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WalrusDavid B. Brunson

SPECIES-SPECIFIC PHYSIOLOGY

Walruses are one of the most difficult marine animals to anesthetize. The large size, limited vascular access, and propensity for sudden physiological changes during chemical restraint have led to their reputation of high anesthetic risk. High mortality rates have been reported with both opioids and dissociative anesthetics but are most likely due to severe respiratory and circula-tory compromise when the animals are anesthetized out of the water. Respiratory arrest is commonly reported during immobilization with potent opioids. Additionally, walruses have died during chemical immobilization; they have developed circulatory failure despite effective ventilatory support.

There are two subspecies of walruses; the Atlantic walrus, Odobenus rosmarus rosmarus, and the Pacific walrus, O. rosmarus divergens. A third subspecies, the Laptev sea walrus, O. rosmarus laptevi, has been pro-posed but this is not commonly recognized. The walrus is a large and powerful pinniped. The skin is typically a cinnamon-brown color which is covered by short course hair. As the animal ages, the skin becomes lighter. The name rosmarus comes from the reddened color due to cutaneous vasodilatation when warmed by the sun while the animal is out of the water. Walruses have enlarged upper canine teeth commonly referred to as tusks. The Atlantic subspecies, the males measure 300 cm and weigh 1200 kg. Females measure 250 cm and weigh 750 kg. At birth, pups are 140 cm long and weigh 50 kg. The Pacific walrus is slightly larger with males being up to 360-cm long and weighing up to 1600 kg; Pacific walrus females are 260-cm long and weigh approximately 1250 kg. Young Pacific walrus pups measure 140 cm and weigh 60 kg (Garlich-Miller & Stewart 1999).

Problems with chemical immobilization of marine mammals and, in particular, walruses have been associ-ated with elicitation of the dive reflex. Clinical signs during chemical immobilization appear similar to this classic physiological reflex. Since walruses can sub-merge for extended time periods for foraging at the sea bottom, they are capable of prolonged breath holding. Additionally, the simultaneous occurrence of low heart rates with breath holding has close similarities to the classic “dive reflex.” However, the ability to dive for long periods of time is not the same as the adverse reactions seen by marine mammals to anesthetic or immobilization drugs. Unlike the normal diving physi-ology, apnea and bradycardias during chemical immo-bilization are associated with high mortality. Normal dive responses are not associated with lasting metabolic derangements or cardiac failure. It appears that during chemical immobilization or general anesthesia, the physiological dive mechanisms do not function nor-mally. A necropsy finding of a captive walrus showed disseminated myocardial fibrosis and atherosclerosis which was attributed as the cause of death. Underlying myocardial disease would increase the risk of anesthe-sia (Gruber et al. 2002).

The upper airway of walruses is characterized by small nostrils with large muscles to close the openings during submersion. The nasal passages are small and the nasal turbinates are such that the nasal passages lack a large meatus. This adaptation prevents water from passing through the nose into the pharynx and trachea. The oral cavity is large and characterized by a high arched hard palate. The walruses tongue is large and thick. Because the tongue lacks thin edges, stan-dard veterinary pulse oximetry probes are not easily placed on the tongue. The lower jaw moves between the large upper canine teeth (tusks). Even in very large

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Zoo Animal and Wildlife Immobilization and Anesthesia, Second Edition. Edited by Gary West, Darryl Heard, and Nigel Caulkett.© 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.

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syringe can be placed on the needle and gentle aspira-tion used to determine entrance into the vein.

Vascular access from the flippers has been described in phocid seals but not walruses. Attempts to obtain blood samples following the landmarks described for seals would be as follows. At the level of the proximal edge of the interdigital webbing of the second and third digits, insert a 37-mm (1 1/1 in) 19-gauge needle at a 10°–20° angle until the blood is aspirated into the syringe. Blood obtained will most often be of mixed arterial and venous origin (Geraci 1978).

An alternative vascular access location is the caudal gluteal vein. A needle is inserted lateral to the sacral vertebra approximately 1/3 of the distance from the femoral trochanter to the base of the tail. In adult wal-ruses, a 12.5-cm-long needle is necessary to reach the vessel. The needle should be held perpendicular to the skin and advanced slowly until blood can be aspirated into a syringe.

Endotracheal IntubationIntubation of walruses is relatively easy to perform. Intubation is easier if the neck is extended to straighten the oral-laryngeal axis of the head and neck. This is facilitated by positioning the walrus in sternal recum-bency. If the animal has large tusks, they can be used to prop up the head and extend the neck. Small ropes or towels placed in the mouth facilitate positioning for intubation. Because of the highly arched hard palate, using a separate rope or towel around each tusk allows better access to the oral cavity than a single one rope passed across the oral cavity. Palpation of the larynx with one hand while directing the endotracheal tube into the trachea with the other hand is recommended. Because of the small size of the oral cavity of even large walruses, it may be difficult to pass a large endotracheal tube while palpating the larynx. In these cases, the use of a small diameter stylet is recommended. The stylet must be at least twice as long as the endotracheal tube. Palpate the larynx as described earlier and pass the stylet through the larynx and into the trachea. The endotracheal tube is then fed over the stylet and into the trachea. Blind intubation is not recommended because of the difficulty in correct placement and the potential for the endotracheal tube to enter the esopha-gus or pharyngeal pouch.

Tracheal intubation is facilitated by muscle relax-ation during immobilization and anesthetic induction. Opioids and dissociative immobilization techniques are frequently associated with extensive muscle rigid-ity. Small dosages of propofol (40–60 mg) via the EIV will produce muscle relaxation and facilitate endotra-cheal intubation.

Preanesthetic ConcernsBecause of the large size and physical power of wal-ruses, most procedures are performed with the animal

individuals, the rostral opening to the mouth is no more than 10–12 cm in width and height. Because of the limited size of the opening, smaller walruses are difficult to manually direct an endotracheal tube into the larynx as is done in cattle.

Walruses have a unique pharyngeal pouch that can be inflated to provide buoyancy while resting on the ocean surface. Although this adaptation seems to be more highly developed in wild free-ranging walruses, it may also present a potential problem for airway man-agement in captive raised walruses (Fay 1969). During tracheal intubation, it is important to direct the tube through the larynx and not into the pharyngeal pouch.

PHYSICaL CaPtURE anD REStRaInt

Physical restraint of untrained wild adult walruses is impractical. Captive walruses can be conditioned to enable physical examination including oral examina-tion and thoracic auscultation. Cargo nets are used for young animals but have limited effectiveness in animals greater than 100–150 kg of body weight.

CHEMICaL REStRaInt anD anEStHESIa

Vascular accessThe heavy subcutaneous fat layer and the thickness of walrus skin prevent identification and access to veins in the cervical region or the appendages. Venous access can be obtained through the dorsal extradural intraver-tebral vein (EIV) and the gluteal vein. Placement of a catheter into the dorsal EIV has been described (Stetter et al. 1997). The epidural IV access can be used for fluid administration, emergency drugs, and for intravenous anesthetic administration.

Needle placement for intravenous access into the epidural venous sinus is identical to placement of epi-dural needles or catheters via the lumbo-sacral space in other species. Palpate the wings of the ilea on each side of the spinal column and the midline of the walruses back to identify the approximate location of the last lumbar vertebra. In the author’s experience, it is easiest to find these landmarks with the animal in sterna recumbency. Even in animals with normal body fat and in healthy condition, these landmarks for needle place-ment can be found. If an epidural catheter is being used for IV access, a tuohy needle is recommended to assist in directing the catheter cranially into the vein. The needle will be placed on the midline approximately 3–5 cm behind a line transecting the midline from the cranial aspects of each ileum. This should be approxi-mately at the third to fourth lumbar vertebra (L3–L4). The landmarks for the proper access site are the same as those used for epidural anesthesia in the dog or cat. In adult walruses, a 16- or 17-gauge 8.9-cm-long needle is necessary to access the EIV. Advance the needle into the animal at a perpendicular angle to the skin. A

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MOnItORInG anEStHESIa anD SUPPORtIVE CaRE

Baseline heart rate and respiratory rates should be taken prior to drug administration. Changes associated with the onset of the effects of immobilization drug can be used to determine the time to approach and to intervene.

Respiratory FunctionApnea is a common sequel to anesthetic or immobiliza-tion drugs. Ventilatory support is essential when working with these animals. End tidal gas monitoring is recommended to assess both the efficacy of ventila-tion and to determine the relative anesthetic concen-trations in the animal. Spontaneous ventilation is likely to be inadequate and methods for assisting ventilation should be available. In contrast to the conscious ani-mal’s ability to breath hold for long time periods, apnea during immobilization and anesthesia are associated with respiratory and metabolic acidosis, arrhythmias, and poor anesthetic delivery. For this reason, controlled ventilation coupled with expired gas monitoring is important for immobilization or anesthesia. Ventila-tion rate is essential to monitor during all phases of handling. Adequacy of ventilation is best determined by measurement of end tidal carbon dioxide (EtCO2) or by blood gas measurements. CO2 can be monitored with battery operated side-stream style monitors with a catheter placed into the animal’s airway or from an endotracheal tube in the intubated animal. Blood gases can be measured on blood samples taken from any vessel including the epidural intervertebral sinus.

Monitoring the Cardiovascular SystemCaptive walruses are usually immobilized with a com-bination of midazolam (0.1 mg/kg) and meperidine (2.2 mg/kg) (Klein et al. 2002). Atropine (0.04 mg/kg) is also recommended to prevent vagal induced bradycar-dia. Intramuscular injections into the hip or epaxial muscles are the recommended injections sites. A long needle (3–4 in.) is necessary to ensure effective drug injection into muscle and rapid absorption.

Heart rate during the onset of immobilization and anesthesia is usually between 80 and 100 beats/min. As anesthesia deepens, the heart rate slows to around 60 beats/min. Heart rate can be determined by observa-tion of movement of the animal’s chest wall. Once at the animal’s side, auscultation of the heart with a stethoscope or use of an electrocardiograph (ECG) to determine the rate and rhythm of the heart is essential to detect early changes in cardiac function. The use of a pulse oximeter has proven difficult because of the lack of places to position the sensing probes. The author has found that a reflectance style pulse oximeter probe placed against the oral mucosa or rectum has worked intermittently.

immobilized with drugs or general anesthetics. If the animal is well trained and will allow handling while awake, surgical procedures can be performed using local anesthetics. The thick skin and heavy layer of insulating fat present mechanical challenges for effec-tive regional nerve blocks. However, infiltration of the skin can be performed to provide effective local anes-thesia. Both lidocaine HCl and bupivicaine HCl are effective for blocking pain.

Opioid analgesics have been used for capture and immobilization in walruses (DeMaster et al. 1981; Gales 1989; Griffiths et al. 1993; Lynch et al. 1999). As with other marine mammals, analgesia in addition to immobilization is assumed. Of the opioids analgesics, meperidine and butorphanol have been used with ben-zodiazepines or alpha-2 agonists. Assessment of the analgesic effects of opioids has been limited to the responses observed during immobilization.

Drug DeliveryThe walrus, like seals and sea lions, have relatively small pelvic limbs. The pelvis, femur, and tibia are short and the associated muscles are small in relations to the size of the animal. The femur of an adult 1500-kg pacific walrus is approximately ∼12–14 in. in length. In addition, the upper pelvic limb is not well demarcated, resulting in injections often occurring at or below the knee joint. The muscles of the hind limbs and pelvis can be used for IM injections but are relatively smaller than the epaxial muscles.

The best sites for intramuscular injections are the large muscles of the back. The epaxial muscles are large and well developed. The areas caudal to the last rib and cranial to the pelvis on both sides of the vertebral column provide a large target area for remote drug injection. The muscles of the front limbs can also be used for IM drug administration. Caution should be used to ensure that injections do not occur into the large cervical blood vessels which are located cranial to the front legs.

During immobilization and anesthesia, injections can be made into the base of the tongue. Sublingual injections have the advantage of rapid absorption due to the high blood flow to the tissue. In emergencies, sublingual injections provide a readily available route for cardiac and respiratory support drugs. Alternatively, emergency drugs can be administered by flushing the drugs into the lungs via the endotracheal tube. A long semirigid piece of plastic tubing can be passed through the ET tube and the medications flushed into the airway. Dilution of the drugs or flushing the tubing with sterile water is recommended to ensure that the entire drug dosage has been administered. Antagonists for immobilization drugs may also be given by these routes when the end of the immobilization has been reached.

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nist (atipamezole) and availability of a concentrated formulation which minimized dart size and injection volume. The addition of medetomidine with carfent-anil would likely decrease the opioid dose and improve muscle relaxation (Mulcahy et al. 2003). A medetomi-dine dose of 30 mg was combined with carfentanil doses that varied from 0.06 to 3.0 mg per animal. At the 0.06-mg carfentanil dose, immobilization was inad-equate, while a dose at the 3.0 mg caused severe respira-tory and circulatory depression that was judged to be life threatening. Carfentanil doses of 0.15 and 0.3 mg with 30 mg of medetomidine enabled approach and handling, but apnea occurred and immediate reversal of both drugs was administered. Due to the low numbers of animals, it was not possible to further evaluate this combination. It should be noted that the addition of medetomidine did markedly decrease the carfentanil needed for immobilization although respiratory depres-sion was still severe. Dexmedetomidine is now avail-able and is a purified form of medetomidine. The dosage is 1/2 of the medetomidine dose due to the elimination of the “left handed” isomer levomedeto-midine. Thus it would be appropriate to use1/2 of the milligram medetomidine dosage if using dexmedeto-midine. Atipamezole is an effective and recommended reversal agent for dexmedetomidine.

Seventeen walruses were immobilized with carfent-anil alone with immediate reversal with naltrexone. Carfentanil (2.4- to 2.7-mg total dose) was adminis-tered via dart to the muscular area of the rump. Seda-tion occurred in 7–21 minutes. Muscle twitching of the muzzle was noted with full body tremors lasting a few seconds, leading to muscle rigidity and immobilization (Tuomi et al. 2002). Naltrexone at 175- to 350-mg total dose was administered into the muscular area of the lip as soon as the walrus was safe to approach. Reversal occurred between 10 and 20 minutes after administra-tion. One animal had a rapid reversal in approximately 5 minutes following injection. An additional dose of naltrexone (175–350 mg) was administered to each animal at the completion of the desired work time to ensure complete reversal of the carfentanil. Use of gas anesthesia provided additional anesthesia when needed (Tuomi et al. 2002).

Field immobilizations with etorphine HCl have had similar effects. Dosages between 4.0 and 15.5 mg/adult male based on estimated body weight were used in field immobilizations of Pacific walruses, with or without reversal with either diprenorphine or naloxone. A dose of 7 mg/adult male was used for the standard dose. Overall mortality rates varied but were in the range of 15–20% (Griffiths et al. 1993; Hills 1992).

Because of the rapid onset of severe respiratory depression associated with potent opioid administra-tion in walruses, an opioid antagonist should be available. Naltrexone is recommended because the antagonism will last beyond the expected effects of

PHaRMaCOLOGICaL COnSIDERatIOnS

Whether immobilizing a walrus in captivity or in the wild, the attendant problems are similar. The drugs must be administered via intramuscular injection and from a distance. For this reason, drug volumes must be small and choices are restricted to alpha-2 agonists, benzodiazepines, opioids, and dissociative classes of drugs. All of these drug groups have been used success-fully; however, mortalities have also been associated with all drug groups. The reversibility of alpha-2 ago-nists, benzodiazepines, and opioids has made them the preferred drugs.

MeperidineBased on early studies within marine mammals, meper-idine was shown to be an effective sedative and immo-bilization drug in a variety of species (Joseph & Cornell 1988). Meperidine was administered by hand injection to 10 walruses at a dosage of 0.23 and 0.45 mg/kg. Sedation/restraint was moderate without apparent det-rimental effects. If used as an analgesic, meperidine was associated with obvious sedation (Cornell 1978). As a result of clinical experience, a higher dose of meperi-dine (2.2 mg/kg) is frequently used to immobilize captive walruses when combined with midazolam (0.1 mg/kg). It is recommended, because of the high incidence of bradycardia during sedation, that atropine (0.04 mg/kg) IM, SQ, or IV be included in the immobi-lization technique (Klein et al. 2002). A long needle (3–4 in.) is necessary for effective drug injection to ensure rapid complete absorption.

Potent Opioids +/– alpha-2 agonistsThe use of highly potent opioid analgesics for immo-bilization of walruses has proven to be less than ideal. Complications include apnea, muscle spasms, rigidity, and death. Adult male Atlantic walruses on a land haul out were effectively immobilized with carfentanil with an intramuscular dose of 2.7–3.0 mg. Muscle spasms were associated with the onset of immobilization. All of the animals had extended periods of apnea; tracheal intubation was not possible due to muscle rigidity (Lanthier & Stewart 2002; Lanthier et al. 1999). The estimated effective dosage range was 4–5 μg/kg.

Many factors make accurate drug dosing difficult in field situations, the body weight can only be estimated prior to drug administration, dart placement, and vas-cularity of the injection site all affect the ultimate success of remote drug delivery. Pacific walruses injected with similar carfentanil dosages were not adequately immobilized despite the presence of apnea and muscle spasms.

A field study was preformed to test combinations of the alpha-2 agonist medetomidine with opioids carfen-tanil in adult Pacific walruses. Medetomidine was selected because of the presence of an effective antago-

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ciated muscle relaxation appears to facilitate spontane-ous ventilation. Similarly, ultrashort-acting barbiturates (thiamylal sodium) have been successfully used to produce general anesthesia in walruses. Dosages of 3.3–4.8 mg/kg with 1/2 of the calculated dose adminis-tered initially and the additional drug given to effect as needed (Walsh et al. 1990).

Volatile anestheticsIsoflurane has been used to maintain general anesthe-sia in walruses. However, because of the low blood : gas solubility of sevoflurane, both uptake and elimination will be faster and may improve the ability to reach surgical anesthesia and have shorter recovery times than with isoflurane. Since the cardiovascular and respiratory depressive effects of isoflurane and sevoflu-rane are similar, the lower solubility of sevoflurane would be of significant advantage in walruses. Oxygen flow rates and vaporizer settings will be similar to equine anesthesia. Oxygen should be set at >4 L/min to ensure delivery of anesthetic and oxygen for meta-bolic needs. Initial vaporizer settings will likely be between 3.0 and 4.5%; however, vaporizer settings should be reduced in anticipation of equilibration of the animal with the anesthetic machine. Maintenance levels of 2.0–2.5% isoflurane and 2.5–3.0 sevoflurane are expected.

anaLGESIa

Analgesic methods used in other marine mammals should be considered to decrease stress associated with painful procedures or conditions. As previously dis-cussed, local anesthetics are effective if injected around the surgical site or on sensory nerves to a region. As with other species, total local anesthetic dosages should be kept below 4 mg/kg to avoid potential local anes-thetic toxicity. Opioid analgesics have been used as a part of chemical immobilization techniques are believed to provide reduction in pain perception, mod-ulation, and transduction. Chronic pain management has not been addressed in walruses and no recommen-dations can be made for the use of anti-inflammatory drugs at this time.

REFEREnCESCornell LH. 1978. Capture, transportation, restraint and marking.

In: Zoo and Wild Animal Medicine (ME Fowler, ed.), pp. 573–580. Philadelphia: WB. Saunders.

Cornell LH, Joseph BE. 1985. Anesthesia and tusk extraction in walrus, Abstracts. Annual Proceedings of the American Associa-tion of Zoo Veterinarians, p. 97.

DeMaster DP, Faro JB, Estes JA, Taggart J, Zabel C. 1981. Drug immobilization of walrus (Odobenus rosmarus). Canadian Journal of Fisheries and Aquatic Sciences 38:365–367.

Fay FH. 1969. Structure and function of the pharyngeal pouches of the walrus (Odobenus Rosmarus L.). Extrait de Mammalia 24(3):361–371.

opioid antagonist. A naltrexone dose of 0.1 mg/kg of the estimated body weight has been used in field immobilization situations with complete reversal occurring in less than 5 minutes. If the immobilization technique includes the use of an alpha-2 agonist such as medetomidine, atipamezole should be administered if full immediate recovery is desired. A 5 : 1 ratio of atipamezole to medetomidine is recommended for complete reversal.

Dissociative anestheticsTiletamine and ketamine have been used to immobilize free-ranging walruses. The principal problem associated with dissociative chemical restraint is the long duration of effect and the lack of a reversal agent. Since walruses are often immobilized near water, it is imperative that conditions prevent the partially anesthetized animal from entering water.

Tiletamine and zolazepam (Telazol or Zolatil) at a dose of 1.4- to 2.2-mg/kg IM have been studied for chemical restraint of walruses. Induction time ranged from 14 to 29 minutes, and the duration of immobiliza-tion lasted from as short as 75 minutes to as long as 220 minutes. The induction and recoveries were reported to be smooth. Apnea was not reported although one of the three animals died during recovery (Griffiths et al. 1993).

Atlantic walruses have also been immobilized with a combination of Telazol and medetomidine. A total dose of Telazol and 100 mg of medetomidine (approxi-mately 3.3 mg/kg Telazol and 17 μg/kg medetomidine) were administered by dart to two animals. The respira-tion rate varied between 10 and 12 breaths/min throughout the procedure. Despite oxygen supplemen-tation via a nasal cannula, pulse oximetry showed rela-tively low oxygen concentration (37–79%) (Lanthier & Stewart 2002). Atipamezole was used to antagonize medetomidine. Three additional walruses (one male and two female) were injected with a total dose of 2 gm of ketamine and 90 mg of medetomidine (approxi-mately 2.5–4.0 mg/kg of ketamine and 11–18 μg/kg of medetomidine). In each case, the respiration rate appeared to be normal (rate and depth). Immobiliza-tion occurred in 15 minutes. None of the walrus showed muscle spasms. Procedure length varied between 16 and 40 minutes. Atipamezole was injected intrave-nously and intramuscularly 50/50 as the antagonist (approximately 9.5–19.0 μg/kg). Recoveries were smooth and rapid (6–23 minutes) (Lanthier & Stewart 2002).

PropofolThe author has used small boluses of propofol (40–60 mg) via the EIV sinus to provide unconscious-ness, muscle relaxation, and to facilitate endotracheal intubation. The use of propofol decreased the need for higher doses of the immobilization drugs and the asso-

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Lanthier C, Stewart REA. 2002. Chemical restraint of walruses: a Canadian perspective. Report of workshop on the chemical restraint of walruses edited by Chadwick V. Jay, pp. 5–7, San Diego, U.S. Geological Survey.

Lanthier C, Stewart REA, Born EW. 1999. Reversible anesthesia of Atlantic walrus (Odobenus rosmarus rosmarus) with carfentanil antagonized with naltrexone. Marine Mammal Science 15:241–249.

Lynch MJ, Tahmindjis MA, Gardner H. 1999. Immobilization of pinniped species. Australian Veterinary Journal 77:181–185.

Mulcahy DM, Tuomi PA, Garner GW, Jay CV. 2003. Immobiliza-tion of free-ranging male pacific walruses (Odobenus Rosmarus Divergens) with carfentanil citrate and naltrexone hydrochlo-ride. Marine Mammal Science 19:846.

Stetter M, Calle PP, McClave C, Cook RA. 1997. Marine Mammal Intravenous Catheterization Techniques. Proceedings of the American Association of Zoo Veterinarians, pp. 194–196.

Tuomi P, Mulcahy D, Garner G. 2002. Immobilization of Pacific walrus (Odobenus rosmarus divergens) with carfentanil, nal-trexone reversal and isoflurane anesthesia. Proceedings of the WSAVA Congress.

Walsh MT, Asper ED, Andrews B, Antrium J. 1990. Walrus biology and medicine. In: CRC Handbook of Marine Mammal Medicine: Health, Disease, and Rehabilitation (LA Dierauf, ed.), pp. 594–595. Boca Raton: CRC Press LLC.

Gales NJ. 1989. Chemical restraint and anesthesia of pinnipeds: a review. Marine Mammal Science 5:228–256.

Garlich-Miller JL, Stewart REA. 1999. Female reproductive pat-terns and fetal growth of Atlantic walrus (Odobenus rosmarus rosmarus) in Foxe Basin, Northwest Territories, Canada. Marine Mammal Science 15:179–191.

Geraci JR. 1978. Marine mammals. In: Zoo and Wild Animal Medi-cine (ME Fowler, ed.), pp. 523–552. Philadelphia: W.B. Saunders.

Griffiths D, Wiig Ø, Gjertz I. 1993. Immobilization of walrus with etorphine hydrochloride and Zoletil®. Marine Mammal Science 9:250–257.

Gruber AD, Peters M, Knieriem A, Wohlsein P. 2002. Atheroscle-rosis with multifocal myocardial infarction in a pacific walrus (Odobenus Rosmarus Divergens Illiger). Journal of Zoo and Wildlife Medicine 33(2):139–144.

Hills S. 1992. The effect of spatial and temporal variability on population assessment of Pacific walruses. PhD Thesis, Univer-sity of Maine, Bangor, Maine, p. 217.

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Klein L, Calle P, Raphael B, Cook R. 2002. Anesthesia in captive pacific walrus. Report of workshop on the chemical restraint of walruses edited by Chadwick V. Jay, pp. 18–25, San Diego, U.S. Geological Survey.