Nature: Risso’s dolphins make twisting dives that allow them to ‘drill’ deep through the water

To gobble up fish, squid and crustaceans that live more than 600 meters deep in the ocean, Risso’s dolphins ‘drill’ through the water using a spinning diving technique.

This is the conclusion of experts led by the University of Amsterdam, who became curious after seeing the mammals turning on the surface of the water before diving.

To find out why, the team attached special sensors to seven dolphins off the coast of Terceira Island, in the Azores, and recorded a total of 226 dives.

They found that the dolphins — whose prey only comes close to the surface at night — use different types of dives depending on the depth they need to reach to feast.

The ‘spindive’, the researchers explained, allows them to hunt at depth while consuming a minimal amount of energy and oxygen.

To gobble up fish, squid and crustaceans that live more than 600 meters deep in the ocean, Risso’s dolphins (pictured) ‘drill’ through the water using a spinning diving technique

THE DEEP SPREADING LAYER EXPLAINED

In their study, Dr. Visser and colleagues found that Risso’s dolphins use different dives because they hunt for prey that live at different depths depending on the time of day.

These creatures (including fish, squid and crustaceans) form the so-called ‘deep scattering layer’.

The layer bears this name because it reflects sonar beams to give the impression of a false seafloor.

The scattering layer moves closer to the surface at dusk, when the inhabitants migrate upwards to feast on plankton.

However, when the day comes, the layer sinks back to the depths to avoid predation.

In fact, unlike Risso’s dolphins, most cetaceans can only access the scattering layer during the day when it is closer to the ocean surface.

The research was conducted by behavioral biologist Fleur Visser of the University of Amsterdam and her colleagues.

“Deep dives are costly for air-breathing marine predators because the higher temporal and energetic travel costs, combined with physiological constraints, limit the effective foraging time at depth,” the researchers explained in their paper.

“Minimizing travel costs is thus essential for optimal foraging and deep-diving cetaceans have developed specialized diving, oxygen-conserving and biosonar strategies to target and locate deep-dwelling prey.”

In their study, the team used biologgers to record various forms of data — including depth measurements, orientation and sound recordings — on 226 dives Risso’s dolphins made between May and August each year from 2012-2019.

According to the team, the dives ranged in depth from about 66–2,043 feet (20–623 m) and were divided into deep “spider dives” and shallower dives with no twists or turns.

Each spin dive appeared to begin with a deep exhalation – to reduce buoyancy – along with an intense blow from dolphins’ fins turning their bodies in a typical clockwise rotation before taking a rapid, twisting 60-degree descent. ° entered.

This would be followed by a rotating, free-sliding phase, reaching an average speed of 9 km/h. Each dive would see as many as three full turns.

It wasn’t until they finished spinning — at an average depth of 426 m and usually about 36 seconds into the dive — that the dolphins began echolocation to help them detect their prey.

This, explained Dr. Visser explains that the dolphins “planned” these maneuvers knowing they would find food at the bottom of each dive.

In total, each spin dive lasted approximately 10 minutes — including time spent hunting the nadir of each deep dive.

In their study, the team used biologgers to record various forms of data — including depth measurements, orientation and sound recordings — on 226 dives Risso's dolphins made between May and August each year from 2012-2019.  According to the team, the dives ranged in depth from about 66 to 2,043 feet (20 to 623 m) and were divided into deep

In their study, the team used biologgers to record various forms of data — including depth measurements, orientation and sound recordings — on 226 dives Risso’s dolphins made between May and August each year from 2012-2019. According to the team, the dives ranged in depth from about 20-623m and were divided into deep ‘spider’ dives (left) and shallower dives where there was no twisting or turning at all (right). ). Pictured, from top to bottom: the dolphin at the start of the dive, orientation during the first descent, depth, acceleration, orientation, forward speed and energy consumption

The shallower, spin-free dives, meanwhile, usually lasted just 6 minutes and saw the dolphins descend to an average depth of 584 feet (178 m) at a speed of about 7 km per hour, with echolocation starting almost from boarding.

Both types of dives saw the mammals reach their prey in about the same amount of time — however, the team reported that spider dives only occurred during the day, while shallower dives typically occurred from dusk to dawn.

According to the team, the way the dolphins vary their dives may be necessary to catch their favorite food, which belongs to the deep scattering layer.

This part of the ocean – first detected by sonar as a ‘false seafloor’ during World War II – is home to an abundance of marine life that comes close to the surface in the evenings to feed before retreating to the depths at dawn. to prevent predation.

The team found that the dolphins — whose prey only comes close to the surface at night — use different types of dives depending on the depth they need to feast.  Pictured: the frequency of no-spin diving (top) and spin diving (bottom), compared to the depth of the so-called deep scattering prey layer on which Risso's dolphins feast

The team found that the dolphins — whose prey only comes close to the surface at night — use different types of dives depending on the depth they need to feast on. Pictured: the frequency of no-spin diving (top) and spin diving (bottom), compared to the depth of the so-called deep scattering prey layer on which Risso’s dolphins feast

Most cetaceans feed only on prey that forms the deep scattering layer when it gets close to the surface after sunset.

Therefore, Risso’s dolphins are distinguished by having developed a tactic that allows them to tap into this food source at any time.

“They really know in advance where they are going and what kind of dive they have to use to get there,” Dr. Visser told the new scientist.

The study’s full findings were published in the journal Royal Society Open Science.

For their study, researchers attached sensors to seven dolphins off the coast of Terceira Island, in the Portuguese Autonomous Region of the Azores, and recorded data on 226 dives.

For their study, researchers attached sensors to seven dolphins off the coast of Terceira Island, in the Portuguese Autonomous Region of the Azores, and recorded data on 226 dives.

WHAT ARE RISSO’S DOLPHINS?

Risso dolphins are found worldwide in temperate and tropical oceans, usually in deep water close to land.

They average 3 meters in length and feed almost exclusively on schools of ocean squid, which usually hunt at night.

While hunting dives are typically short, lasting less than five minutes, the mammals can reach depths of 300 meters.

Risso dolphins are found worldwide in temperate and tropical oceans, usually in deep water close to land.  Older persons usually appear white and are often covered with scars from social interactions (artist impression)

Risso dolphins are found worldwide in temperate and tropical oceans, usually in deep water close to land. Older persons usually appear white and are often covered with scars from social interactions (artist impression)

Older individuals usually look white and are often scarred from social interactions.

The dolphins usually travel in groups of 10-50, but these can grow into huge collectives reaching 400.

Smaller social subgroups often form within larger groups, and the species is also known to travel with other marine mammals, including gray whales.

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