Monitoring Vermont Furbearers

Harvest data is critical for understanding trends in Vermont’s furbearer populations and is the most cost-effective way to advance our ecological knowledge. The information gathered by trappers is a piece of the larger statewide and regional population dynamics information gathered for furbearers from Maine to Virginia.

The trapper surveys we receive each year allow us to track the sex ratio of our furbearer populations and model the likely abundance of furbearers across our state. The carcass samples trappers must provide for bobcat, fisher, and otter also let us gather sex and age data and gives us the opportunity to measure environmental toxins that some species may be exposed to. These samples also ensure the department has high quality data, because trapper harvests are kept in good condition compared to other sources of carcasses like roadkill.

Trappers are acting as community scientists when they provide us this data, meaning their time and resources are just as essential as the actual information they provide. Trappers buy and maintain traps, set them up and check them every day. If the department were to do this work, it would require buying and maintaining traps and using staff time. Not only does trapper sourced data give us our most comprehensive data on these species, it does so in an extremely cost effective manner—in fact, trappers PAY us for the data THEY collect for us. 

The department uses roadkill data collected from a variety of sources including game wardens, partners at the US Department of Agriculture, and data collected from the VT ROaDS and Wildlife App. The app allows anyone to report roadkill sightings and is one of many avenues for reporting and gathering information. Visit https://anr.vermont.gov/content/vt-roads-and-wildlife for more information and directions on how to download the app.

As a result of this work, the department's roadkill data goes back decades. Although the data we get from roadkill helps inform landscape movement and connectivity, the data we get from roadkill is not enough information to provide trends in population size. For example, smaller mammals decompose fast and the damage from vehicle collisions makes it difficult to get any useful information about the animal. That is why the department pairs roadkill data collection with other datasets, including trapper harvest data. 

The department is actively engaged in multiple studies using remote camera monitoring of furbearing mammals. One of these studies involves working with the University of Vermont (UVM), the Nature Conservancy, and Vermont Agency of Transportation, and is focused on assessing movements and road crossing safety for furbearers. The department also partnered in recent years with Central Connecticut State University and the US Forest Service to monitor American marten occupancy in the southern Green Mountains.

Another camera monitoring study is a partnership between the department, UVM, the US Forest Service, and 11 other partners across the Northeast region with more than 600 cameras in total, over 100 of which are in Vermont. This is part of a long-term monitoring study employing cameras paired in part of Vermont with acoustic recording devices.

However, camera monitoring also requires a significant amount of time, money to purchase equipment, and staff time to set up and review camera images. The department currently does not have enough funding or staff to conduct a camera monitoring program that would provide the same amount of data on the same diversity of species we receive from trapper surveys and tissue samples.

The large remote camera monitoring study described above also incorporates autonomous recording units (ARUs). ARUs are self-contained audio recording devices that are used for audio monitoring. There are currently more than 50 ARUs in Vermont. Similar to the camera monitoring project described above, this study is in development. So, it will be a few years before all the collected data can analyzed to determine what species it may be applicable to.

Most of the data collected using ARUs in this study is geared towards birds, but recordings can be scanned for coyote howls. Most mammals, including coyotes, will likely have much higher detection from camera photos.

iNaturalist is a source of some useful data, but there are not enough observations submitted for each species to meet our monitoring needs for some furbearers. iNaturalist also has complications with data reliability. Making iNaturalist data usable for the kind of state-wide population monitoring the department is responsible for would require funding and staff that we do not have. It would require investing in coding platforms to compile and measure potential biases and analyze results along with error estimates.

The department uses one community science platform that is extremely useful and provides the department with its main source of long-term population monitoring data for all species of furbearers: trapper catch-per-unit-of-effort (CPUE). This is the number of animals caught compared to the number of traps and the number of hours those traps were in the field. Each year, trappers in Vermont report this data to us in their mandatory trapper survey.

The CPUE data is useful because it directly connects amount of effort put into trapping with the number of animals trapped. The department has used this source of data to track trends in furbearer populations for more than 20 years and uses the harvested individuals of bobcats, fisher, otter, and the pelts of muskrats to measure trends in age and sex for those species. This method recently had an exciting development – the data was used to build population models to track and predict trends like abundance, reproduction, and survival.
 

These models are still in development, but recent research on matrix models applied to bald eagles is showing promising results.

Observation surveys work well for birds and other easily observable species. Furbearer species are highly camouflaged and mostly active at night or dawn and dusk when light levels are low, making it difficult for the human eye to see them. Aerial surveys can be used for estimating beaver populations, but it is expensive and only gives us a snapshot in time. It would need to be repeated every few years to gain meaningful information.

Night vision tools like Forward-Looking Infrared (FLIR) are extremely expensive, making this method cost prohibitive. Even with an ability to see animals in the dark, furbearer species do not form large aggregations on the landscape, tend to range over large areas, especially when compared to something like colonial nesting birds, and furthermore they do not advertise their presence in the way that birds do by singing or displaying. For this reason, aerial/drone surveys are practically an unheard-of sampling method for furbearers, particularly for those inhabiting forested habitats.

Drone surveys have similar limitations as observation surveys. Furbearers can be very difficult to see and the Federal Aviation Administration requirement that drone operators maintain line of sight when using drones makes this method not useful for monitoring furbearer species.

The relatively large home ranges of many furbearer species require coverage of large expanses. Despite advances in drone battery life, maximum run times for Pro versions of some of the most trusted and popular drone brands (like DJI) last only 21 minutes and have a range of 8 miles. Coyotes can have home ranges of 15 square miles. Even smaller-bodied fisher can have home ranges this size. For this reason, drones are not a viable option for monitoring furbearers.

Scat can also be used to identify species by its size and shape, and this method of identifying species can be very simple for certain species such as deer, moose, and bear. However, research has indicated varying accuracy with this method for furbearing species, ranging from 90 percent accuracy for coyote in Alaska (see Prugh and Ritland) to 54 percent accuracy for coyote in West Virginia (see Morin et al.).

This variation makes it difficult to use this method consistently with confidence. Scat detection dogs provide a potential solution to increase accuracy. The department has explored this option in Vermont for bear, fisher, and bobcat scat and though it was accurate for bear and fisher (see Long et al.), the cost is consistently higher than using remote cameras (see Clare et al.). When appropriate, as in our moose health studies, we have used scat monitoring—but that case reinforced the significant costs and challenges to using this method at the scale of a single species.

Hair traps are effective sampling tools for some furbearers but not others. Design innovations in hair traps and tubes have vastly increased detection rates for both un-baited and baited traps. This method works best for single species or at most a few species, because the traps need to be specifically designed according to the morphology of an animal. Furthermore, detectability is low for some species that are averse to entering tubes or enclosures or stepping through devices.

Many feline species are difficult to trap for this reason, as illustrated by the Long et al. study referenced above in the scat survey section. This study found very low detection for bobcat with hair traps. Hair traps are sometimes used in conjunction with camera trap surveys but have been found to contribute little to research objectives. The department will continue to partner with researchers to explore novel ways of increasing detection for hair snares.

Track surveys can be helpful for rare species, but like most methods, there are limitations. One of the methods we currently use to detect the presence of lynx in the state is through systematic track surveys.  Limitations exist for collecting enough information from track surveys, including requirements for good fresh snow on the ground and high intensity of sampling.

The department does not use capture-mark-recapture for furbearers. This method can yield useful information, but it involves capturing a lot of animals and requires resighting them multiple times. Particularly for elusive camouflaged animals like furbearers, resighting marked animals can be extremely difficult.

Capture-mark-recapture often employs GPS or radio tracking devices as the main mechanism to make relocating and resighting the animal feasible. These devices are very expensive to purchase and operate, and varying body morphologies of smaller mammal species can make it difficult to design tracking devices such as collars or tags that will stay on and not cause harm to an animal.

This method can be difficult to use on smaller mammals and is also very expensive. However, this method can yield useful information on smaller scales for single species. This does not provide broad information on population trends on a statewide basis, but it can provide answers to specific questions regarding movements, behavior, habitat use, and survival.

This method was employed for moose and bobcat in previous research Vermont Fish & Wildlife collaborated on with UVM. For moose, a two-year study centered on the Northeast Kingdom area of Vermont assessed moose survival and habitat use. For bobcat, a three-year study examined bobcat habitat requirements in the Champlain Valley region of Vermont. These studies did provide useful information for these individual species but the high cost, single species focus, and snapshot in time nature of this method limits its utility to addressing specific questions about movement, behavior, or habitat use for a single species. Abouelezz et al. 2018, Farrell et al. 2018, and this summary article highlights the results of the bobcat study.