How do ocean giants find tiny prey?

© The Marine Mammal Center

Leviathan Press:

The North Atlantic right whale is one of the most endangered mammals in the world. Although the whaling that led to their extinction has been banned, they are still threatened by accidental collisions with ships or entanglement in fishing gear. The North Atlantic right whale can reach 60 feet in length and weigh up to 70 tons, making it the third largest whale in existence. Their lifespan is comparable to that of humans, and they can live to be hundreds of years old.

As this article explains, despite strict speed limits of 10 knots in some protected areas and new regulations limiting the number of lines between buoys and crab and lobster traps on the seafloor, conservationists worry that's not enough. Climate change is also exacerbating the problem, as warmer North Atlantic waters make the fatty crustacean, a staple food of right whales, increasingly scarce in their habitat that stretches from Florida to Canada.

Baleen whales such as humpbacks are among the largest creatures on Earth, but their prey is among the smallest in the ocean. © Gifer

When it is time to forage, humpback whales head to polar waters. Their goal is to eat as much as they can—eat until they are fat and full. They must store energy, and they have to eat a ton of fat a week. This fat is their "power bank" on the way from polar and sub-polar feeding grounds to warm breeding waters.

It’s a journey that can take months and thousands of miles, and when they arrive at their breeding grounds, they must be well prepared. Perhaps nature loves contradictions, as these deep-sea monsters, about 18 meters long and weighing 40 tons, feed on the smallest marine creatures, including krill and shrimp-like crustaceans (which are found in oceans around the world but are concentrated in cold waters at high latitudes).

© Giphy/BBC

Humpback whales are known to eat by the same method. They filter water using baleen, keratin plates that run along the roof of their mouths and resemble the bristles of an old toothbrush. To get the thousands of pounds of food they swallow every day, humpbacks must seek out places where crustaceans congregate. Once they find a large school of krill, they cleverly employ a cooperative feeding strategy, surrounding their prey while blowing out columns of bubbles to form a "fishing net." Then they begin to feed, opening their mouths wide and swooping down on the tightly packed prey, using their wrinkled throat pouches to gulp down thousands of gallons of krill-filled water and filter it through their baleen.

© The Conversation

Despite all the research on these charismatic sea beasts, no one knows how baleen whales find food in the first place. Their toothed whale relatives (which include sperm whales, belugas, and dolphins) use ultrasonic sonar systems to detect prey, but baleen whales (which include humpback whales, blue whales, fin whales, and sei whales) lack this ability. Yet somehow they are able to find tiny prey in the vastness of the ocean.

Scientists are eager to solve this mystery, in part because we still know very little about these giants, but also because the question of how baleen whales find food has important conservation implications, especially for the North Atlantic right whale.

North Atlantic right whale. © International Fund for Animal Welfare

The North Atlantic right whale, a plump, dark-skinned species that feeds on copepod crustaceans the size of a grain of rice, is one of the most endangered mammals. Commercial whaling nearly wiped it out in the early 20th century. In 1935, the League of Nations banned all hunting of right whales. Unlike other species whose populations have plummeted due to whaling, the North Atlantic right whale has not recovered since the ban. The species' feeding grounds, which overlap with human ranges in coastal areas of New England and the Canadian Maritimes, have taken a heavy toll from collisions with ships, entanglement in fishing nets, and climate change that has affected its habitat and food.

The latest estimates put the North Atlantic right whale population at less than 350, of which only 70 are females of reproductive age. The species is predicted to become extinct in the next few decades. Knowing how baleen whales forage for food can help scientists predict where they will go, so they can better regulate human activities that harm baleen whales in those areas.

Researchers are studying humpback whales in Antarctica to understand how they find krill. © Kate Wong

The research scientists are doing is not about just one species. North Atlantic right whales and other baleen whales are ecosystem engineers that feed in the deep ocean and release nutrients into shallow waters through their feces, which promote the growth of tiny phytoplankton. These phytoplankton in turn feed krill, copepods and other zooplankton, which in turn provide food for larger animals.

Whale tissues are also containers for fixing large amounts of carbon dioxide. On average, each large whale can fix about 33 tons, which helps curb global warming. When a whale dies, its body falls to the bottom of the sea, providing nutrients for various creatures in the deep sea, including sleeper sharks and sulfur-loving bacteria, which rely on so-called "whale falls" for food and shelter. The health of baleen whale populations is the basis for the health of many species.

© SIMoN

The most direct way to understand how baleen whales find food is to track them - by fitting them with beacons that record their underwater movements and foraging behavior. But this method doesn't work for North Atlantic right whales, which are extremely sensitive to human activity, and any direct contact will make things worse. Fortunately, North Atlantic right whales have less endangered relatives, such as humpback whales. The best place to observe the latter feeding is at the bottom of the ocean.

In 2020, two weeks before the World Health Organization declared the outbreak of the coronavirus, I was on a ship to Antarctica with a team of researchers trying to answer how baleen whales forage for food. I was on board as a guest of the cruise operator Polar Latitudes, observing a study conducted by seven scientists on board and giving lectures on whale evolution.

By taking a tourist cruise, an international team of researchers from the United States, Sweden and Japan saved the high cost of traveling to the Antarctic continent. In exchange for three shared staterooms, meals and the use of two durable "Zodiac" rubber boats, these scientists regularly educate passengers on their latest research results, which can also be regarded as a whale expedition project prepared for citizen scientists.

The team is testing a hypothesis about baleen whales that originated from studies of seabirds. Around 1950, Gabrielle Nevitt of the University of California, Davis, discovered that when phytoplankton is eaten by zooplankton, it releases dimethyl sulfide (DMS), a chemical that attracts tube-nosed seabirds (a class of carnivorous birds that includes albatrosses, petrels, and water razors), which in turn prey on the zooplankton. It's a mutually beneficial relationship: the phytoplankton attracts the seabirds through the scent of DMS, and thus receives their protection. Even at the bottom of the food chain, the principle of "the enemy of my enemy is my friend" still applies.

One of the team leaders is Daniel Zitterbart, a physicist at the Woods Hole Oceanographic Institution who uses remote sensing methods to study the behavior and ecology of whales and penguins, and Kylie Owen, a whale behavior expert at the Swedish Museum of Natural History, who was curious whether whales would also be attracted to DMS. If so, then in theory, following higher concentrations of DMS, whales could find krill and other phytoplankton-eating animals more efficiently than if they were just wandering around randomly to forage.

To find out, Zietbart and Irvine teamed up with whale biologist Annette Bombosch of Woods Hole Oceanographic Institution; zooplankton researcher Joseph Warren of Stony Brook University; Kei Toda of Kumamoto University in Japan, who developed the technology to measure DMS, and his graduate student Kentaro Saeki; and oceanographer Alessandro Bocconcelli of Woods Hole Oceanographic Institution, who has pioneered the use of sophisticated digital beacons to study whales.

The team plans to track humpback whales using special equipment, including pressure sensors, accelerometers, magnetic compasses and hydrophones to record underwater behavior, as well as signal transmitters to facilitate tracking. They have obtained a tracking permit, but can only track 5 whales, and must complete it within 5 days, so there is no room for trial and error.

On February 28, we set out from the port of Ushuaia, Argentina, the southernmost city in South America. For two days, we sailed through the Drake Passage, accompanied by albatrosses and petrels. This 620-nautical-mile-wide strait between South America and Antarctica is known as the "Storm Corridor." On March 1, we passed through the Antarctic Convergence Zone and entered the calm and cold waters of the Southern Ocean. This was the first time I saw the shore from the starboard side of the ship since entering the Drake Passage. It was Smith Island, part of the South Shetland Islands.

Away from the seasickness and nausea of ​​the Drake Passage, I no longer needed drowsy medication and could now focus on the stunning scenery around me. Icebergs, ice blocks, ice floes—all kinds of ice connected sea and sky in varying shades of blue. Fluffy Gentoo penguin chicks chased their exhausted parents for food, and white-and-gold crabeater seals lazily sunbathed on ice floes. I let this otherworldly beauty wash over me.

© Antarctica/The Polar Travel Company

At dawn on March 4, I woke up in Paradise Bay, a scenic harbor where whaling ships once anchored. I sat on the Zodiac boat and watched the rising sun break through the clouds and spread golden sunlight over the glaciers in the distance.

We had reached whale country, where pods of whales floated like logs, their breaths rising high into the air and blending with the crash of glaciers and the rumble of avalanches.

The scientists had successfully beaconed their first humpback whale the day before. When they announced the news at breakfast, passengers cheered. Unfortunately, the whale slept through the observation period. But later in the evening, they successfully beaconed a second whale, which was a perfect subject, making several deep dives to a depth of about 260 meters. Sensor data showed that the whale was feeding—exactly the behavior the scientists were hoping to see.

On the morning of the 4th, they were trying to track the third whale, hoping that it would be as active as the second one. Zitterbart is a tall and energetic man who thinks and speaks very quickly. He got up at 5:30 and went to the bridge of the cruise ship to observe whether there were whales around and what the weather was like. It looked like there was hope today. There were whales around and the water was calm, which was suitable for recovering equipment. These equipment would stay on the whale for a few hours, and then they would automatically fall off and float to the surface of the sea.

At 6:45, the research boat was lowered, and they were preparing to install a beacon on a whale they had observed earlier. A carbon fiber pole about 6 meters long was suspended at the stern of the boat, and there were 4 suction cups at the bottom of the device. Once they were within 3 meters of the whale, they used the pole to shoot the beacon at it. Bombo and Bokencheri drove the boat across a mirror-like open water and slowly approached a group of whales. However, this group looked lazy, and they didn't want to waste time on the dozing whales, so Owen and Bombo decided to find another group of more active whales.

© Antarctica Travel Centre

I was in another small boat when two humpback whales appeared. All I could see was their tiny dorsal fins and the very top of their sleek black backs. They didn't look that big, but like an iceberg, the bigger part was underwater. From a distance, you could only sense how big these whales were when they waved their giant fins above the surface, lifted their tails before diving, or split the water to show their entire bodies.

Zitterbart held the heavy pole, one foot on the bow and one foot in the boat, standing tight. Installing the beacon was a thrilling process. To ensure better reception of the transmitter's signal, he had to place the beacon as close to the whale's back as possible, but not too close to the sensitive skin near the blowhole.

As the boat approaches the whale, Zitterbart raises the pole and throws the beacon at it at the right time. Whales sink when they are startled - this is a common reaction. Then the researchers quickly put away the long pole, record the GPS location of the whale, and prepare to monitor it. They have successfully installed beacons on three whales in a row.

Once the whale returns to the surface, they will track it from about 100 meters away to avoid disrupting its normal activity path. They will observe it with their naked eyes for several hours and use a very high frequency receiver to receive signals. After the beacon automatically falls off at the pre-set time, they also need to recover the equipment, after all, it stores animal behavior data and is worth $10,000 each.

Now the team is just hoping that the subject will cooperate. "Ideally, we'll be tracking a whale that hasn't eaten yet and is swimming around looking for food," Owen explains. Then, colleagues on the prey boat will analyze seawater samples to see if the density of krill and DMS increases along the whale's path. If the subject has already eaten, there's nothing they can do.

But humpback whales are unpredictable animals and have their own schedule. "If we want them to do what we want, we have to wait until the sun rises in the west," Owen said.

Baleen whales gulp up water filled with prey and then filter it through their baleen. © Michael S. Nolan/Alamy Stock Photo

Antarctic krill is a favorite food of humpback whales. © Justin Hofman/Alamy Stock Photo

To visit Antarctica is to encounter the powerful forces that have shaped the fate of baleen whales throughout history. Whales evolved from four-legged land animals, and when they moved from land to sea, they underwent one of the most dramatic changes that vertebrates can undergo. Like any species, whales evolved in response to changes in their environment. Whales began to evolve during the Eocene, about 50 million years ago, when the greenhouse effect was very evident.

At that time, the southern hemisphere supercontinent, Gondwana, was disintegrating, and the ancient Tethys Ocean stretched from the Pacific Ocean to the Mediterranean Sea. In warm, shallow waters, early whales underwent their first transformation and became more adapted to life in the sea. Their forelimbs became fins, their noses became breathing holes, and their ears could hear sounds while underwater.

Ten million years after their furry, four-legged ancestors walked the coasts, whales have adapted completely to life in the water and can no longer live on land.

The second phase of whale evolution unfolded when Earth became an icehouse. As Earth entered the Oligocene, tectonic forces dealt the final blow to Gondwana, separating Oceania, South America, and Antarctica. When the separation was complete, the Antarctic Gyre current flowed around Antarctica, isolating it from warmer waters and bringing nutrients from the deep ocean to the surface, where large populations of phytoplankton and zooplankton could feed. In fact, the new current was so powerful that it changed ocean circulation, temperature, and abundance around the globe. During this dire period of great change in geology, climate, and oceans, the ancestors of modern baleen whales emerged. 35 million years ago, they roamed the oceans. Over millions of years, their descendants eventually evolved the baleen and large bodies we are familiar with.

North Atlantic right whales are at risk of extinction. Researchers hope to use DMS to predict where these whales will go to feed, information that will help inform conservation efforts. © Tumblr

From the perspective of evolutionary timescale, baleen whales have undergone adaptive changes due to dramatic changes in the environment and ecology, but this long evolutionary history has not spared modern baleen whales, and they have encountered huge threats from human society in a short period of time. In the 20th century alone, industrial whaling ships equipped with harpoon guns and whaling ships capable of processing whales at sea helped people slaughter more than 2 million baleen whales, pushing many populations to the brink of extinction and destroying the ecology. After the demise of the whaling industry, some species are recovering, but now face a new round of survival threats. Rising sea temperatures and commercial fishing are reducing the zooplankton on which whales depend for survival.

A North Atlantic right whale that died after being struck by a ship in June 2019. © CBC

After spending the first four days observing how the beacons were installed, I joined the quarry boat with Zietbart, Wallen, Kentaro, and Julien Bonnel of the Woods Hole Oceanographic Institution. The researchers decided to use the unplanned stopover to Foley Station, a Chilean research station on King George Island in the South Shetland Islands with an airstrip, to take an injured passenger to the nearest Chilean hospital to map the concentrations of krill and DMS in the shallow bay north of the island.

We were wearing jackets, hats and gloves to keep out the morning chill, but just a few weeks ago, Antarctica reached an unprecedented 18.3 degrees Celsius. The Antarctic Peninsula is one of the fastest-warming regions on Earth. That's caused a lot of ice melt, which is bad for krill, Wallen said. The juveniles take shelter in winter sea ice and feed on algae on the underside of the ice.

Rising temperatures are not the only threat to krill’s survival. Demand for the small crustaceans has soared in the past two decades, driven in large part by the health industry, which promotes krill oil as rich in omega-3 fatty acids that are good for the body, and by aquaculture, which feeds krill to fish. Whether krill fisheries are managed sustainably remains a concern.

In 2020, a survey of krill predators showed that even though Antarctic krill around the Antarctic Peninsula were restricted to less than 1% of the total stock in the southwest Atlantic region of the Southern Ocean, the number of penguins in the area was still declining, perhaps because fishing boats were operating in places where penguins were feeding. As the distribution and amount of krill and other food changed, predators such as whales had to change their foraging routes.

As the boat cleared the ship, the researchers began to adjust their equipment. They used an echosounder to send sound waves that bounced back when they hit animals like krill, and Warren's computer created an image of what was in the bubble column. The lower the frequency of the sound waves, the deeper the sensor can "see," while higher-frequency waves can detect smaller targets. The team used both high- and low-frequency sound waves to search for clusters of tiny krill, which are generally found in the upper 200 meters of the bubble column. Warren's lab mascot, a tiny "screaming chicken" called Mr. Lots of Signals 2, was watching the whole thing. "This is so exciting!" Warren joked as he lowered the echosounder.

A right whale and her calf. © Yahoo News UK

Tracking krill isn't as exciting as tracking whales, but the technology used on hunting boats has improved greatly in recent years. As the boat moves along the route, Kentaro scoops seawater from the side every two minutes for analysis. In two plastic boxes the size of a briefcase are the equipment that measures DMS in the water samples. A bubbler squeezes air into the water to produce gaseous DMS; a dryer removes any remaining moisture; an ozone generator makes sulfur from gaseous DMS; and a photomultiplier measures the light signal released by the sulfur - the amount of light is proportional to the amount of DMS.

Previously, this type of analysis could only be done in the laboratory, but now the researchers have optimized the DMS measuring equipment so that they can perform the analysis on a small boat. "Being able to operate the DMS equipment on a small boat was a major achievement for us this season," said Zitterbart. This allowed them to analyze the water samples on the spot. "We don't know how long the DMS signal will last in the sample, so to be cautious, we analyze it within two minutes," he explained.

Monitor echosounder signals, scoop water, process samples. Repeat. There are no whales around to interrupt the monotony, just clear blue skies, a biting wind, and the hum of sound outside the boat. We were halfway through our survey day when the echosounder picked up krill, crustaceans that normally hang suspended above the seafloor in the bay’s shallow waters. The work was adrenaline-free, but scientifically meaningful.

" No one has ever surveyed these bays, so any data we get is valuable, " Wallen said. They eventually detected two schools of krill and returned to the cruise ship with dozens of seawater samples. These data will help researchers understand the distribution of krill and DMS in the Southern Ocean and lay the foundation for future measurements.

In March, Antarctica's short summer is drawing to a close. Daylight is giving way to night, and the sea ice is about to erode the shore. Soon, the humpback whales will head north to the warm waters off the west coast of South and Central America to breed. Perhaps this is why they are not cooperating as much at this time. Although the researchers successfully tracked five whales, only two were observed foraging, while the other three were either sleepy or wandering around the bay leisurely. In Zietbart's opinion, the whales' lack of interest in foraging means that they need to adjust the time of their next research. "(Humpback whales) are already very large after March, so they need to sleep a lot. It would be better to go earlier because the whales are storing energy in their bodies and will be more active."

Strategies for analyzing seawater samples also need to be adjusted. After analyzing samples brought in by researchers and passengers in the citizen scientist program, the content of DMS in the samples was lower than expected. Maybe it's not because there is not enough DMS in the water. Wallen thinks there is another possibility: melted fresh water flows into the seawater, diluting the concentration of DMS. He said: "The physical movement of water makes the situation more complicated." In order to get more accurate information on the chemical composition, researchers may need to sample deeper seawater.

The North Atlantic right whale is considered one of the most endangered baleen whale species in the world. © Vocal Media

In the future, Zietbart wants to get away from the cruise ship's sightseeing schedule and focus on collecting information about whales in a bay. His plan is to take a downwind ship to the Antarctic research station and stay there, using the Zodiac boat to stay in one place for several days in a row, mapping the distribution of whales, krill and seawater chemicals to see how they change, and then take the return cruise.

First, they need to find a ship that can take them back to Antarctica. The cruise industry still has a lot of backlogs because many guests who have paid in recent years have been unable to travel due to the epidemic. The ships that the team could book in previous years have already been booked. "We expected to need five years of data, and now it's been three years," said Zietbart when talking about the impact of the epidemic on the project. He hopes to be able to travel in 2024. In the meantime, he is also focusing on research in the Arctic, which may help accelerate whale research.

Over the past three years, while awaiting their next trip to Antarctica, Zietbart, Irwin, and their colleagues have been studying the connection between DMS, zooplankton, and baleen whales in the waters off the coast of Massachusetts. Since they cannot track North Atlantic right whales, they are looking for connections between DMS hotspots and right whale populations in Cape Cod Bay. This investigation aims to determine whether DMS can be used as a predictor of whale appearance.

The team did not use tracking devices, but instead made observations by boat and airplane. While the Antarctic survey was designed to determine the specific mechanism by which baleen whales find prey—whether it's tracking krill swarms using changes in DMS or some other method—their work in Cape Cod Bay was simply to determine whether whales are more likely to be in areas with higher DMS levels. If so, whether by detecting DMS concentrations or other DMS-related clues, scientists could theoretically use DMS values ​​to predict where and when whales will appear.

Currently, measures to protect North Atlantic right whales include seasonal speed limits for vessels and visual and acoustic monitoring. For example, from January 1 to May 15, all vessels over 20 meters long must travel at less than 10 knots in Cape Cod Bay, an important breeding ground, to reduce the possibility of hitting or seriously injuring the whales. If whales are seen or heard in the area at any time of the year, vessels of all sizes must reduce speed. A free software called "Watch Out for Whales" provides near-real-time maps of seasonally controlled areas and whale detection data.

David Wiley, an ecologist at the National Oceanic and Atmospheric Administration's Stellwagen Bank National Marine Sanctuary and a colleague of Zitterbart's on the DMS, said such management measures are not yet predictive. Large ships on crowded shipping routes are often unable to change course in time to avoid collisions with slow-moving whales. "If we have a predictive tool like DMS, we can plan ahead instead of reacting."

In 2021, Irwin, Zietbart, and their collaborators published a paper on their research in Cape Cod Bay[1], which showed that DMS concentrations are proportional to the concentration of phytoplankton, so if baleen whales can indeed track DMS, then they can find prey. Now, researchers are investigating whether baleen whales gather in DMS hotspots. Preliminary findings suggest that this is the case for North Atlantic right whales and sei whales.

To consolidate this research, they will measure the concentration of DMS in the whales' swimming routes every two weeks before, when, and when the North Atlantic right whales arrive in Cape Cod Bay and Massachusetts Bay this year. Their goal is to know how concentrated DMS is for the whales to appear. Willy, who is involved in this study, expects that this work will take two years. He said: "We need to find out this threshold, which is closely related to whales in biology."

Blue whales and other baleen whales are ecosystem engineers. The health of their populations affects the health of many other species. © Franco Banfi/Minden Pictures

The researchers' dream is to be able to monitor areas where DMS levels are increasing through satellite imaging, which is likely to be where North Atlantic right whales gather. Wildlife managers could change routes in the area or temporarily close fisheries and wind farms to avoid disturbing the whales until DMS levels in the area drop and the whales leave. Meteorological scientists have long been interested in DMS because it contributes to cloud formation. They have found that the chemical can be detected from space. But predicting the movements of whales requires more accurate satellite data than is currently available.

We are not paying enough attention to the North Atlantic right whale and all the species that depend on it. If nothing is done, says Willey, our generation will see the extinction of the right whale. He believes the plight of this important species is our generation's problem. Perhaps by studying Antarctica's hungry humpbacks, and with the help of curious scientists, the North Atlantic right whale and other endangered baleen whales will one day regain their place as the rulers of the deep.

References:

[1]www.nature.com/articles/s42003-021-01668-3

By Kate Wong

Translated by Yord

Proofreader/Pharmacist

Original article/www.scientificamerican.com/article/no-one-knows-how-the-biggest-animals-on-earth-baleen-whales-find-their-food/

This article is based on the Creative Commons License (BY-NC) and is published by Yord on Leviathan

The article only reflects the author's views and does not necessarily represent the position of Leviathan