September, 1943 Brown & YEAGER: 
trapping being regulated according to the 
degree of control needed. Usually, the 
services of skillful trappers can be enlisted. 
Populations —Of 643 trappers ex- 
pressing an opinion relative to changes in 
muskrat population, for the season of 
SURVEY OF FuR RESOURCE 
455 
bearing on muskrat catches, reproduction 
and dispersal (Errington 19375, 1939, 
1940). For example, the 1938-39 musk- 
rat catch in Illinois was, according to 
numerous experienced trappers, lower than 
that of the preceding year, a drop believed 
Fig. 14—A spring-run type of marsh, highly productive of muskrats, Vermilion County. 
This marsh, less than 2 acres, annually produces 20 to 40 muskrats. 
1938-39, 173 indicated a decrease, 63 no 
change and 73 an increase as compared 
with the preceding season; for the season 
of 1939-40, 207 trappers reported a de- 
crease, +9 no change and 78 an increase 
as compared with the preceding season. 
Many of the older, experienced trappers 
were able to recall fluctuations for the 
5-year period preceding this study. Near- 
ly all of these individuals agreed that a 
general population decline had occurred; 
this decline appeared to be less rapid in 
the northern third than in other parts 
of the state. 
Local muskrat populations were found 
to vary widely in density, a condition 
making unreliable the use of restricted 
observations as criteria for statewide 
trends. Weather has a highly important 
to have been due in part to heavy floods 
at a time destructive to new-born litters. 
The decrease in the 1939-40 catch, which 
was not experienced in 17 northwestern 
counties, is believed to have been due 
mainly to drought over most of the state 
during the late summer and fall of 1939. 
Lack of water forced many animals to 
migrate, and consequently exposed them 
more than usual to predation. he north- 
western corner of the state, in contrast, 
experienced normal weather during the 
spring, summer and fall, and a moist open 
season, conducive to effective trapping. Be- 
cause of normal young-rearing and _har- 
vesting, a normal crop was taken there. 
The writers believe that, except for the 
possible but unknown effect of cycles, the 
following factors are chiefly responsible 
