Abstract

There are few studies on chemical immobilization of leopard seals in the field, and it has become evident that, relative to other phocid species, it is frequently associated with high mortality rates (5–38%).^5,6,9 Due to certain anatomical and physiological characteristics such as lower haematocrit and haemoglobin concentration, lower myoglobin,⁸ smaller spleen,¹ a small aortic bulb,² a single caudal vena cava,⁷ and a collapsible trachea which puts them at risk of hypoxia under the influence of muscle relaxants - and along with the difficulties of an accurate body weight estimate³ - leopard seals anesthesia in the field represent a unique challenge.

Here we present the preliminary results of an ongoing study on leopard seals chemical immobilization, part of the U.S. Antarctic Marine Living Resources (AMLR) pinniped research program at Cape Shirreff (62°28'S, 60°46'W), Antarctic Peninsula. The general objectives for U.S. AMLR program are to monitor population demography and trends, reproductive success, and status of pinnipeds throughout the summer months. To better understand the role of leopard seals ecology within the region and their influence on krill-dependent predators, during the field season 2010–2011 we captured and instrumented 10 animals with time depth recorders (TDR) and ARGOS-linked transmitters and collected biological sample and morphometric measurements.⁴ We chemically immobilized 11 adult female leopard seals (some animals twice), weighing from 380 to 501 kg, with a combination of butorphanol (50 mg/ml, Zoopharm, USA) at 0.1–0.2 mg/kg and midazolam (50 mg/ml, Zoopharm, USA) at 0.1–0.2 mg/kg. The seals were approached when hauling out on the beach, at least 5–10 meters from the water. All animals were monitored for 5–10 minutes before drug delivery and basal physiological parameters (respiration rates and heart rates) were recorded. Body weight was estimated by the 5 team members separately and then decided collectively. The drugs were delivered with a 10-ml, 60-mm long needle Telinject Vario Syringe (Telinject, USA) from short distance with a dart gun (Telinject, USA) in the gluteal muscle. The induction time was on average of 10–15 minutes after injection. Vein access was obtained in all animals by placing a 16-G 5.25-inch needle (MILA International, USA) in the extradural sinus, and IV isotonic saline was delivered throughout the whole procedure at a maintenance rate of approximately 3 ml/kg/h. If necessary, additional 10-mg boluses of midazolam were delivered to maintain the appropriate anesthetic plan. A mask was used to deliver oxygen at 3–5 l/min throughout the procedures. During the anesthesia, respiration rates (mean 7 b/pm), heart rates (mean 77 b/pm), and blood pO₂ saturation (mean 98%) were monitored and recorded. Reversal agents, naltrexone (Zoopharm, USA) at 0.1 mg/kg, and flumazenil (Sandoz, USA) at 0.005 mg/kg, were used to antagonize respectively butorphanol and midazolam.

No mortality events were recorded and no emergency procedures, such as resuscitation, mechanical ventilation were needed. A maximum anesthesia time of 2 hours was achieved with the protocol presented. The animals tolerated well the anesthesia and were all seen in the same beaches the days after captures. This report suggests a novel protocol for safely anesthetizing leopard seal in the field.

Acknowledgments

The National Science Foundation provided support and transportation to the Cape Shirreff field site for the opening camp crew. We thank seabird biologists, Jefferson Hinke, McKenzie Mudge and Alison Larned, for their help with pinniped studies. We are likewise grateful to Anthony Cossio, Christian Reiss, and all the AMLR personnel, and the crew of the R/V Moana Wave for their invaluable support and assistance to the land-based AMLR personnel. All pinniped research at Cape Shirreff was conducted under Marine Mammal Protection Act Permit No. 774-1847-04 granted by the Office of Protected Resources, National Marine Fisheries Service.

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