Sound reveals ‘Ocean giants’ dance with wind to find food

A study by MBARI researchers and their collaborators published today in Ecology Letters sheds new light on the movements of mysterious, endangered blue whales. The research team used a directional hydrophone on MBARI’s underwater observatory, integrated with other advanced technologies, to listen for the booming vocalizations of blue whales. They used these sounds to track the movements of blue whales and learned that these ocean giants respond to changes in the wind.

Along California’s Central Coast, spring and summer bring coastal upwelling. From March through July, seasonal winds push the top layer of water out to sea, allowing the cold water below to rise to the surface. The cooler, nutrient-rich water fuels blooms of tiny phytoplankton, jumpstarting the food web in Monterey Bay, from small shrimp-like krill all the way to giant whales. When the winds create an upwelling event, blue whales seek out the plumes of cooler water, where krill are most abundant. When upwelling stops, the whales move offshore into habitat that is transected by shipping lanes.

“This research and its underlying technologies are opening new windows into the complex, and beautiful, ecology of these endangered whales,” said John Ryan, a biological oceanographer at MBARI and lead author of this study. “These findings demonstrate a new resource for managers seeking ways to better protect blue whales and other species.”

CREDIT:Image: Goldbogen Lab/Duke Marine Robotics and Remote Sensing Lab (NMFS Permit 16111)

The directional hydrophone is a specialized underwater microphone that records sounds and identifies the direction from which they originate. To use this technology to study blue whale movements, researchers needed to confirm that the hydrophone reliably tracked whales. This meant matching the acoustic bearings to a calling whale that was being tracked by GPS. With confidence in the acoustic methods established, the research team examined two years of acoustic tracking of the regional blue whale population.

This study built upon previous research led by MBARI Senior Scientist Kelly Benoit-Bird, which revealed that swarms of forage species—anchovies and krill—reacted to coastal upwelling. This time, researchers combined satellite and mooring data of upwelling conditions and echosounder data on krill aggregations with the acoustic tracks of foraging blue whales logged by the directional hydrophone.

“Previous work by the MBARI team found that when coastal upwelling was strongest, anchovies and krill formed dense swarms within upwelling plumes. Now, we’ve learned that blue whales track these dynamic plumes, where abundant food resources are available,” explained Ryan.

Blue whales recognize when the wind is changing their habitat and identify places where upwelling aggregates their essential food—krill. For a massive animal weighing up to 150 tonnes (165 tons), finding these dense aggregations is a matter of survival.

While scientists have long recognized that blue whales seasonally occupy Monterey Bay during the upwelling season, this research has revealed that the whales closely track the upwelling process on a very fine scale of both space (kilometers) and time (days to weeks).

“Tracking many individual wild animals simultaneously is challenging in any ecosystem. This is especially difficult in the open ocean, which is often opaque to us as human observers,” said William Oestreich, previously a graduate student at Stanford University’s Hopkins Marine Station and now a postdoctoral fellow at MBARI. “Integration of technologies to measure these whales’ sounds enabled this important discovery about how groups of predators find food in a dynamic ocean. We’re excited about the future discoveries we can make by eavesdropping on blue whales and other noisy ocean animals.”

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Clarifying the chaos of narwhals behavior; what are narwhals, how they help [Details]

Researchers have used the mathematical equations of chaos theory to analyse the data from long-term monitoring of an electronically tagged narwhal. They have extracted previously undetected diurnal patterns within what initially appeared to be irregular diving and surface resting behavior, using records extending across 83 days.

“While animal-borne ocean sensors continue to advance and collect more data, there is a lack of adequate methods to analyse records of irregular behavior,” says Hokkaido University geophysicist Evgeny A. Podolskiy, first author of the research published in the journal PLOS Computational Biology.

Podolskiy developed the procedure to find behavioral patterns in seemingly intractable complexity with Mads Peter Heide‐Jørgensen at the Greenland Institute of Natural Resources.

Narwhals (Monodon monoceros) are relatively small whales found in Arctic seas, famous for their long single tusks and called the unicorns of the sea. They are one of the most endangered Arctic species due to climate change, human activity, and predation by such invasive species as killer whales. The narwhals are notable for undertaking dives to extreme depths of more than 1,800 metres. Their life cycle is tightly coupled with sea ice, which is rapidly declining.

A pod of adult male narwhals, Greenland, September 2019 (Photo: Carsten Egevang; This image may exclusively be used in relation to this press release. The image can not be included in media archives for use apart from the above and not be handed over to third parties, without prior acceptance by the photographer)./CREDIT: Carsten Egevang

Podolskiy and Heide‐Jørgensen combined their expertises in signal processing and biologging to understand the full diversity of behaviors of a satellite-tagged narwhal. Mathematical techniques developed as part of chaos theory can interpret complicated and seemingly chaotic behavior in dynamic systems to reveal states called ‘attractors’, which the systems tend to develop towards. In essence, the approach identifies significant patterns that would otherwise be difficult to detect.

The analysis of the behavior of the electronically tagged narwhal, inspired by Podolskiy’s previous work on turbulence, revealed a daily pattern of activity and how it was affected by changing seasons, features of narwhal behavior that were previously unrecognised. The animal rested nearer to the surface around noon, but when they did dive at that time the dives were very deep. During twilight and at night the dives became more shallow but also more intense, possibly due to hunting for squid, which is known for diurnal vertical migration. It was also found that increased sea ice constrains the narwhal’s surface activity, and is correlated with more intense diving.

“Our approach is relatively simple to implement and can map and label long term data, identifying differences between the behavior of individual animals and different species, and also detecting perturbations in behavior caused by changing influences,” the authors suggest.

The researchers expect that their new method may be especially useful for assessing the challenges to narwhals and other Arctic animals posed by climate change and the loss of sea ice. Such information may prove vital in adopting policies to protect endangered species in the face of natural change and increased human activity.

Related: http://dx.doi.org/10.1371/journal.pcbi.1010432

Scientists take a deep dive into how ‘elasmobranchs’ use the ocean depth

Using sophisticated electronic tags, scientists have assembled a large biologging dataset to garner comparative insights on how sharks, rays, and skates – also known as “elasmobranchs” – use the ocean depths. While some species spend their entire lives in shallow waters close to our shores on the continental shelf, others plunge hundreds of meters or more off the slope waters into the twilight zone, beyond where sunlight penetrates. This new understanding of how elasmobranchs use the ocean will enable policymakers and resource managers the opportunity to examine the threats these animals face, and guide future management and conservation plans.

A study published Aug. 19 in Science Advances, led by Stanford University and ZSL (Zoological Society of London) researchers, is the largest global investigation of where and when a diverse group of elasmobranchs move vertically. A team of 171 researchers from 135 institutions across 25 countries brought together two decades of data from satellite and archival tags that remotely tracked the movements and behaviors of 38 species in oceans across the globe.

“For the first time, we have a standardized, global database that we used to fill important knowledge gaps about the diving behaviors of sharks and rays,” said Samantha Andrzejaczek, co-lead author of the study and a postdoctoral research fellow at the Hopkins Marine Station of Stanford University. “This will enable better understanding of what fisheries interact with elasmobranchs and how to improve management of many of these long-lived animals.”

Australia Coral Reef Experiment Shows Acidification from CO2 stems growth

Ocean acidification will severely impair coral reef growth before the end of the century if carbon dioxide emissions continue unchecked, said new research on Australia’s Great Barrier Reef led by Carnegie’s Ken Caldeira and the California Academy of Sciences’ Rebecca Albright.

Their work, published in Nature, represents the first ocean acidification experiment in which seawater was made artificially acidic by the addition of carbon dioxide and then allowed to flow across a natural coral reef community. The acidity of the seawater was increased to reflect end-of-century projections if carbon dioxide from greenhouse gas emissions are not abated.

Two years ago, Caldeira and Albright, then at Carnegie, published a landmark study providing evidence that ocean acidification is already slowing coral reef growth.

In that work, they made a coral reef community’s seawater chemistry more alkaline–essentially giving the reef an antacid–and demonstrated that the coral’s ability to construct its architecture was improved under these conditions. It was the first time that seawater chemistry was experimentally manipulated in a natural coral reef environment.

They once again altered seawater chemistry of reef flats surrounding One Tree Island off the coast of Australia. But this time they gave the reef heartburn, increasing acidity by adding carbon dioxide to seawater flowing over a coral reef community.

“Last time, we made the seawater less acidic, like it was 100 years ago, and this time, we added carbon dioxide to the water to make it more acidic, like it could be 100 years from now,” Caldeira explained.

When coal, oil, or gas is burned, the resulting carbon dioxide is released into the atmosphere. It is well established that these emissions are the culprit of global climate change, the warming from which has a negative impact on coral reefs. But this atmospheric carbon is also absorbed into the ocean, where it remains for millennia.

A chemical reaction between the seawater and these soaked-up carbon emissions produces carbonic acid, which is corrosive to coral reefs, shellfish, and other marine life. Reefs are especially vulnerable to this ocean acidification, because their skeletons are constructed by accreting calcium carbonate, a process called calcification. As the surrounding water becomes more acidic, calcification becomes more difficult.

“Our findings provide strong evidence that ocean acidification caused by carbon dioxide emissions will severely slow coral reef growth in the future unless we make steep and rapid reductions in greenhouse gas emissions,” said first author Albright.

Furthermore, by working in controlled areas of a natural reef community, Caldeira, Albright, and their team were able to demonstrate how acidification affects coral reefs on the ecosystem scale, not just in terms of individual organisms or species, as other studies have done.

They say this approach is crucial to understanding the full scope and complexity of ocean acidification’s impact, as well as to predicting how acidification will affect the coastal communities that depend on these ecosystems.

“Coral reefs offer economic opportunities to their surrounding communities from fishing and tourism,” Caldeira said. “But for me the reef is a beautiful and diverse outpouring of life that we are harming with our carbon dioxide emissions. For the denizens of the reef, there’s not a moment to lose in building an energy system that doesn’t dump its waste into the sky or sea.”