2.2 Survey Methods
We aimed to replicate the historic survey methods as closely as possible
to obtain comparable contemporary results. We undertook this task by
following the detailed descriptions and survey maps provided in the old
reports to re-locate the same survey areas for each site. At Point Pelee
National Park, muskrat surveys were initiated in the 1950s by Parks
Canada staff and standardized in 1963 when the marsh was divided into 14
survey zones, with a 15th zone added in 1971 (Reive,
1978). The zones remained consistent in subsequent years; however, a few
boundary adjustments were made in 1979 to accommodate changes in the
vegetation structure of the marsh (Reive, 1979). The surveys were
conducted annually until 1980, after which time no further muskrat
surveys were undertaken (Bremner
and Reive, 1980). At Matchedash Bay-Gray Marsh, muskrat surveys were
initiated in late fall 1978 by Ontario Ministry of Natural Resources
Midhurst (formerly Huronia) District staff and continued annually until
1986, with the exception of 1982 when no surveys were conducted
(LaFrance, 1986).
At both sites the basic survey method was a winter count of all active
muskrat houses found within the respective search areas (survey zones).
At Pelee the survey method covered only a portion of the entire marsh
(the perimeter of all ponds along the open water-vegetation edge which
is typically where muskrat house-building occurs) while at Matchedash
the survey method aimed for a complete census of the entire marsh. We
surveyed Pelee for muskrat houses during the winters of 2014 and 2015
and during the springs of 2017 and 2019 (four surveys spanning six
years). We surveyed Matchedash for muskrat houses during the winter of
2014 and the springs of 2014-2018 (five consecutive survey years).
At Pelee we conducted surveys the first two years in winter (March 2014
and February 2015) on foot with two observers walking on the ice along
the inner edge of all marsh areas (i.e., the perimeter of mapped
ponds, as well as small channels and accessible pools) within each
mapped survey zone, as depicted in the historic survey reports.
Observers searched from the edge of the emergent vegetation to
approximately 15 m inwards from the open water edge (or as far into the
vegetation stands as we could detect muskrat houses) and counted all
muskrat houses seen. This is the same survey method used in the historic
surveys at Pelee, except that in some years the observers rode an ATV
rather than walked. The latter two years of our surveys (2017 and 2019)
were conducted by canoe, as ice conditions were not adequate in the
winters of those years to allow surveyors to safely traverse the marsh.
The canoe-based surveys followed the same routes covered on foot in the
first two survey years, with the exception of one survey zone which was
conducted on foot in chest waders as it did not contain enough open
water to paddle a canoe through. When surveys were conducted by canoe,
the lead observer was stationed in the bow in a kneeling position and
frequently stood up to ensure a comparable search height to the surveys
conducted on foot in winter. As well, spring surveys were conducted in
early spring before new growth and green-up of the marsh vegetation so
that visibility of muskrat houses was comparable to surveys conducted in
winter.
At Matchedash we conducted the very first survey in late winter (March
2014) on a combination of foot and snowmobile with two observers
covering the entire marsh (both perimeter and interior areas); however,
the snow cover was very deep (over 5 feet) and we were concerned that
muskrat houses might be buried and thus not detected. We verified this
suspicion by returning to the site early the following spring (2 months
later) and paddled the perimeter of the entire marsh by canoe, counting
several muskrat houses that we missed due to snow depth in the winter.
We know that these were not houses that were newly built in the interval
between our winter and spring visits because we conducted the survey in
May only a few weeks after ice-out which is not a time when muskrats
conduct house-building in this region. Muskrats typically build houses
in the fall before freeze-up and these remain in place over the winter
until they start to deteriorate (if no longer in use) the following
spring-summer (Dozier, 1948). Over the following four years (2015-2018)
we conducted all surveys at this site in May by canoe as the Matchedash
area typically receives heavy snow accumulation and thus we knew that
detection of houses would continue to be hindered by snow cover in
winter. The historic surveys, however, were conducted every year in
winter by snowmobile (except for 1981 when it was conducted by boat in
spring) and covered the entire marsh area, not just the perimeter of the
vegetation stands which the canoe-based surveys were restricted to (even
though this is where most muskrat houses typically occur). We
compensated for this difference in survey coverage by using
high-resolution aerial photographs of the study area (collected
ourselves in March 2018 during low snow cover conditions) to count
muskrat houses in the areas we could not access by canoe. We also used
our aerial count of houses found in these inaccessible areas of the
marsh, in combination with aerial verification of the houses found by
ground survey, to calculate a percentage of the total count of houses
missed by the spring ground survey (i.e. a field detection rate).
We then applied that detection rate to the previous four survey years’
counts as a correction factor, yielding what we feel should represent
the most accurate final house count possible for each year based on all
available data. In other words, for each survey year we have a minimum
count of muskrat houses observed from only the early spring canoe
surveys and an estimated maximum count based on a combination of the
canoe and aerial surveys (the latter presumably representing greater
detection accuracy). We considered the estimated maximum count as our
final house count for each year.
At Pelee we also used imagery-based house counts in 2017 and 2019 to
supplement our field counts in the survey areas we could not access,
similar to how we handled the Matchedash data. For this we used
high-resolution imagery available from the County of Essex (Essex
County, 2020) as well as imagery collected and shared by Point Pelee
National Park. We used our final house counts (i.e. our estimated
maximum counts) at both Pelee and Matchedash in all survey years to
compute an average house count at each site for the contemporary survey
period. Because we chose to use our estimated maximum house counts that
were based on a combination of aerial and ground census methods, rather
than our minimum ground-only observations (which were incomplete in some
cases), we are confident that we have not underestimated the current
muskrat population at either site by potential detection errors
(i.e. missed houses) resulting from site conditions or incomplete
survey coverage.
Historic house counts dating back to 1957 are available for Pelee,
however a detailed breakdown of the count results by survey zone was not
provided until 1967. Examining these data, we realized that not all
areas of the marsh were surveyed consistently from year to year and that
some years suffered from poor survey conditions (e.g., due to deep snow)
which likely decreased house detection rates. Because we could not
supplement the historic field data with imagery as we did for our recent
surveys, we decided that a trend over time analysis of house counts
would be more accurate if we omitted the survey years and zones where
survey coverage was reportedly poor, incomplete or simply unknown due to
a lack of information in historic reports. We therefore began our
analysis of the Pelee muskrat population with the 1968 data and removed
the years 1977-1978, as well the survey zones 14 and 15 in all years,
from our final analyses to create a reduced but more consistent and
comparable set of time series data. However, we also computed a mean
house count for the 1968-1980 period at Pelee without excluding the two
survey zones to compare with our final results. At Matchedash survey
coverage was generally more consistent from year to year (with a couple
minor exceptions) and thus we used all seven survey years and the annual
results obtained for the entire survey area to calculate a mean house
count for the historic period at that site.
As was done for the historic surveys, we used our annual counts of
muskrat houses as indices of abundance for each site, a common method
first described by Dozier (1948) and since undertaken by many
researchers as a means to infer abundance and track annual change in
muskrat populations (e.g., Greenhorn et al., 2017; Kroll and Meeks,
1985; Proulx and Gilbert, 1984; Toner et al., 2010).