University of Tasmania
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Chemical restraint of southern elephant seals (Mirounga leonina)

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posted on 2023-05-27, 13:07 authored by Woods, Rupert

This study examined the use of anaesthetic agents in southern elephant seals (Mirounga leonina). The aim was to improve their safety especially when used for the routine chemical restraint required for biological investigations. This was to be achieved by more accurately refining drug dose rates, determining appropriate drugs to use and developing techniques for the early recognition and treatment of complications.

Monitoring to detect complications early A system of monitoring was developed from reflexes and responses used to measure the depth of anaesthesia in terrestrial animals. Heart rate, respiratory rate, rectal temperature, head response, palpebral response, withdrawal response, caudal flipper response, muscle top.e, righting response, and capillary refill were used as indicators of anaesthetic depth and the status of the animal's respiratory, cardiovascular, and thermoregulatory systems. It was more difficult to elicit responses when pethidine based drug combinations were used, or with large seals (> 1000 kg), and apnoea made assessment of level of restraint unreliable. However, the responses measured usually followed predictable sequences as level of restraint increased and the system allowed early recognition of complications and comparisons to be made between animals restrained with different drug combinations.

Methods to more accurately estimate animal masses and anaesthetic drug doses Length-mass relationships were established by linear regression of data gathered from southern elephant seals which had been weighed and measured at Macquarie Island, South Georgia and Heard Island. The relationships differed between locations and with sex and stage of yearly cycle. Although the variability was often large, these relationships offer the best current estimate of mass based on morphometric data and can be used to improve the accuracy of drug dosage.

For certain well defined categories of seals, nominal dosages of ketamine and diazepam were determined which could be administered when an accurate assessment of mass is not possible. Nominal masses, snout-tail lengths, as well as respiratory rates, heart rates and other variables used during monitoring were found for each category of seal. This improved the ability to accurately assess the anaesthetic episode by allowing recognition of abnormal responses and facilitated their early treatment.

Pharmacokinetic studies were carried out in animals at different stages of their yearly cycle. Ketamine concentrations were measured in venous blood after administration of a single intravenous bolus of ketamine to animals which had been sedated with pethidine. The data indicate that apparent volume of distribution, total body clearance and half life may not vary with stage of yearly cycle and that additional intravenous ketamine dosage is best based on lean body mass. Recommendations were developed for additional drug administration which enable more precise drug dosage, and lessen the problem of inappropriate drug dosage.

Finding the most appropriate anaesthetic drug or combination of drugs A comparison was made of the responses of groups of seals to the commonly used anaesthetic agents ketamine combined with diazepam, xylazine or midazolam, and tiletamine combined with zolazepam. It was found that all of the drug combinations could be safely used to restrain southern elephant seals. However, for people with little experience with anaesthetics, tiletamine and zolazepam offered some advantages over other combinations with respect to formulation and predictability. However, the results indicated that the safety margin for tiletamine plus zolazepam may not be high and its use should be approached with caution in animals whose mass is not known.

Other anaesthetic drugs which had not previously been administered to southern elephant seals, but had proved useful in other animals, were also examined. These were combinations of midazolam, pethidine, ketamine and thiopentone, and medetomidine combined with ketamine. Pethidine based combinations were very useful and versatile, and were rapidly reversible with naloxone or naltrexone. However, in some cases apnoea was induced after administering ketamine or thiopentone intravenously and the time to maximum sedation was longer than when using cyclohexamine based combinations. Hyperthermia, vomiting and unpredictable responses were associated with the use of medetomidine combined with ketamine and for this reason ketamine combined with xylazine appears preferable.

Treatment of complications A study was carried out to detennine whether antagonists could be used to improve control of the anaesthetic episode. Yohimbine antagonised ketamine combined with xylazine, but a,ntagonism was unnecessary for the cyclohexamine based drug combinations (those containing ketamine or tiletamine) due to the normally rapid recovery from these agents. The use of 4-aminopyridine and sannazenil to antagonise ketamine combined with diazepam appears to be contraindicated as their administration was associated with prolonged recovery or blindness. Antagonists were not effective in reversing apnoea.

An attempt was made to treat apnoea by firstly determining an effective dose of the respiratory stimulant doxapram in breathing animals and then administering it by a variety of routes to apnoeic animals. Doxapram was not always effective, however endotracheal administration appeared to be of benefit in some cases and is worth trying if facilities to administer positive pressure ventilation are not available. Supportive techniques such as intubation and positive pressure ventilation remain the treatment of choice for apnoea. Pharmacokinetic studies of ketamine in apnoeic and breathing seals indicated that apnoea affects apparent volume of distribution and total body clearance of this drug during anaesthesia. Recommendations were developed for the prevention and treatment of apnoea.

This study fulfilled its aims of improving anaesthetic safety for southern elephant seals. However, it is still difficult to treat apnoea in large southern elephant seals and techniques for positive pressure ventilation in these animals need to be determined.



  • PhD Thesis


xxv, 225, 22 pages


School of Pharmacy


University of Tasmania

Publication status

  • Unpublished

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Date of Event (Start Date)


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Copyright 1995 the author.

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