Evaluating unoccupied aerial vehicles for estimating relative abundance of muskrats

Abstract

Due to muskrat (Ondatra zibethicus) population declines in North America, it is important to develop rapid, safe, and economical tools for estimating abundance over large spatial and temporal scales. During November 2020-February 2021, we assessed unoccupied aerial vehicles (UAVs) at Bear Creek Flooding State Wildlife Management Area in the northern Lower Peninsula of Michigan. We examined aerial red-greenblue (RGB) and thermal imagery for estimating the total number of muskrat houses during a snow-free and snowcover period relative to ground surveys. Muskrat house counts were similar between the December snow-cover UAV RGB image survey and February ground survey (V = 70, P = 0.559), but 132% and 136% lower for November snow-free UAV survey compared to snow-cover UAV (V = 203.5, P < 0.001) and ground (V = 196, P < 0.001) surveys, respectively.Unoccupied aerial vehicles can rapidly, safely, and economically estimate and track relative abundance of muskrats in wetlands that have some snow cover and should be evaluated further in more systems at different times of the year. Effective use of UAV imaging for muskrat house surveys is dependent on time of day, temperature, and weather conditions.Conducting UAV surveys in tandem with ground surveys would improve estimates of relative abundance if active muskrat houses are better defined.

The use of orthoimagery and stereoscopic aerial imagery to identify muskrat (Ondatra zibethicus) houses

“…To a lesser extent, it is also possible that deep snow cover led to houses being missed and to misidentification of other raised features on the orthoimagery. While Cline et al (2022) found greater detection accuracy of muskrat houses from aerial surveys conducted on a wetland under snow cover, which they attribute to the contrast and shadow cast by snow‐covered houses against the surrounding vegetation, the total snow depth at the time of their imagery collection was 15 cm. Snow depths in our study sites ranged from 5 cm to 35 cm during the dates of our imagery collection; houses in the wetlands under deeper snow cover may have had less definition and less visible contrast against their surroundings.…”

“…The use of other types of imagery also holds promise, either for alternative methods of automatic classification or to assist human aerial imagery observers with the classification of ambiguous structures. For example, Cline et al (2022) experienced some success in using thermal imaging to identify the vent holes at the top of muskrat houses, thus confirming the presence of active houses, although they noted that the technique was not reliable in all snow and temperature conditions.…”

“…Aerial imagery presents some clear advantages for muskrat house census over traditional ground surveys; however, the costs of aerial surveys can be substantial depending on the extent of coverage desired and whether a research team needs to hire an external provider to collect the imagery or is able to collect it themselves (i.e., with their own aircraft and photography equipment). Small and/or close proximity wetlands can be efficiently captured using an unoccupied aerial vehicle (e.g., Cline et al 2022), but multiple large sites located across a wider geographic region are typically more efficiently captured from a piloted aircraft capable of long‐distance sustained flights. The two types of aerial imagery we collected for this study have their own associated costs and benefits.…”

“…Aerial photographs were used to count muskrat beds in a 1945 study in Texas (Lay 1945) and the possible advantages of this approach to muskrat census, including cost savings and unimpeded views, have been discussed by other researchers (Dozier 1948, Leedy 1948). More recently, Cline et al (2022) compared ground‐based muskrat house counts with aerial counts from orthoimagery obtained by unoccupied aerial vehicles (UAVs) at a fine scale and presented some of the benefits of this approach. However, to the best of our knowledge, orthoimagery is still not commonly used for muskrat house counts, and the use of stereoscopic aerial imagery for this purpose has not been investigated.…”

A Novel Scouring Method to Monitor Nocturnal Mammals Using Uncrewed Aerial Vehicles and Thermal Cameras—A Comparison to Line Transect Spotlight Counts

“…Monitoring mammals using drones has been extensively studied [22][23][24][25][26][27][28][29][30][31][32][45][46][47][48][49][50][51]. Witt et al [32] compared spotlight counts and point counts to counts using a drone equipped with thermal cameras for direct detection of koalas (Phascolarctos cinereus) while flying the drone in predetermined transects.…”