First-ever seal disentanglement with new darting technique

Rescuing any animal from an entanglement is a challenge, regardless of the species. Wary of our presence and unaware of our good intentions, they respond with fight or flight, and we are forced to capture them, restrain them, assess their health, and quickly extricate them from the life-threatening entanglement.

This challenge is only further complicated when the focus of a rescue effort is a marine mammal, such as a seal.

In our response area, the seals haul out onshore in large groups and flush back into the water with the slightest disturbance. If we are to tranquilize them, we need the seal to be sedate enough to catch, but not so sedate that it fails to swim to the surface to breathe.

Now, with a new, innovative technique in which we use a sedation dart with an integrated acoustic tag, we have successfully darted and tracked a gray seal (Halichoerus grypus) and rescued it from its life-threatening entanglement.

I have devoted a lot of my professional life to disentangling marine animals – primarily large whales and sea turtles, but many seals, too—and I see this as a major milestone for our work.

Early Wednesday morning, our IFAW team left the docks in Chatham in three boats loaded down with specialized disentanglement and veterinary equipment, along with a talented team of biologists and veterinarians from The Marine Mammal Center, the Center for Coastal Studies, National Marine Life Center, and National Oceanic and Atmospheric Administration.

We were on a mission to locate and rescue an entangled seal before it succumbed to its deadly entanglement. It wasn’t long before we found the right animal. A light-colored, small female on the edge of the haul-out would give us our best shot.

The team visually estimated her weight from a distance so that the veterinarians could determine the right amount of drugs to use. We estimated that she was approximately 220 pounds. The pressurized dart gun was loaded and we slowly eased our boat to within range of the hauled out seals.

Too close and we would flush the seals before the shot. Too far away and the dart would not have enough power to properly inject the drugs.

A perfect shot! We immediately placed a hydrophone in the water to listen for the dart’s signal. The specialized dart, conceived and developed by The Marine Mammal Center, integrates a small acoustic transmitter into the sedation dart.

The transmitter emits a pinging sound that can be picked up by the hydrophone, allowing us to track the seal when it dives.

Time, of course, was still of the essence. We had about one hour to capture the seal and remove her entanglement before the drugs would wear off and she would again be uncatchable.

Upon the dart’s impact, the seal flushed into the water along with some of the surrounding seals. At first, her signal was clear and then it suddenly disappeared.

These twenty quiet minutes of searching seemed like an eternity, but a recent acoustical tracking experience made us think of something:

She must have gone around the sandbar!

We were right. As we rounded the corner, we could hear the pinging again on the hydrophone.

Now was the time to catch her. Our first choice was a hoop net about three feet in diameter with a five foot handle, but the seal was avoiding our boat and we couldn’t get close enough to net her this way. So we next deployed our seine net (300-foot long, 20-foot deep) from the skiff and quickly maneuvered to encircle her.

Four of us worked together to pull the seal onboard and we quickly motored to nearby North Beach.

We carried her to land and the team immediately began to carefully remove the monofilament fishing net that was wrapped around her neck using a scalpel and hemostats.

We applied antibiotic ointment to her deep, open lacerations, gave her an injection of long-lasting antibiotics to help with the infection and performed a health assessment. The team closely monitored her throughout the procedure; her heart rate and respirations were normal for the circumstances and the in-field blood analysis looked good. The veterinarians determined that we had caught the entanglement in time and that she could be released on scene.

We affixed identification tags to both rear flippers and attached a small temporary satellite tag to her so that we could monitor her recovery.

We watched as she returned to the water. Later we decided that we would name this seal “Sausalito” in honor of the California town where The Marine Mammal Center is located, and for the amazing help we have received from the TMMC team.

The tracking looks promising so far.

As I write this, we have a team on the water heading  to her most recent known location in one of the main haul-outs near Strong Island in Chatham to monitor “Sausalito” and she how her wounds are healing.

The battery should last for about four months before she sheds the tag this winter with her molt, providing us with copious data to help inform our understanding of the movements and behaviors of gray seals.

This is a significant step forward in responding to entangled seals locally.  The information we gather on drug efficacy, capture tools and techniques can also help other entangled animals around the US and in other parts of the world.

--BS

All activities were performed under NMFS permit #18786 and supported in part by a John H. Prescott grant.

For more on the NOAA Marine Mammal Health and Stranding Response Program, click here

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