SNAMP Pub #19: The use of radiotelemetry in Martes research: techniques and technologies
Article Title: The use of radiotelemetry in Martes research: techniques and technologies
Authors: Thompson, Green, Sauder, Purcell, Sweitzer, and Armeno
This is a summary of the use of radiotelemetry (Ground-based VHF, Aerial VHF, GPS and Argos) in research on the Martes species (marten, fisher or sable). It reviews 129 distinct research projects that used radiotelemetry to investigate various aspects of the ecology of 7 of the 8 recognized species and represents the work of 22 countries from 1972 to the present.
- It provides a background on each method, information on the various tracking techniques involved, accuracy, efficiency, costs, current and future applications and the strengths and limitations of each of the different monitoring techniques.
- Information is provided on the risks to study animals and recommendations for minimizing those risks while using collars for radio-tracking, such as break-aways and spacers. Risks of implant transmitters, under the skin or in the body cavity are also addressed.
Radiotelemetry, the tracking of animals by radio collar, was first used on a Martes species in 1972. The first Global Positioning System (GPS) collars became available in 1994 and the first deployment of one on a Martes species in the southern Sierra Nevada of California occurred in February 2009. Radiotelemetry is used to examine habitat uses such as: home ranges, core-use, resting, denning, dispersals, reintroductions, effects of disturbance of harvest practices, mortality rates and their causes.
Ground-based telemetry is labor- and time-intensive, particularly with highly mobile animals in rugged terrain. However, it is well suited to investigate questions related to habitat use at all spatial scales, and is particularly applicable to fine-scale habitat selection.
The primary benefit of aerial telemetry is the ability to consistently locate a large number of study animals and to monitor long-distance movements in inaccessible terrain. Its weaknesses are reduced location accuracy, the loss of additional insights/data gleaned by tracking animals on the ground, and the high cost of flight time.
The Global Positioning System (GPS) allows telemetry units to record accurate locations and store the data until the collar is recovered or the data is remotely downloaded. The challenge of using GPS transmitters on Martes species is that the size and weight of the transmitter is often too large. Miniaturized units, small enough to be deployed on the larger Martes species, are new and relatively untested.
Argos satellite telemetry works differently than GPS telemetry in that the unit on the animal is a beacon that is tracked by satellites. Locations are recorded and transmitted directly to the researcher via email. The application of Argos telemetry to Martes species is currently limited because of the minimum weight and bulk of the collars, and while data collection is greatly simplified, location accuracy can be highly variable.
Since telemetry was first used in 1972, collars have been the overwhelming choice of attachment method by Martes researchers. Of the 131 telemetry based research projects we identified, 92% used collars exclusively.
Limitations on collar weight as a percentage of body weight of 1-5% are commonly adhered to for most mammals.
Aerial telemetry reports detection distances of 6 – 9 miles in the direction of flight, and 4 – 6 miles using side-mounted antennas. The optimal elevation for locating radio-collared Martes species is between 500 – 750 feet above the ground. Location error increases with flight speed, elevation above ground level, signal reflection in rugged topography, and decreases with increasing levels of experience of both the pilot and biologist.
The ground-based studies reviewed used three primary techniques to locate radio-marked animals: omnidirectional antennas, directional antennas, and homing. Most studies used hand-held l H, 2-, 3-, or 4-elementYagi antennas. These directional antennas are used to identify a compass bearing to the collared animal, then multiple bearings are used to triangulate the animal’s location. The accuracy of locations acquired via ground-based triangulation is highly variable and depends on the terrain, observer experience, animal movement, proximity to the animal, number of bearings taken, and the angle of intersection between subsequent bearings. Researchers used homing techniques, following a signal to it’s source, to locate mortalities, dens, or resting structures.
GPS collars for tracking Martes species range in weight from 1.14 to 2.64 ounces in weight, which includes the battery, epoxy, and collar material. Accuracy and efficiency are significant concerns with GPS technology as Martes species typically live in structurally complex habitats where GPS accuracy and fix rates may be compromised. GPS collars currently have much shorter lifespans than VHF collars, typically on the order of 2 to 3 months versus 2 to 3 years, meaning animals must be recaptured to change the collar. Features are available to remotely download data from larger GPS collars whenever an animal comes within range of a cellular tower using a Global System for mobile communication.
The Argos Satellite System has expanded markedly in the last 30 years. Currently, movements of 300 species are monitored with it.
Three areas of current technological development may benefit Martes researchers: battery technology, signal-processing technology, and the combination of digitally encoded signals and automated receiving systems. The development of lithium-ion batteries has already greatly increased the lifespan and range of transmitters.
Technological advances such as the miniaturization of ARGOS and GPS collars open new opportunities for use in species that weigh over 4.5 pounds. Research into Argos telemetry concluded that the moderate location accuracies of this system limit its suitability to studies of species that move long distance. Evolution of transmitter design, satellite configuration, and data-processing techniques will hopefully improve its use with smaller animals.
The supplementation of the GPS satellite constellation with additional satellite or terrestrial networks would create a product capable of communicating with multiple satellite systems, greatly increasing coverage and reducing the time-to-fix, resulting in more successful locations. However it will likely be a number of years before this technology is miniaturized to the degree required for use on Martes species.
Potential harm to study animals from radiocollaring requires that researchers weigh the risks associated with capture, handling, and radiotagging of animals against the benefits of the data to be collected. When animals must be handled, researchers should maximize collection of data on age, sex, reproduction, physiological state, and other covariates that may influence resulting data. Consideration of the species’ ecology and morphology should inform the selection of collar shape, size, material, color, the use of break-aways or spacers and the ultimate timed removal of collars.
Problems with implant transmitters appear to have been resolved and researchers using them in fishers report comparable results in signal strength, range, and lifespans to radiocollars. Problems with mortality switches, attributed to the gradual accumulation of moisture inside the transmitter. have led to premature transmitter failures though the problem appears to have been corrected in newer products. The greatest risk involves the surgical procedure, the recovery time while the incision heals and the avoidance of infection. It is recommended that all implants be surgically removed during the operational life of the batteries.
Thompson, Green, Sauder, Purcell, Sweitzer, and Armeno. 2012. The use of radiotelemetry in Martes research: techniques and technologies. Proceedings of the 5th International Martes Symposium, Seattle, Washington. In Aubry, K.B., W.J. Zielinski, M.G. Raphael, G. Proulx, and S.W. Buskirk, (editors), Biology and conservation of martens, sables, and fishers: a new synthesis. Cornell University Press, Ithaca, New York, USA.
The full paper is available here.
For more information about the SNAMP project and the Fisher team, please see the: Fisher Team Website.