Monitoring the ocean’s deepest diving animals into the twilight zone

There’s just one phrase for it: indescribable. “It’s a kind of superior experiences you’ll be able to’t put into phrases,” says fish ecologist Simon Thorrold. Thorrold is making an attempt to clarify the way it feels to dive into the ocean and fix a tag to a whale shark — essentially the most stupendous fish within the sea. “Each single time I do it, I get this big adrenaline rush,” he says. “That’s partly concerning the science and the mad race to get the tags fastened. However a part of it’s simply being human and amazed by nature and big animals.”

Whale sharks are one in every of a choose group of huge marine animals that scientists like Thorrold, of the Woods Gap Oceanographic Establishment in Massachusetts, have signed up as ocean-going analysis assistants. Fitted with digital tags incorporating a set of sensors, monitoring units and infrequently tiny cameras, they collect data the place human researchers can’t. They’ve revealed outstanding journeys throughout total oceans, they usually have proven that diving deep is just about ubiquitous amongst giant marine predators of every kind.

Many commonly plunge a whole bunch and generally 1000’s of meters — to depths the place the water will be dangerously chilly and in need of oxygen, there’s little or no mild aside from the sparkles and flashes of bioluminescent organisms, and the strain is immense, placing some animals liable to deadly decompression illness.

To perform at such depths, deep-diving species have developed an array of anatomical and physiological variations — thick, insulating blubber, as an illustration, or blood vessels reworked into heat-exchange programs, collapsible lungs and oxygen-storing muscle mass, and ultra-sensitive eyes, to call however a couple of. However what drove these nice predators to accumulate their outstanding diving expertise?

For many biologists, the reply is a no brainer: Meals. But that’s been remarkably laborious to show. After a long time of tagging research, there’s sufficient circumstantial proof to be assured that many prime predators do dive deep seeking prey. However even now, just one species has been seen in motion. The northern elephant seal (Mirounga angustirostris) is now one thing of a famous person, due to a pioneering sequence of mini-movies that includes its personal snout and whiskers and a supporting forged of deep-sea fish and squid.

Meals, although, may not be the deep sea’s solely attraction, says Thorrold, coauthor of an article that examines the motivation of diving predators within the 2022  Annual Assessment of Marine Science. Dives and diving habits range: Some animals dive many occasions an hour, others sporadically. Most stick with depths between 200 and 1,000 meters, a area formally named the mesopelagic however higher often known as the twilight zone; others plunge far deeper. The shapes of dives trace at a couple of perform, too. A fast downward plunge and equally steep ascent, as an illustration, suggests a distinct objective from an extended, gradual, flat-bottomed dive. “If the identical particular person does several types of dive at totally different occasions,” says Thorrold, “then that’s good proof they’re for various functions.”

There isn’t any scarcity of ideas for what these functions is perhaps. Deep, dimly lit waters might present refuge from different predators; someplace to chill an overheating physique; navigational cues for these in a position to detect them; even a long-distance communication channel. “All these concepts are in play,” says Thorrold. “The very fact we will’t rule out any of them displays how mysterious lots of these giant pelagic animals are to us.”

Welcome to the deep-sea diner

Diving deep has developed in almost each sort of ocean-going vertebrate. Large bony fish, resembling tuna and swordfish, do it. Cartilaginous sharks and rays do it. So, too, do air-breathing animals — penguins, sea turtles, toothed whales and seals, all of which attain extraordinary depths on a single breath.

Most dive as far down because the twilight zone, the place the dim mild from above quickly dwindles to nothing. Some go into the blackness of the midnight zone, the bathypelagic realm between 1,000 and 4,000 meters. The present record-holder is Cuvier’s beaked whale: In 2014, one tagged whale reached 2,992 meters off the coast of Southern California. The file for a fish is held by a whale shark that reached 1,928 meters within the Gulf of Mexico in 2010.

Biologists of occasions previous would by no means have dreamt that deep waters would have a lot to supply a prime predator. Within the nineteenth century, naturalists believed that little lived deeper than 500 meters or so — however within the Nineteen Forties, Navy sonar operators found the deep scattering layer, a zone the place their sonar bounced off multitudes of mesopelagic organisms. This food-packed layer moved up and down as fish and tiny invertebrates migrated towards the floor to feed at evening and retreated to the relative security of deep water in the course of the day.

The ocean’s twilight zone turned out to be an unexpectedly well-stocked larder, crammed with strange gelatinous creatures, muscular squid, the ever-present and extremely nutritious lanternfish and the spiny-toothed bristlemouth, reckoned to be the world’s most plentiful vertebrate. In 1980, fisheries scientists estimated the worldwide biomass of mesopelagic fish at a billion metric tons, based mostly on surveys with nets. In 2014, a research based mostly on acoustic surveys put the determine seven to 10 occasions greater.

As but, there is no such thing as a international estimate of the life within the chill, black depths of the midnight zone, however a research within the waters over the Mid-Atlantic Ridge discovered a good higher mass of potential prey there. “Diving to forage is sensible if deep water is the place the biomass is,” says Thorrold.

Till very lately, although, all of the supporting proof for foraging was circumstantial. Mesopelagic fish, squid and crustaceans turned up within the stomachs of tuna, swordfish and blue sharks, whereas sperm whale stomachs contained the indigestible beaks of deep-sea squid, together with the enormous squid Architeuthis. Tagging research persistently put predator and prey in the identical place on the similar time. They’ve proven that enormous fish and mammals commonly and repeatedly dive into the deep scattering layer, and infrequently dive deeper in the course of the day when potential prey has migrated additional into the gloom. Some tagged tuna and swordfish observe exactly the day by day migration of potential prey.

With the event of more and more subtle tags, biologists are constructing an ever extra detailed image of what these animals are doing within the depths. Hooked up to fins and flippers, heads and jaws, they gather and retailer a variety of knowledge over many months. Tags that embody sensors for strain, temperature and light-weight have enabled researchers to reconstruct actions by the water and the depth and profile of dives. The previous few years have seen the emergence of progressive extras — accelerometers that log the twisting and turning of a head, sensors that detect the motion of jaws, sound-detecting hydrophones, even a sensible video digital camera that shoots solely moments value recording.

Getting these tags onto prime predators is massively difficult. “Whale sharks are uncommon and elusive, however we’ve now gotten a superb variety of tags on them,” says Thorrold. Fearsome sharks like the nice white are difficult for a distinct purpose. Free diving shouldn’t be an choice, and if you wish to tag the creature’s dorsal fin, you want a ship with a hydraulic platform to raise the shark aboard, the place the operation will be carried out safely.

Swordfish are notably difficult to tag. They’re laborious to search out, unpredictable and harmful, as Thorrold’s coauthor Peter Gaube, an oceanographer on the College of Washington, can testify. “While you do catch one, it’s a must to maintain it alongside the boat and attempt to repair the tags whereas it tries to whack you or bash a gap within the boat with that razor-sharp sword.”

The satan’s within the particulars 

A number of the finest proof of foraging has come from sudden quarters — such because the Chilean satan ray (Mobula tarapacana), an enormous however mysterious fish with a “wingspan” of almost4 meters. Most sightings of Chilean satan rays come from floor waters, the place they usually appear to bask within the solar, a behavior that led to the idea that they like life nearer the floor.

Curious to know extra about them, Thorrold and colleagues made two tagging expeditions to the Azores, the place giant numbers of satan rays collect across the Princess Alice seamount for a couple of months every year. In 2011, the workforce tagged 4 rays; in 2012, they tagged 11 extra. The tags logged the rays’ actions for as much as 5 months earlier than transmitting their information again to Woods Gap.

The outcomes had been staggering: Not solely did the satan rays journey 1000’s of kilometers at a cracking tempo, they steadily plunged 1,000 meters and extra. The deepest recorded dive was 1,896 meters. The sun-soaking, surface-dwelling rays are something however: They’re among the many deepest diving fish within the sea, and all the pieces factors to meals because the attraction.

Many of the satan ray dives had an uncommon stepwise profile. “They dive deep, then degree for a bit, come up a bit and degree once more, and so forth,” says Thorrold. “When you have a look at sonar, it appears they cease off the place there are skinny however dense layers of prey. We haven’t been in a position to see what they’re doing, however that is sturdy proof that they’re foraging.”

That may clarify why Chilean satan rays have a community of well-developed blood vessels of their mind cavity, very similar to that present in some deep-diving sharks, the place it features as a heat-exchange system to forestall the mind from rising too chilly. Biologists had at all times questioned why a fish that lives within the sunlit higher waters of the ocean would want one. “That is extra proof that satan rays forage at depth and so have to preserve their mind and sensory programs energetic within the chilly,” says Thorrold.

As for basking within the solar — that, Thorrold suggests, is to heat up earlier than and after deep dives.

Like satan rays, tag-toting sharks have been offering intriguing glimpses of habits that lends extra weight to the concept that they hunt in deep waters. Most of what we all know of sharks comes from research in coastal waters — but many migrate huge distances throughout the open ocean. Away from the seal-studded coasts, prey turns into patchy and thinly unfold. So how do large sharks get sufficient to eat?

Latest analysis means that some sharks have a sensible technique to realize entry to the ocean’s largest buffet. Information from  two nice white sharks and 15 blue sharks as they traveled the North Atlantic confirmed that they reap the benefits of eddies, swirling plenty of water that break free from the Gulf Stream. Eddies that spin off the northern fringe of the Gulf Stream entice hotter water from the south; eddies fashioned from the southern edge carry cool water southward. Each white and blue sharks confirmed a marked choice for warm-hearted eddies.

These heat eddies include the next density of mesopelagic prey, acoustic surveys have proven. And with anomalously heat water extending a whole bunch of meters, sharks can forage a lot deeper and for longer. “Heat eddies can present sharks entry to deeper meals sources that might in any other case be inaccessible,” says Gaube, a coauthor of the analysis.

Sound and imaginative and prescient 

The closest factor to proof that marine predators developed excessive diving expertise to use a wealthy however in any other case inaccessible supply of meals is coming from animals sporting tags with further bells and whistles.

Within the case of the short-finned pilot whale (Globicephala macrorhynchus), that meant a sound recorder. Pilot whales emit a sequence of clicks whereas they hunt, listening for echoes bouncing off prey. As they shut in on a goal, the clicks come so thick and quick they merge to a buzz. Natacha Aguilar de Soto, a marine biologist on the College of La Laguna, Tenerife, in Spain’s Canary Islands, determined to snoop on native pilot whales throughout their dives and fitted 23 of them with sound-recording tags.

The tagged whales dived deep, reaching a most of 1,019 meters, clicking as they went. Simply earlier than the deepest level of a dive, the clicks turned to buzzes — an indication {that a} whale was about to launch its assault. On the events when a whale dived very deep, it made a remaining, high-speed sprint earlier than it buzzed, which Aguilar de Soto interprets as an additional push in pursuit of fleeing prey, one thing giant sufficient to be value drawing on a quickly dwindling oxygen provide, resembling a Grimaldi scaled squid (a meter lengthy plus tentacles) and even maybe a large squid.

Listening to the sound of the hunt is convincing, however it’s nonetheless not proof. “We have to see what these predators are doing,” says Thorrold. For now, biologists should content material themselves with the brief snippets of movie shot by the northern elephant seal.

Feminine northern elephant seals make good analysis assistants, particularly these belonging to the colony in Año Nuevo State Park, simply north of Santa Cruz. Biologists from the College of California, Santa Cruz (UCSC), have been operating a analysis program there for greater than half a century. The Año Nuevo elephant seals have the benefit of being accessible: Like others of their type, they haul out on land within the winter to pup and mate, and once more within the spring or summer season to molt.

In between, the males stay in coastal waters, however the females migrate 1000’s of kilometers throughout the Pacific and again once more, diving constantly as they journey. It’s rather a lot less complicated to suit tags and retrieve them later than it’s for, say, an important white shark, and there are knowledgeable elephant seal wranglers available to assist. “It may be harmful, although,” says Japanese biologist Taiki Adachi, who has labored with the seals for greater than a decade and is at the moment based mostly at UCSC. “They’re very large and aggressive, and are particularly scary within the breeding season, when the moms have to guard their pups and males are defending their harem.” 

Adachi lately reported that migrating feminine elephant seals dive nearly constantly for 20 or extra hours every day. “They principally dive to 400 to 600 meters, the depth the place small fish — particularly oil-rich lanternfish — are very plentiful,” he says. Typically they go far deeper, persistently diving 800 meters or extra: The utmost recorded for this species is 1,735 meters.

At Japan’s Nationwide Institute of Polar Analysis, Adachi’s colleague Yasuhiko Naito developed an ingenious jaw-motion recorder that logs a seal’s makes an attempt to grab prey from the water. Fitted to the seal’s decrease jaw, the system logged 1,000 to 2,000 makes an attempt a day. The clincher, although, got here from one other of Naito’s improvements: a sensible video tag that’s connected to the seal’s jaw or head. The digital camera and lights are triggered by a mixture of depth and the actions attribute of a strike, a system that makes the a lot of the tag’s restricted battery energy.

The first elephant seal selfies, taken by one particular person seal and revealed in 2017, confirmed it making an attempt to catch fish some 800 meters down. The 21 fuzzy photographs confirmed elements of what had been later recognized as giant, deep-sea ragfish. With the assistance of extra camera-carrying seals, the workforce ultimately had 48 hours of footage from 240 meters to greater than 1,000 meters deep and capturing nearly 700 assaults. The standard was adequate to determine not simply the kind of prey however in some circumstances the species. They included the little lanternfish, ragfish and a kind of hake, plus half a dozen totally different squid, together with cockeyed squid and glass squid.

A spot of higher security

Except for diving for meals, there may be proof that dives, particularly extra excessive ones, serve different functions. Escaping from extra formidable predators is a particular contender.

Take yellowfin tuna (Thunnus albacares), which spend most of their time within the ocean’s higher 200 meters. In 2020, fisheries biologist Tim Lam, from the College of Massachusetts Boston, reported that  six of 17 tuna he tagged within the waters off Hawaii appeared to have had a run-in with a predator. 4 dived deep — three of them to round 1,000 meters — after which misplaced their tags, probably throughout frantic maneuvers as they tried to flee, Lam suggests. A fifth tuna plunged immediately from 134 meters to 1,592 meters — a splash interpreted as a doable try to outrun an enemy. When it returned to the floor, it appeared to have the jitters, spending the entire day close to the floor earlier than resuming regular exercise.

And, then there was the one which didn’t get away: Information from its tag confirmed that it reached a depth of 326 meters after which all the pieces went darkish and the temperature rose, most likely as a result of it was contained in the abdomen of a false killer whale or short-finned pilot whale.

Elephant seals additionally appear to reap the benefits of the dimly lit depths to keep away from their enemies. The most typical reason behind demise for these seals is considered predation by sharks or killer whales whereas at sea, says physiological ecologist Roxanne Beltran, who works on the Año Nuevo elephant seal program. “However we see a number of seals come to shore with contemporary or therapeutic shark bites, so clearly it’s doable to evade their predators.” 

A technique is to go downward. An early trace that the seals do exactly that got here from a check run of a novel tag greater than a decade in the past. As a part of analysis on the impacts of underwater noise, bioacoustician Selene Fregosi of Oregon State College fitted younger elephant seals at Año Nuevo with a prototype tag that performed again recorded sounds. The thought was to show seals to brief bursts of varied noises and see how they react. The playlist included echolocating clicks and whistles of killer whales and sperm whales; each despatched elephant seals into steep dives. If a seal was already diving, it accelerated; if it was on its return to the floor, it turned tail and dived deeper, in a single case nearly doubling its authentic depth.

Final yr, Beltran and colleagues reported that elephant seals don’t simply flee into the darkness, in addition they relaxation there. Elephant seals usually tend to be killed within the brightly lit higher ocean, the place sharks and killer whales are widespread. They’ve time just for brief breaks from foraging, drifting effortlessly for 10 to twenty minutes at a time. Their choice, Beltran found, is to relaxation a number of hundred meters beneath the floor. And the fatter and fitter they turn out to be, the deeper they go seeking higher security.

What, then, of different instructed causes for diving into the deep?

Navigation seems extremely seemingly. Virtually all giant marine predators migrate lengthy distances in some unspecified time in the future of their lives. Some — together with sharks and turtles — are recognized to be able to detecting cues offered by Earth’s magnetic area, sensing gradients in magnetic depth and anomalies created by geological options resembling seamounts. “Animals that may sense these cues may dive deep, the place the alerts are stronger,” says Thorrold. Leatherback turtles make excessive dives solely throughout lengthy migrations, suggesting they is perhaps checking they’re on the correct route. Hammerhead sharks within the Gulf of California are thought to search out their strategy to and from seamounts by sensing the native magnetic “panorama.”

There’s a single instance of a species that seems to dive to chill off. Atlantic bluefin tuna spend months every year in chilly temperate waters and have developed a extremely efficient strategy to preserve their physique warmth — however they spawn within the subtropical Gulf of Mexico, the place that’s a handicap. In an obvious technique to keep away from overheating, the tuna dive beneath 500 meters as they enter and depart the Gulf and keep beneath 200 meters whereas they spawn.

That leaves the chance that the deep sea is an effective place to speak. There’s a zone that stretches from a depth of some hundred meters to greater than a thousand, the place sound travels additional, making it excellent for long-distance communication. When blue and fin whales are within the zone, they are often heard an estimated 1,700 kilometers away, although nobody is aware of in the event that they go there for that particular objective.

“There’s a lot we don’t know, even with the expertise that’s turn out to be accessible prior to now 20 years,” says Thorrold. The longer term tech want listing is lengthy. Thorrold and his colleagues are testing a prototype tag that may find an animal’s place within the water column with a lot higher accuracy. They’d like to have tags that ship again solely related information from the huge portions recorded over many months at sea.

High of the listing, although, are small, sensible cameras. If elephant seals could make such nice motion pictures, why couldn’t different prime predators? “We’d like higher fish-cams which can be sufficiently small to mount on tuna, sharks and swordfish,” says Thorrold. “They should be miniaturized however high-resolution, in a position to function in low mild ranges and log information in the course of the lengthy intervals they’re touring the open ocean.”

In brief, seeing is believing. Apart from, who wouldn’t need to watch an important white shark’s dwelling motion pictures?