Immobilization of captive and free-ranging pinnipeds is often required for biological studies, translocation or the examination of sick or injured animals. However, pinnipeds present unique problems when using chemical immobilization agents because they have evolved specific adaptations in their respiratory, cardiovascular and thermoregulatory systems enabling them to dive for extended periods. These adaptations can exacerbate problems associated with chemical immobilization procedures [1
]. This physiological "dive response" is characterized by profound bradycardia, shunting of blood away from peripheral tissues, and periods of prolonged apnoea [3
] that can be aggravated by the presence of immobilizing agents in the blood and tissues. This may result in relatively high concentrations of drug being transported to central organs, particularly the brain, which also affects the level of immobilization and recovery time [4
]. The physiological status of an animal has also been shown to have a profound effect on sensitivity to immobilization and on the ability to metabolize chemicals [5
]. Furthermore, the number of previous captures and immobilizations can increase recovery time [6
]. Therefore, knowledge of the physiological (e.g., total body lipid) and anatomical characteristics (e.g., in some species the trachea is flat and has incomplete cartilaginous rings which may increase the risk of respiratory obstruction), methods of administration, and species-specific response to particular drugs are important for the effective, safe and optimal application of chemical immobilization in free-ranging wildlife species.
Intramuscular (IM) injection has been one of the most commonly used routes for administration of immobilizing agents in pinnipeds [see 1
], and it is thought to be relatively safe and easy compared to other methods. Immobilization by intravenous (IV) injection has recently become more common with some species [6
]. Although physical restraint is required prior to the administration of drugs using IV methods, smaller doses and better control of the intensity and duration of immobilization are generally achieved compared to IM injection methods. Pinnipeds that received the same drugs by IV and IM injection have been reported to have shorter induction and recovery times and less variable responses when IV methods were used [8
Weddell seals (Leptonychotes weddellii
) are deep-diving (> 500 m) predators that have been the subject of many studies requiring immobilization. Many of these studies have reported varying responses to immobilizing agents [11
], and most have reported mortality rates ranging from 10 to 31 %, indicating that Weddell seals may be particularly sensitive. More recently, a safe method of gas anaesthesia (zero mortality) of Weddell seals has been reported by Kusagaya & Sato [16
] and Bodley et al
] (n = 9, n = 11, respectively); however, this procedure is not always practical for field situations due to the cumbersome equipment required. Therefore, a reliable, safe and direct technique of immobilization is still required for this species.
A 1:1 mixture of tiletamine and zolazepam (available commercially as Telazol®
, Fort Dodge, Castle Hill, Australia, or Zoletil®
, Virbac, Peakhurst, Australia) has been characterized by rapid, smooth induction, good analgesia (unresponsive to painful stimuli), maintenance of pharyngeal and laryngeal reflexes, and a smooth recovery phase [18
]. The tiletamine:zolazepam mixture has been used successfully to immobilize a range of domestic and wild mammals, including some pinniped species [6
]. In its experimental stage (2:1 ratio mixture of tiletamine and zolazepam), Telazol was effective with Weddell seals [14
]; however, some complications (i.e., apnoea leading to death) were encountered in a later study by Phelan & Green [12
]. This may have been due to the method of administration rather than the drug itself (see Discussion). The 1:1 mixture of tiletamine:zolazepam potentially offers a safe and effective method for immobilizing Weddell seals in the field.
In this study we investigated the use of tiletamine:zolazepam (Telazol®
) for Weddell seals. We compared performance between IV and IM injection, and examined the relationship between age, body condition (total body lipid, TBL) and stage of lactation on induction and recovery time. We hypothesized that variation in these parameters among individuals would influence drug sequestration and recovery time and that this may be more pronounced than at other stages in this species' life history due to the physiological pressures and energetic constraints of lactation [5
]. In addition, we examined differences in recovery time due to the number of previous immobilizations. We hypothesized that recovery time would decrease for individuals that had been chemically immobilized previously, as has been found in other species [6