The ancient Norse sagas are filled with mythical creatures, but some of these sea monsters may have been whales displaying behaviours that have modern-day scientists mystified.

In 2021, a social media post of hundreds of anchovies jumping into a whale’s gaping mouth as it was treading water in the Gulf of Thailand went viral.

The whale in the social media post, a species known as Bryde’s whale (Balaenoptera brydei), was using a technique known as trap or tread water feeding. 

This behaviour was first documented 10 years earlier in a pair of humpback whales (Megaptera novaeangliae) off the west coast of Canada.

“It was assumed that this was a new technique that whales had just developed,” said Erin Sebo, an expert in historic manuscripts at Flinders University.

But an examination of ancient texts by Dr Sebo and colleagues published today in the journal Marine Mammal Science, suggests medieval seafarers may have witnessed the behaviour and woven it into stories of a sea monster called the Hafgufa more than 1,000 years earlier.

Dr Sebo said she hoped the information found in the manuscripts could help us better understand whale behaviour.

“By finding these descriptions in medieval and ancient manuscripts, we were actually able to give scientists a lot of evidence that they assumed was lost.”

Stories of sea monsters 

The first hints of a sea creature that trapped fish appeared in Greek manuscripts from Egypt around 300 AD, which were copied and embellished in several languages for centuries after.

An Icelandic story from 1200 AD starts: Es hualr i sø̨ er heiter aspedo (there is a whale in the sea called the aspido).

It is believed this formed the basis of 13th-century depictions of the Hafgufa, which was said to emit a smell and trap fish in its mouth, in a text known as Konungs skuggsjá (the King’s Mirror).

“The Norse sagas are particularly accurate and probably reflect the skill of Viking seafarers,” Dr Sebo said.

By the 17th and 18th centuries the legendary creature had grown — literally — to be the size of a small island.

Many people argued the Hafgufa was an optical illusion caused by an underwater volcano, or light at a particular time of year interacting with different atmospheric conditions.

“We have been able to demonstrate fairly clearly that it’s a whale, and also that medieval people recognised that it was a whale.”

Sit, wait and gulp tactic

Humpbacks and Bryde’s whales, which are a subgroup of baleen whales known as rorquals, use a number of feeding techniques.

Most commonly they turn onto their side, open their mouth and lunge at their prey, explained Vanessa Pirotta, a whale expert at Macquarie University. 

“Baleen feeding 101 is essentially engulf your prey and water at the same time, then get rid of the water later on,” Dr Pirotta, who was not involved with the study, said.

Humpbacks in Antarctica and the east coast of Australia also work in groups to blow bubbles that encircle their prey in a technique known as bubble-net feeding.

But where these two techniques are very active, trap or tread water feeding is a very passive sit, wait and gulp tactic, she said.

Just why the fish jump into the whale’s gullet is a mystery.

Takashi Iwata, who identified the first instance of the behaviour in whales in the Gulf of Thailand in 2017, believes fish in the top layers of the ocean may be pulled in by a current created by the shape of the whale’s mouth.

Another idea is that the whale may belch undigested food that attracts the fish, and gives off a smell like that described by the ancient mariners.

“It’s an interesting idea but there is no clear evidence,” Dr Iwata of the University of Japan said.

Others suggest the fish may be tricked into seeking sanctuary in the whale’s dark maw from seabirds.

Why do whales do this?

So far the technique has only been seen in humpbacks off the coast of Vancouver and Bryde’s whales in the Gulf of Thailand as well as in Beibu Bay in China.

But the behaviour is becoming more common in all these areas.

While only two humpbacks used the technique in 2011, there have now been 30 sightings off the west coast of Canada, said Christie McMillan of the Marine Education and Research Society, which has been monitoring the whales.

“We believe the whales are learning the strategy from each other,” she said.

In Thailand, growing numbers of whales are seen using the technique throughout the year, but particularly in October to November.

Dr Sebo said one reason scientists might only just be seeing this behaviour is the huge dip in whale populations that occurred in the 19th century. 

“It is really interesting that ancient and medieval people were seeing enough whales and getting close enough to whales to actually be able to observe this behaviour as accurately as they did when modern people haven’t,” she said.

Dr Pirotta said looking to the past and to First Nations knowledge of whale behaviours prior to whaling could be very powerful, while scientific observations provided an understanding of what whales were doing now.

It’s not the first time that new behaviour has suddenly been spotted in humpbacks recovering from whaling.

Just two years ago, mass supergroups were detected bubble-net feeding off the east coast of Australia for the first time.

“If [trap feeding] is popping up independently like bubble-net feeding, and we’re seeing two different populations doing it, there must be some motive or mechanism as to why the animals are choosing to do this,” Dr Pirotta said.

Dr Iwata believes the whales in Thailand and China could be using it to skim scarce food from the top of the ocean in increasingly polluted areas.

Meanwhile, Dr McMillan said whales in Canada tend to use the technique more when schools of fish are smaller, and less dense.

“We cannot say whether the increasing number of individuals using trap-feeding is due to declines in herring in this area, or any other environmental changes, but this is something that we are continuing to look into,” she said.