Abstract Introduction

In addition to being a Canadian icon, the beaver (Castor canadensis) still has important economic and social value for the First Nations in Quebec. Hydroelectric projects in the James Bay basin can locally alter the beaver’s habitat. Translocation of beavers from sites of future hydroelectric development to non-altered habitats has been advocated as a management strategy to preserve local populations of this fur animal. However, these translocations should be closely monitored to assess successes and failures. One of the objectives of our pilot study was to evaluate if ear-tag transmitters designed for use in wild ungulates could be successfully attached to the tail of beavers and, therefore, used to assess dispersal and survival of translocated animals. If reliable, these external devices would represent a less invasive alternative to the surgically implanted abdominal transmitters currently used in this semi-aquatic rodent species.

Methods

Twenty beavers (15 adults, 4 subadults, and 1 juvenile) ranging from 4–19 kg were live-captured during summer 2003 near Eastmain River, James Bay, Quebec, Canada. With the exception of one juvenile male, all the other juvenile beavers captured (10) were judged to be too small to accommodate the transmitters available. Each animal was anesthetized with an intra-muscular injection of a combination of ketamine and xylazine (either 0.32 mg/kcal of ketamine + 0.03 mg/kcal of xylazine or 0.23 mg/kcal of ketamine + 0.07 mg/kcal of xylazine, for an average of 9.7 mg/kg of ketamine + 2.1 mg/kg of xylazine). When the beaver was sufficiently immobilized for handling, the attachment site on the tail (approximately 12 mm thick) was disinfected with several passages of 2% chlorhexidine. These attachment sites were located between 2 and 4 cm laterally to the midline of the tail and 5–10 cm caudally from the base of the tail depending on the size of the animal. The surgical site was then transmurally infiltrated with 0.3 ml of bupivacaine 0.5%. A transmural anchor hole was made using a sterile 5.5 mm surgical drill bit powered by a cordless drill. The transmitters used in this study were manufactured by ATS (model M3530, Isanti, MN). Each transmitter (35 g; 47x16x43 mm; 11 cm antenna) was affixed by ATS to an AllFlex “global female” ear tag (Allflex USA, Inc., Dallas/Fort Worth Airport, TX). The transmitters were fixed on the dorsal aspect of the tail using a sterile attachment button, composed of a 12-mm-long plastic post perpendicularly linked to a 25 mm diameter circular flat disk. This button was inserted ventrally in the predrilled hole and secured to the “global female” ear tag using a Universal Total Tagger (Allflex U.S.). The radio tags were set up with an activity switch that monitors the motion of the animal and increases the pulse rate after 24 hours of inactivity. After the completion of the procedure the anesthesia was partially reversed with 0.2 mg/kg of atipamezole IM and the animals were transferred by helicopter to the relocation sites. The beavers were then housed in retention pens and released either the same day or up to five days post-capture. Telemetric surveys started a few days after the translocation of the tagged beavers. The relocation sites were surveyed by helicopter at seven occasions during the three-month period following the translocation.

Results and Discussion

All the beavers survived the procedure and recovered well. The level of anesthesia obtained was sufficient for the procedure, and the recoveries were fast and uneventful. The broadcast distance of transmitters was evaluated to be 1.5 km at an altitude of 100 m. Increased pulse rates indicative of immobility were recorded for eight of the 20 transmitters within the first three months. Two of these beavers died. One was preyed upon by a black bear while still in its retention pen and the other was shot by a hunter. Four of the six other immobile tags were found on the ground without any sign of predation. Examination of the fallen tags suggested that they were either pulled by the beavers or dislodged from their tail after being entangled in branches or bushes. The two last transmitters were located but couldn’t be retrieved since they were underground on edges of lakes, most likely in underwater tunnels. This study shows that attachment of transmitters on the tail of beavers is a relatively non-invasive procedure that can be easily performed in the field. Nevertheless, this procedure, which involved the surgical drilling of a well-innervated structure, should only be performed under general anesthesia. The anesthetic protocol used in this study was sufficient for the level of pain induced by the procedure and well-tolerated by the animals. However, this level would have been insufficient for abdominal implantation of transmitters. In this study, the working life of these ear tags mounted on beaver’s tails was not optimal. Indeed, when removing mortalities (2), at least 25% (4 out of 16) of the tags were lost within three months. It is likely that this figure will increase over the winter. This will be assessed by telemetric surveys planned for the spring of 2004. Some modifications of the attachment buttons, such as increasing the diameter and rigidity of the disk, might increase the working life of these tags. In addition, posts of adjustable lengths (up to 30 mm) would enable the fixation of the tags in the thickest area of the tail (closer to the base and the center of the tail). This should increase the strength of the anchorage and would, therefore, likely decrease the rate of lost tags encountered in our pilot study. Further work will have to be conducted to evaluate if the working life of these tail-mounted tags would reach an acceptable level with these modifications.

Acknowledgment

We would like to thank George Coonishish and Matthew Wapachee (Cree trappers), and Joël Poirier (Wildlife Technician) for their help with the field work. This study was financially supported by the Société d’énergie de la Baie James (Eastmain-1 project).