex- 



cellent opportunity to make obser- 

 vations on the types of vegetation 

 and cover, changes in the abundance 

 of nmskrat foods, and predation 

 and disease. 



House counts caai be made from 

 the ground or from an airplane and 

 are most accurate after a killing 

 frost has leveled the rank marsh 

 vegetation. On the Atlantic coast 

 the muskrats begin pushing up 

 their houses in early September, 

 with the arrival of the high tides. 

 Construction of new houses and 

 patching of the old ones continue 

 through the first week in November. 

 This is the ideal time for a count. 

 At this time, theoretically, the 

 maximum muskrat population, in- 

 cluding the young-of-the-year, is 

 present, and provides the truest 

 basis for estimating populations 

 and establishing trapping quotas. 



Transects and counts from road- 

 sides have been used by some tech- 

 nicians to estimate muskrat popu- 

 lations. Although acceptable on 

 some areas, these methods generally 

 are unsatisfactory. 



Unquestionably, the most accu- 

 rate population estimate can be 

 made from the ground by a "strip" 

 count, if trained personnel is avail- 

 able to cover the area adequately. 

 The various units must be well 

 delimited by natural boundaries or 

 by markers. Reference to a field 

 map of the area to be covered is 

 useful as knowledge of the natural 

 features is necessary for an effective 

 survey. Weather and tides must be 

 considered when choosing the time 

 to make a count, but for compara- 

 tive purposes the units should be 



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