By Daniel Cressey

Only around 300 endangered right whales remain in the North Atlantic, and a number of them end up tangled in fishing gear off the east coast of the United States. The nets and ropes cut into the animal's flesh, and can in rare cases lead to a painful and protracted death. Marine biologist Michael Moore, of the Woods Hole Oceanographic Institution in Massachusetts, is one of those who attempt to free these animals. He tells Nature about a new technique that aims to raise the success rate of disentanglement efforts by shooting whales with drug-filled darts.

Why did you start thinking about sedating whales?

It all started in 1999 when we had a right whale that was chronically entangled, with a fishing net wrapped around both flippers and over its back. We were unable to disentangle it, and it died months later with the blubber being dissected off the back by the net and the line cutting down into the animal. This was a case that struck to the heart of many of us, in terms of animal welfare and the pain the animal went through.

How would a team normally go about cutting free a whale?

They'll physically restrain the animal by drag--by putting more buoys on the animal, slowing it down and tiring it out so they can get near it. That works very well for humpbacks but it doesn't work so well for right whales.

Right whales are fundamentally less stoppable. They're stronger, they're attitudinal, they're bigger animals that are harder to work with. It became clear that the disentanglement crew would be very interested in chemically enabling the process [of slowing down the whale].

So what happened last year with whale number 3311, which had rope cutting into its head and lower lip?

We had a tracking tag so we knew where the animal was moving and we had about three days within a three-month period where we would gather in a hotel, get up early in the morning, launch the boat and also head out with a plane to relocate the animal.

You're working in a boat that's no longer than than 20 feet; it's a rubber boat with a tuna tower [a raised platform] on it. All kinds of things that are very easy to do on a lab bench suddenly become far more complicated when you're bouncing up and down in 3-foot seas.

Essentially one of the issues is to safely and efficiently load a syringe with a drug cocktail, get it into the barrel [of the delivery system] and get it all set up so that you can safely discharge this. It's quite a challenge (see video).

There was a real sense of preparedness, but also the unknown, because no one had ever done what we were aiming to do. We had endless conversations about what was going to happen, and we were about to find out.

What happened when you finally darted the animal?

We were very concerned that we only sedated and not anaesthetized this animal. There was a significant, calculated unknown.

We would only do this work on an animal that would otherwise die in a very painful way. If all we'd ended up doing was euthanizing it, then that in itself was not something we would regret.

It was very gratifying to see the animal doing exactly what we wanted, which was to change from a very evasive animal to the behavior essentially of a drunk, where we were able to drive near it in the boat without it veering away from us.

And that's when the disentangling crew in a second boat got to work?

Once they realized the whale was approachable, they immediately went into disentanglement mode to cut the gear off.

There are a number of varieties of how that works, depending on the entanglement. In this case, the rope was embedded so you couldn't really hook it. We used a spring-loaded knife. When we put the knife's release catch against the side of the whale, the knife popped in like a little guillotine by about 3 inches, so it could actually cut the blubber and get into the flesh to the embedded line.

Did it work?

We removed about 900 feet of line off the whale. We left a small amount embedded in the head because, had we tried to pull it out, we may well have caused a major fatal hemorrhage.

The operation was a success. Did the patient live? We do not know. The weather was deteriorating rapidly so essentially we had to kiss the whale goodbye and say good luck. We have not had a sighting dead or alive of that whale ever since. We're waiting and hoping that someone will see it, but we don't know.

Are you ready to do the same thing again?

We've got a new system. We've got more hardware and we've got more drugs. We're more ready than we ever were.