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Blue Planet II

Uncovering the secrets of “the boiling sea”

The boiling sea is a rarely witnessed phenomenon. A feeding frenzy that is the stuff of ocean legend.

The boiling sea is a rarely witnessed phenomenon. A feeding frenzy that is the stuff of ocean legend.

Small fish are rounded up and driven to the surface by larger predators and as these hunters close in on their prey, crashing through the ocean’s surface, the sea turns a bubbling white; creating the illusion that it is boiling.

After months of searching, the Blue Planet II team became the first to capture this unusual spectacle on camera in full. Not only did they but the footage has unlocked new information around mobular ray feeding behavior.

The Blue Planet II team off Costa Rica, in their efforts to film the fabled the 'boiling sea' (Credit: Erik Beita Cortez)
The Blue Planet II team off Costa Rica, in their efforts to film the fabled the 'boiling sea' (Credit: Erik Beita Cortez)

The biggest challenge facing the crew was simply finding their subject – the prey. Despite being one of the most populous and widely distributed fish in the ocean, the tiny lanternfish (Symbolophorus barnardi) proved difficult to find. They live in the deep sea during the day, but at night they rise into the warmer shallow waters to feed and at certain times of the year they rise during the day to spawn.

“We launched a big expedition to the coral sea off the north east coast of Australia, and spent three weeks at sea and didn’t see a splash,” said series producer Mark Brownlow.

On the first attempt to film the feeding frenzy the crew weren’t successful owing to unseasonably high sea temperatures off Australia, which affected the expected spawning behavior of the lantern-fish. Subsequently the team would discover this was the beginning of an El Nino weather event.

Mobula rays feed mainly on plankton, but the Blue Planet II filmed them for the first time eating fish (Credit: BBC 2017)
Mobula rays feed mainly on plankton, but the Blue Planet II filmed them for the first time eating fish (Credit: BBC 2017)

Eighteen months later, the team heard reports of sightings of a ‘boiling sea’ from the other side of the Pacific, just off Coast Rica. This time they decided to hunt the predators rather than prey – the spinner dolphin (Stenella longirostris).

Working from the research vessel Umbra, the aim was to film the feed from below, capturing the dolphins corralling the lanternfish, driving them tighter and tighter together and ever closer to the surface. Cameraman Roger Munns captured the underwater action, while Brownlow was able to capture the drama from above.

After weeks of searching, Brownlow said it was a massive relief to finally witness the phenomena.

“Spotting them was like finding a needle in a haystack,” said Brownlow.

“Through a combination of aerial filming and an underwater team we were able to capture this epic phenomenon.”

The team frequently spotted signs the 'boiling sea' only to find the action was all over by the time they dived in (Credit: Erik Beita Cortez)
The team frequently spotted signs the 'boiling sea' only to find the action was all over by the time they dived in (Credit: Erik Beita Cortez)

The spinner dolphins began the fest and were soon joined by yellow tuna. The surprise was that mobula rays (Mobula tarapacana) also arrived in large numbers to devour the lanternfish. It had previously been thought that mobula rays only fed on plankton.

Earlier scientific papers suggested that small fish had been found in the stomachs of dead mobula rays, but it had been assumed these fish had been eaten by accident until the Blue Planet II footage was shared with Josh Stewart from The Manta Trust & Scripps Institution of Oceanography, University of California.

A new paper about their findings has been published. In a paper published in Ecology, Stewart confirmed it was the first time mobula rays had been witnessed actively targeting fish.

“Given the number of hours that the Blue Planet II film crews were out in the wild waiting for these spectacular events to occur, it's really no surprise that they captured tons of new behaviours that haven't been recorded before,” he said.

“Documentary productions like Blue Planet II are extremely important for communicating science and conservation messages to the public, and I'm very happy to see the inverse happening as well, where the documentary is helping to inform our scientific understanding of the oceans.” 

**Our Blue Planet is a collaboration between the BBC’s Natural History Unit and OceanX Media. Join the conversation on twitter; Follow us @OurBluePlanet

Find out more about the series by downloading the Blue Planet II podcast: http://www.bbc.co.uk/programmes/p05jv5zk**

By Martin Poyntz-Roberts & Kara Segedin