Seal darting research progress - a new tool for helping cape fur seals

Cape fur seals are playful and curious animals. Because of this, they are prone to becoming tangled in loops of plastic pollution or discarded fishing line, which they find interesting, or which may smell like food. Over time, these nooses work their way down a seal’s neck, becoming tighter, incising into their skin and flesh, and ultimately resulting in severe infections and death.

Human intervention is the old way to disentangle these seals, but relies on a large degree of stealth or luck for a rescuer to be able to approach and help a seal without being injured or the seal fleeing into the water – a problem that is especially difficult to overcome in complex, built environments like the V&A Waterfront. A tool that sea rescuers have long wanted is safe tranquiliser darts that can be used on hard-to-reach seals – but these carry their own set of risks, and until recently, little information existed to help rescuers.

Anaesthetising seals is considered higher risk than darting a land mammal. Because seals are always near to water, there is the risk that a partially sedated seal could escape into the water and potentially drown or injury itself. Seals are also physiologically adapted to life in the water, and it has been unclear how these adaptions might influence the effectiveness of common anaesthetic drugs and their reversal agents.

The Two Oceans Aquarium Foundation has, over the past two years, worked closely with veterinarian Brett Gardner of Zoos Victoria, and the Department of Forestry, Fisheries and Environment to trial a protocol for the use of sedative darts and stimulants on high-risk Cape fur seals. The paper Disentanglement of Cape fur seals (Arctocephalus pusillus pusillus) with reversible medetomidine-midazolam-butorphanol published in the Journal of the South African Veterinary Association is a reflection of this collaboration.

How did the study work?

Based on a best-understanding of fur seal sedation, from the review of similar studies on the South American fur seal, New Zealand sea lion and California sea lions, draft protocols for the sedation of Cape fur seals were drawn up. No veterinary guidelines existed for this type of work with seals in South Africa, and this work appears to be the first-ever use of this drug on wild Cape fur seals.

The study was done on eight Cape fur seals identified by the seal patrol in the V&A Waterfront and by members of the public in the Hout Bay harbour. When a seal was darted, two boats were kept in the area – a smaller one to follow it and confirm that the dart hit, and a larger boat that would allow a veterinary support team and scuba divers to enter the water to recover the seal should the sedation kick in while it was swimming, free it of entanglement and administer the reversal agent.

What were the results of the study?

The study found that the sedative took approximately 4 minutes to act on a seal, that the average time from darting to the handling of the seal was 20 minutes, and that it took seals about 27 minutes to fully rouse after being given the reversal agent. The study also revealed that partially sedated seals could be stimulated by noisy neighbours and by the approaching boats, so protocols for the administration of additional sedative were necessary.

Of the eight high-risk seals that were part of this study, six were successfully retrieved and disentangled, and one could not be sufficiently sedated, but was able to fully recover without the need for the reversal agent. A final seal, unfortunately, was able to hide or escape after being administered the sedative and could not be found by the divers deployed to recover it – this, unfortunately, was found dead several days later, but an independent necropsy could not determine the cause of death. Despite this unfortunate death, these initial findings do suggest that the use of MMB has a lower mortality rate than other drug combinations used in previous Cape fur seal studies, and it’s important to keep in mind that these seals would not be able to survive without an attempt to free them of their entanglements.

In short, this study proved that the dosages for sedative and reversal agents were reliable, and has helped to better define a safe protocol for the darting and retrieval of severely injured and entangled Cape fur seals.