In summer of 1995, 17 adult male Pacific walruses at Cape Peirce, Togiak National Wildlife Refuge, Alaska were immobilized by Alaska Science Center researchers for deployment of transmitters. Selection of drug protocol for immobilization was based on 1) rapid induction time, 2) small volume to permit delivery by commercially available darts, 3) availability of a specific drug antagonist for reversal of the immobilization drug, and 4) duration of effect to allow for 30 to 45 minutes of working time for each animal. A variety of drugs have been used in walrus in the past (Gales 1989, Walsh 1990), including ketamine (Hagenback et al. 1975), phencyclidine (SernalynC, DeMaster et al. 1981), etorphin (Hills 1992, Griffiths et al. lO93), TelazolC (Stirling and Sj are 1988, Griffiths et al. 1993), carfentanil (Hills 1992) and isoflurane (Jones, M. pers. comm.). Because etorphine HCl was no longer available in the United States, carfentanil was chosen to meet the criteria listed above. Carfentanil had previously been used in a small number of wild walruses (DeMaster et al. 1981). More importantly, carfentanil can be reversed with naltrexone, a new investigational reversal agent with prolonged duration of activity which reduces the possibility of renarcotization experienced with the previously used drug, naloxone. Isoflurane gas was used to maintain the walruses under anesthesia for longer periods than possible using only the injectable agents and to increase the safety of immobilization in these animals.

Carfentanil (2.4 to 2.7 mg) administered via dart to the muscular area of the rump of mature bulls provided reliable immobilization within 7 to 21 minutes. The walruses showed little disturbance after being struck by the darts, shifting position of the rear flippers or sometimes moving a few feet closer into the nearby herd. Fine twitching of the muzzle and whiskers occurred within 5 minutes followed by disorientation with extension of the head upwards, then whole body tremors lasting a few seconds and eventually complete muscular rigidity and stupor.

As soon as the animal could safely be approached, naltrexone (175 to 350 mg) was injected into the muscular tissue of the lip. This resulted in smooth reversal of the carfentanil effects within variable time periods (10 to 20 minutes). One walrus demonstrated unusually rapid recovery and moved away from the work area in less then 5 minutes after administration of the naltrexone. This may have been the result of inadvertent injection of the naltrexone intravenously into one of several large vessels present in the muzzle of these animals.

A commercially available large animal gas anesthesia machine was adapted for field work to provide portable capability and to supply regulated amounts of oxygen and isoflurane gas. The flutter valves, soda lime canister and a Fluotec C vaporizer were mounted on an aluminum freighter backpack frame along with an E cylinder of medical oxygen and pressure reducing valve. A cuffed silastic endotracheal tube (30 mm diameter) and standard flexible hoses and 15 liter rebreathing bag completed the closed anesthetic circuit. The strong muscular contraction of the jaws while under the effects of carfentanil made it impossible to place a mouth gag in the immobilized walrus until naltrexone reversal began. When muscular relaxation and return of voluntary respiration permitted, we attempted to pass a cuffed endotracheal tube into the trachea of each walrus and administered isoflurane gas at 3 to 5% vapor concentration in oxygen. Difficulty in incubation occurred due to position in some walrus and inability to visualize the larynx with equipment available on site as well as a relatively short "window of opportunity" as the animals completed naltrexone reversal.

Good endotracheal tube placement and anesthetic control was achieved in 4 animals which were maintained for up to 30 minutes after intubation for completion of transmitter attachments, blood and tissue sampling, and a variety of body measurements. Voluntary respiratory rates of 8 to 12 per minute with tidal volume near 15 liters were higher than previously reported in captive walrus (Walsh et al. 1990) probably due to the larger size of the wild bulls treated in this study. Body temperatures determined with a deep flexible rectal probe ranged from 36.2 to 39° C. Heart rates (as determined by pulse oximeter or visualization of the cardiac movement through the chest wall) ranged from 41 to 120 bpm under carfentanil immobilization and 52 to 80 bpm while on isoflurane.

Additional doses of naltrexone (175 to 350 mg) were given intravenously or intramuscularly at the completion of work on each animal to ensure complete reversal of carfentanil effects prior to its return to the water. All but one walrus remained for several hours on the haul out near the procedure site, resting in sternal or lateral recumbency, or moving intermittently back toward the remainder of the herd. Several transmittered bulls were observed swimming near shore and returning to the haul-out within 1 to several days. No adverse effects from immobilization or transmitter attachment were observed in these animals.

This protocol provided adequate and safe immobilization of the study animals for the procedures desired and fulfilled the requirements set forth at the beginning of this paper. Problems occurred 1) when larger or smaller doses of carfentanil were used, 2) with inadequate immobilization at the 2.4 to 2.7 mg dose in some animals, possibly due to very large body size or less than ideal dart placement, and 3) when administration of naltrexone was either delayed or resulted in very rapid reversal of immobilization. Modifications to the isoflurane intubation and delivery systems were necessary due to the large tidal volume and oral anatomy of the wild walruses.

References

1.  DeMaster, D.P., J.B. Faro, J.A.Estes, J. Taggert, and C. Zabel. 1981. Drug Immobilization of walrus (Odobenus rosmarus). Can. J. Fish. Aquat. Sci. 38:356-357.

2.  Gales, N.J. 1989. Chemical Restraint and anaesthesia of pinnipeds: A review. Mar. Mammal. Sci. 5:228-256.

3.  Griffiths, D.,O. Wiig, and I. Gjertz. 1993. Immobilization of walrus with etorphin hydrochloride and Zoletil. Mar. Mammal Sci. 9:250-257.

4.  Hagenback, C.C., H. Lindner, and D. Weber. 1975. Fiberoptic gastroscopy in an anesthetized walrus (Odobenus rosmarus). Aq. Mamm. 3:20-22.

5.  Hills, S. 1992. The effect of spatial and temporal variability on population assessment of Pacific walruses. Ph.D. Dissert., Univ. Maine, Orono. 217pp. Stirling, I., and B. Sjare. 1988. Preliminary observations on immobilization of male Atlantic walruses (Orobenus rosmarus rosmarus). Mar. Mamm. Sci. 4:163-168.

6.  Walsh, T.W., E.D. Asper, B. Andrews, and J. Antrim. 1990. Walrus biology and medicine. In: CRC Handbook of Mar. Mamm. Med., L. Dierauf, ea., CRC Press, F1. pp 591-599