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

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


A number of the ocean’s greatest predators dive means down into the chilly, darkish depths. Animals-turned-oceanographers are serving to biologists discover out what they do once they get there.

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 attempting to elucidate the way it feels to dive into the ocean and fasten a tag to a whale shark — probably 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 mounted. However a part of it’s simply being human and amazed by nature and large animals.”

Whale sharks are certainly one 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 gadgets and sometimes tiny cameras, they collect data the place human researchers can’t. They’ve revealed outstanding journeys throughout whole oceans, and so they have proven that diving deep is just about ubiquitous amongst massive marine predators of every kind.

Many commonly plunge lots of and generally 1000’s of meters — to depths the place the water could be dangerously chilly and wanting oxygen, there’s little or no gentle apart from the glints and flashes of bioluminescent organisms, and the strain is immense, placing some animals prone to deadly decompression illness.

To perform at such depths, deep-diving species have advanced an array of anatomical and physiological variations — thick, insulating blubber, for example, or blood vessels remodeled into heat-exchange methods, collapsible lungs and oxygen-storing muscular tissues, 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 arduous to show. After a long time of tagging research, there’s sufficient circumstantial proof to be assured that many high predators do dive deep searching for 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 collection 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 Evaluation of Marine Science. Dives and diving habits fluctuate: Some animals dive many instances an hour, others sporadically. Most follow depths between 200 and 1,000 meters, a area formally named the mesopelagic however higher often called the twilight zone; others plunge far deeper. The shapes of dives trace at multiple perform, too. A fast downward plunge and equally steep ascent, for example, suggests a distinct objective from a protracted, gradual, flat-bottomed dive. “If the identical particular person does several types of dive at completely different instances,” says Thorrold, “then that’s good proof they’re for various functions.”

There isn’t any scarcity of options for what these functions is perhaps. Deep, dimly lit waters may 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 quite a lot of these massive pelagic animals are to us.”

Welcome to the deep-sea diner

Diving deep has advanced in practically each sort of ocean-going vertebrate. Massive 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 gentle 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 instances previous would by no means have dreamt that deep waters would have a lot to supply a high 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 night time and retreated to the relative security of deep water throughout the day.

The ocean’s twilight zone turned out to be an unexpectedly well-stocked larder, stuffed with bizarre gelatinous creatures, muscular squid, the ever-present and extremely nutritious lanternfish and the spiny-toothed bristlemouth, reckoned to be the world’s most ample 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 instances increased.

As but, there isn’t any world 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 fair better mass of potential prey there. “Diving to forage is sensible if deep water is the place the biomass is,” says Thorrold.

Till very just 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 constantly put predator and prey in the identical place on the identical time. They’ve proven that giant fish and mammals commonly and repeatedly dive into the deep scattering layer, and infrequently dive deeper throughout the day when potential prey has migrated additional into the gloom. Some tagged tuna and swordfish observe exactly the each day migration of potential prey.

With the event of more and more refined 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 embrace sensors for strain, temperature and light-weight have enabled researchers to reconstruct actions via 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 wise video digital camera that shoots solely moments value recording.

Getting these tags onto high 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 cause. Free diving will not be an choice, and if you wish to tag the creature’s dorsal fin, you want a ship with a hydraulic platform to carry the shark aboard, the place the operation could be carried out safely.

Swordfish are significantly tough to tag. They’re arduous to search out, unpredictable and harmful, as Thorrold’s coauthor Peter Gaube, an oceanographer on the College of Washington, can testify. “If you do catch one, you must 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 greatest proof of foraging has come from surprising 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 belief 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 massive numbers of satan rays collect across the Princess Alice seamount for a couple of months every year. In 2011, the group 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 knowledge again to Woods Gap.

The outcomes have 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 every little thing factors to meals because the attraction.

A lot of the satan ray dives had an uncommon stepwise profile. “They dive deep, then stage for a bit, come up a bit and stage once more, and so forth,” says Thorrold. “For those who take 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 robust proof that they’re foraging.”

That will clarify why Chilean satan rays have a community of well-developed blood vessels of their mind cavity, very like that present in some deep-diving sharks, the place it capabilities as a heat-exchange system to stop the mind from rising too chilly. Biologists had at all times puzzled 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 maintain their mind and sensory methods lively 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 huge sharks get sufficient to eat?

Latest analysis means that some sharks have a wise technique to realize entry to the ocean’s greatest buffet. Knowledge from  two nice white sharks and 15 blue sharks as they traveled the North Atlantic confirmed that they make the most 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 lots of of meters, sharks can forage a lot deeper and for longer. “Heat eddies can present sharks entry to deeper meals sources that will 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 advanced excessive diving expertise to use a wealthy however in any other case inaccessible supply of meals is coming from animals carrying 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 collection 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 closing, high-speed sprint earlier than it buzzed, which Aguilar de Soto interprets as an additional push in pursuit of fleeing prey, one thing massive sufficient to be value drawing on a quickly dwindling oxygen provide, resembling a Grimaldi scaled squid (a meter lengthy plus tentacles) and even maybe an enormous squid.

Listening to the sound of the hunt is convincing, but it surely’s nonetheless not proof. “We have to see what these predators are doing,” says Thorrold. For now, biologists should content material themselves with the quick 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 working 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 time 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 repeatedly as they journey. It’s so much easier to suit tags and retrieve them later than it’s for, say, an ideal white shark, and there are knowledgeable elephant seal wranglers readily 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 present based mostly at UCSC. “They’re very huge 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 just lately reported that migrating feminine elephant seals dive nearly repeatedly for 20 or extra hours every day. “They largely dive to 400 to 600 meters, the depth the place small fish — particularly oil-rich lanternfish — are very ample,” he says. Generally they go far deeper, constantly 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 machine logged 1,000 to 2,000 makes an attempt a day. The clincher, although, got here from one other of Naito’s improvements: a wise video tag that’s hooked up to the seal’s jaw or head. The digital camera and lights are triggered by a mix 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 attempting to catch fish some 800 meters down. The 21 fuzzy pictures confirmed components of what have been later recognized as massive, deep-sea ragfish. With the assistance of extra camera-carrying seals, the group finally had 48 hours of footage from 240 meters to greater than 1,000 meters deep and capturing nearly 700 assaults. The standard was ok to establish not simply the kind of prey however in some instances the species. They included the little lanternfish, ragfish and a sort of hake, plus half a dozen completely different squid, together with cockeyed squid and glass squid.

A spot of better security

Other than 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, presumably throughout frantic maneuvers as they tried to flee, Lam suggests. A fifth tuna plunged all of the sudden from 134 meters to 1,592 meters — a touch 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: Knowledge from its tag confirmed that it reached a depth of 326 meters after which every little thing 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 make the most of the dimly lit depths to keep away from their enemies. The most common explanation for demise for these seals is regarded as 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 method is to move 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 concept was to show seals to quick bursts of assorted 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 unique 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 frequent. They’ve time just for quick 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 change into, the deeper they go searching for better security.

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

Navigation seems to be extremely seemingly. Nearly all massive 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 subject, 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 appropriate route. Hammerhead sharks within the Gulf of California are thought to search out their method 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 advanced a extremely efficient method to keep 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 likelihood that the deep sea is an efficient 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 best 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 know-how that’s change into obtainable previously 20 years,” says Thorrold. The long run 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 better accuracy. They’d like to have tags that ship again solely related knowledge 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 films, why couldn’t different high predators? “We want higher fish-cams which might be sufficiently small to mount on tuna, sharks and swordfish,” says Thorrold. “They must be miniaturized however high-resolution, in a position to function in low gentle ranges and log knowledge throughout the lengthy intervals they’re touring the open ocean.”

Briefly, seeing is believing. Apart from, who wouldn’t need to watch an ideal white shark’s house films?

This text initially appeared in
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