4 Discussion
Our surveys demonstrate substantial declines in muskrat populations at
two large coastal wetlands in Ontario based on direct field observations
spanning 40-50 years. The data we present are not confounded by trapper
effort like harvest data often are, but they correspond to reported
trends in provincial harvest data and are consistent with anecdotal
evidence of muskrat decline. Ahlers and Heske (2017) called for direct
evidence of muskrat population trends to avoid confounds with harvest
effort, and we provide such evidence here.
It has been suggested that most survey techniques for muskrats are only
feasible across small geographic extents and may not be useful for
examining large-scale population trends (Roberts and Crimmins,
2010). While this may be true, we have two independent data
sets from widely separated wetlands illustrating declines of similar
magnitude over a similar time frame, which are also coincident with
declines in our provincial harvest data over the same approximate time
period. Collectively this suggests a broader trend of decline in muskrat
populations across Ontario.
The greater than 90% decline in muskrat house abundance at each of our
study sites is startling, yet we are confident the survey data indicate
a true decline of this magnitude. In many wildlife surveys there is
potential for over-estimating or under-estimating abundance as a result
of observer error. In the case of muskrat house surveys, some houses may
be missed, and some structures may be falsely labelled as active houses
when they are in fact only feeding structures or inactive houses, or
vice versa. However, we assume that in our study, on average, detection
probabilities were similar from year to year (except the survey years
that were deliberately eliminated from the analyses because the
surveyors reported poor conditions that hindered their ability to detect
houses), and that in most cases muskrat structures were correctly
identified. We attempted to replicate the historic survey methods as
closely as possible so that the results we obtained could be largely
attributed to actual population change and not simply methodology
differences. Furthermore, considering that our final house counts for
each recent survey year at each site were adjusted based on imagery data
that were used to supplement our field counts, and it is more difficult
to distinguish new houses from old ones on air photos, it is more likely
that we have over-estimated contemporary muskrat abundance by including
houses in our tally that are no longer in use. Therefore, our estimates
of the magnitude of muskrat population change at each site are likely
conservative.
Muskrats have been shown to fluctuate periodically, thus it is possible
that low house counts may be reflecting a low phase in a normal
population cycle. Erb et al. (2000) examined ninety-one historic time
series of muskrat harvest data from boreal, taiga and southern Arctic
regions of Canada and found overall that mean period length ranged from
3.7 to 8.6 years. If the same pattern holds true in southern Ontario and
the start of our surveys corresponded to a population low phase, we
would expect to have seen a greater increase in our recent house count
numbers over the five to six years we conducted the surveys. We observed
no such increasing trend, and similarly, there is no increasing trend
evident in muskrat harvest data collected over the same time period.
Instead, it appears that muskrat populations at Point Pelee and
Matchedash Bay-Gray Marsh are currently persisting at very low levels
and are much reduced from the levels seen 30-50 years ago. Furthermore,
harvest data suggest that these declines are widespread.