In 2007, Professor Jonas Teilmann from the Department of Ecoscience at Aarhus University traveled to Iceland with a team of colleagues. Their plan was to capture minke whales and measure their hearing in the icy waters along the coast.

But that was easier said than done.

Although minke whales are the smallest of the baleen whales – a kind of little brother to the giant blue whales – it is not easy to catch them and keep them still and calm while subjecting them to a hearing test.

And back in 2007 it failed.

For the next 15 years, no one attempted to measure the hearing of minke whales. Until Jonas Teilmann and his colleagues tried again.

Last year, a research team from Denmark, the US, and Norway succeeded in capturing and measuring the hearing of two minke whales. Notably, without the whales suffering any harm. When the result appeared on the computer, however, the researchers were surprised.

- No one has measured the hearing of a baleen whale before, and we actually didn't think they were able to hear high-frequency sounds. When they communicate, it's in the low-frequency range. But it turns out that they hear much better than we humans do, says Jonas Teilmann.

Baleen whales are the only group of marine mammals never to have their hearing tested. Until now. In the past, researchers have tested the hearing of seals, toothed whales, otters, and sea cows.

This makes the new results very important, says the professor.

- Anatomical studies of baleen whales have previously indicated that this group of whales could not hear high-frequency sounds. Now we show that those studies were wrong. This means that we need to think more about the noise we make in the ocean, because motorboats, ferries, and the installation of wind turbines may disturb minke whales more than we previously thought.

Given that it took Jonas Teilmann and his colleagues 15 years to successfully capture and measure the hearing of a minke whale, it's clear that catching a whale is quite a feat. Especially when you don't want to harm it.

Today minke whales are still hunted and consumed in Norway, Iceland, and Greenland. The species is not endangered, and the hunters legally hunt them for their meat and blubber.

However, catching a minke whale without using a harpoon is a completely different matter, explains Jonas Teilmann.

- It was very important to us that the whales wouldn’t be harmed in any way. Therefore, we spent a lot of time figuring out a safe way to capture them.

Every June for the past four years, the research team has been in Lofoten, Norway, studying the whales. At that time of the year, minke whales migrate north along the coast, and many of them swim through a narrow strait between two islands in the archipelago. It’s one of the only places in the world where minke whales swim so close to the shore, making it the perfect spot to catch them, he explains.

- The first year we spent monitoring their swimming habits so that we knew how to place the nets to gently catch them, he says.

The passage between the two islands is so narrow that the researchers could place a net across it. When the whales swam between the islands, the researchers closed the net behind them from two motorboats.

- Typically, we took turns scouting the ocean from the cliffs, keeping an eye out for whales. When a whale was spotted, we coordinated the effort over the radio. We had 24-hour surveillance to catch the few whales that fell into the trap and to be able to react quickly if the whales were to be injured in the net, he says.

Once the net was closed behind the whale, the motorboats slowly followed the whale and gently nudged it into a section of the now-closed basin where another net was placed. The researchers had borrowed this net from a local salmon farmer.

With the whale safely in the salmon net, the researchers would slowly raise the net and lift the whale to the surface of the water by hand. Here they attached a suction cup with an electrode and measuring equipment on the whale.

- We had two veterinarians with us who constantly monitored the whale's well-being. They looked for stress symptoms like irregular breathing, whether it was shaking or opening its eyes and mouth wide. Several times we had to stop measuring because some of the whales were simply too agitated. When that happened, we released it again, he says.

Jonas Teilmann and his colleagues ended up measuring the hearing of two whales, while they had to release three before they managed to measure anything.

How do you measure a whale's hearing? You can't, as we do when testing humans, ask it if it could hear this or that sound.

Well, the principle is the same as when measuring the hearing of infants, who can't say whether they heard something or not, explains Jonas Teilmann.

- We measured what's called the auditory brainstem response (ABR). We send a sound to the whale's ear and then measure its brainwaves, which emit small electrical signals. When the whale registers a sound with its ear, it sends a signal to the auditory center of the brain. And that's the signal we're trying to pick up.

If the whale hears the sound, the signal from the ear to the brain will appear in the researchers' measurements. This way, by playing sounds at different frequencies, the researchers can learn more about what the whales are actually capable of hearing.

And this is where they were surprised.

- We started with the low frequencies and worked our way up. We were surprised that the whales were able to hear such high frequencies. In fact, they are capable of hearing higher frequencies than humans. With that in mind, we need to consider the baleen whales when we’re at sea, he says.

On top of that, examining the seabed to assess if it’s stable enough for wind turbines to be installed is very noisy. There are a whole lot of activities that could potentially affect the minke whales

- Jonas Teilmann, professor at the Department of Ecoscience

Although the minke whale primarily lives further north, it is also found in Danish waters. In the North Sea, the whales are often observed around oil platforms, but it happens that they stray into the inner Danish waters.

With the whales being near human activity we need to consider which types of activities we do at sea, says Jonas Teilmann.

- The military uses high-frequency sounds in search of submarines, fishing vessels use sonar to hunt for fish, and the oil industry uses high-frequency sounds when mapping the subsurface in the search of new oil fields, he says, continuing:

- On top of that, examining the seabed to assess if it’s stable enough for wind turbines to be installed is very noisy. There are a whole lot of activities that could potentially affect the minke whales and probably other baleen whales such as the blue whale or humpback whale as well. That’s something we haven’t been aware of. 

How much baleen whales are affected by the high-frequency sounds, we do not yet know. That is something we must investigate in the future, concludes Jonas Teilmann.