A dense and vigorous plant community is neces- 

 sary to support a large muskrat population, and when 

 the population increases beyond the growth capacity 

 of the plants, ' eatouts ' (large areas devoid of vegeta- 

 tion) occur. When eatouts become severe, the muskrat 

 population may collapse. 



No special reproductive requirements have been 

 reported for muskrats; they are monogamous, and 

 are sexually active year-round. The gestation period is 

 26 to 28 days, and up to 5 or 6 litters may be pro- 

 duced each year. The average litter size is 4, and a 

 lodge or burrow may contain as many as 3 litters in 

 different stages of development (O'Neil 1949). 



The presence of preferred food plants is no assur- 

 ance that an area is suitable muskrat habitat. Many 

 areas in the Chenier Plain have an abundent growth of 

 preferred plants, but do not support muskrats, whereas 

 other areas with lower quality food plants do support 

 muskrat populations (O'Neil 1949). This observation 

 suggests that some factor other than food is regulating 

 Chenier Plain muskrat populations. For example, 

 excessive flooding and drying of marshes are known to 

 affect muskrat abundance adversely (O'Neil 1949). 

 Diseases and parasites reduce muskrat numbers when 

 muskrat numbers are high. Commercial trapping has 

 not been demonstrated to greatly affect Chenier Plain 

 muskrat populations (O'Neil 1949). 



5.2.4 NUTRIA (Myocastor coypus) 



Nutria are usually active during the early morning 

 and late evening hours and at night (Chabreck 1962b). 

 In Chenier Plain marshes, activity takes place within a 

 circular home range of about 0.78 km or 0.30 mi^ 

 Adams 1956, Kays 1956). 



Nutria living in the vicinity of agricultural areas of- 

 ten move into these areas to feed on crops. Evans 

 (1970) studied nutria in sugarcane fields in southwestern 

 Louisiana and found that only about 10% of the nutria 

 using these fields actually made their homes there and 

 only 50% of these remained year-round. 



Nutria were introduced into Louisiana in 1938 and 

 populations increased rapidly. Within a period of 20 

 years, the animals dispersed across the Chenier Plain 

 (Davis 1960, Lowery 1974b). Although no systematic 

 studies have been made of nutria populations in dif- 

 ferent habitat types, Palmisano( 1972a) analyzed trap- 

 ping records and concluded that greatest population 

 densities occurred in fresh and intermediate marshes. 

 Brackish marshes carried lower populations but still 

 produced a sizable harvest. Salt marshes support con- 

 siderably lower nutria populations than the other 

 marsh types. 



Large numbers of nutria feed in rice fields during 

 the rice-growing season. The animals move into rice 

 fields from adjacent fresh marshes. Most return to the 

 marsh after the rice has been harvested. Nutria that re- 

 main in rice-growing areas during the winter months 

 occupy irrigation canals, drainage ditches, and im- 

 poundments (Evans 1970). 



Swamp forests, where water is readily available, 

 usually support nutria. Under favorable conditions, 

 swamp forests will produce population densities similar 

 to those of coastal marshes (Palmisano 1961, Nichols 

 1974). 



Nutria feed chiefly on plants and consume 1 to 1 .5 

 kg (2 to 3 lb) of vegetation per day. Preferred plants 

 in fresh and intermediate marshes are pickerelweed, 

 cattail, southern wild rice, alligatorweed, sawgrass. 

 pennywort, giant bulrush, and spikerush. In brackish 

 marshes, they feed heavily on Olney's three-corner 

 grass, big cordgrass, saltmeadow cordgrass, and leafy 

 three-square. Important foods in salt marshes are 

 smooth cordgrass and saltgrass (Atwood 1950, Palmi- 

 sano 1961). Submerged pond plants such as pondweed, 

 southern naiad, and parrots' feather are also consumed. 



Nutria reproduce year-round. The gestation peri- 

 od is from 130 to 134 days and females will often 

 breed two days after young are born (Atwood 1950). 

 Of 224 adult females examined on Rockefeller Refuge, 

 91% were pregnant (Kays 1956). The number of em- 

 bryos ranged from 1 to 1 1 and averaged 5. In studies 

 elsewhere on the Chenier Plain, Atwood (1950) and 

 Harris (1956) examined different nutria populations 

 and found that the average number of embryos ranged 

 from 4 to 6. Harris (1956) noted that 5% of the total 

 embryos were in the process of resorption, and the 

 percentage of resorbed embryos seemed to be associated 

 with increased nutria populations and dwindling of the 

 food supply. 



The rapid spread of the nutria througliout the 

 Chenier Plain after its release in 1938 indicated that 

 the species adapted well. The extent to which diseases 

 and parasites have increased since that time has not 

 been studied in detail, but it is likely that diseases and 

 parasites have become increasingly important as limiting 

 factors. Lowery (1974b) reported that 80% to 90% of 

 the nutria in Louisiana are infected by the nematode 

 Strongyloides myopotami, which restricts reproduction 

 and causes mass mortality. 



The alligator is the main predator, other than man, 

 and apparently consumes large numbers of nutria 

 (Lowery 1974b). Valentine et al. (1972) reported that 

 nutria are the major food of large alligators on the 

 Chenier Plain. The young nutria are also eaten by turtles, 

 gar, snakes, and birds of prey. 



Severe freezes which occasionally strike the Chenier 

 Plain sometimes cause high mortality. Young animals 

 are more seriously affected than adults. Greatest losses 

 occur where shelter in bank burrows or dense vegeta- 

 tion is sparse. 



Annual harvest of nutria for fur over the past sev- 

 eral years has been about equal to recruitment, so that 

 fall populations have remained fairly static. Increased 

 harvest rates coupled with losses due to predators, to 

 diseases and parasites, and to habitat destruction from 

 increased saltwater intrusion and marsh drainage could 

 result in serious population declines. 



228 



