What's quieter than a fish? A school of them

Swimming in schools makes fish surprisingly stealthy underwater, with a group able to sound like a single fish.

The new findings by Johns Hopkins University engineers working with a high-tech simulation of schooling mackerel, offers new insight into why fish swim in schools and promise for the design and operation of much quieter submarines and autonomous undersea vehicles.

"It's widely known that swimming in groups provides fish with added protection from predators, but we questioned whether it also contributes to reducing their noise," said senior author Rajat Mittal.

"Our results suggest that the substantial decrease in their acoustic signature when swimming in groups, compared to solo swimming, may indeed be another factor driving the formation of fish schools."

The work is newly published in Bioinspiration & Biomimetics.

The team created a 3D model based on the common mackerel to simulate different numbers of fish swimming, changing up their formations, how close they swam to one another, and the degrees to which their movements synched.

The model, which applies to many fish species, simulates one to nine mackerel being propelled forward by their tail fins.

The team found that a school of fish moving together in just the right way was stunningly effective at noise reduction: A school of seven fish sounded like a single fish.

"A predator, such as a shark, may perceive it as hearing a lone fish instead of a group," Mittal said.

"This could have significant implications for prey fish."

The single biggest key to sound reduction, the team found, was the synchronization of the school's tail flapping -- or actually the lack thereof.

If fish moved in unison, flapping their tail fins at the same time, the sound added up and there was no reduction in total sound.

But if they alternated tail flaps, the fish canceled out each other's sound, the researchers found.

"Sound is a wave," Mittal said. "Two waves can either add up if they are exactly in phase or they can cancel each other if they are exactly out of phase. That's kind of what's happening here though we're talking about faint sounds that would barely be audible to a human."

The tail fin movements that reduce sound also generate flow interaction between the fish that allow the fish to swim faster while using less energy, said lead author Ji Zhou, a Johns Hopkins graduate student studying mechanical engineering.

"We find that reduction in flow-generated noise does not have to come at the expense of performance," Zhou said.

"We found cases where significant reductions in noise are accompanied by noticeable increases in per capita thrust, due to the hydrodynamic interactions between the swimmers."

The team was surprised to find that the sound reduction benefits kick in as soon as one swimming fish joins another.

Noise reduction grows as more fish join a school, but the team expects the benefits to cap off at some point.

"Simply being together and swimming in any manner contributes to reducing the sound signature," Mittal said.

"No coordination between the fish is required."

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Biggest Holocene volcano eruption found by seabed survey

A detailed survey of the volcanic underwater deposits around the Kikai caldera in Japan clarified the deposition mechanisms as well as the event's magnitude. As a result, the Kobe University research team found that the event 7,300 years ago was the largest volcanic eruption in the Holocene by far.

In addition to lava, volcanos eject large amounts of pumice, ashes and gases as a fast-moving flow, known as "pyroclastic flow," and its sediments are a valuable data source on past eruptions.

For volcanoes on land, geologists understand the sedimentation mechanism of pyroclastic flows well, but the sediments themselves get lost easily due to erosion.

On the other hand, for volcanoes on oceanic islands or near the coast, the pyroclastic flow deposition process is largely unclear, both because the interaction with water is less well understood and because reliable data is difficult to obtain and therefore sparse.

For these reasons, it is difficult to estimate the impact of many past eruptions on the climate and on history.

A Kobe University research team around SEAMA Nobukazu and SHIMIZU Satoshi took to the seas on the Kobe University-owned training vessel Fukae Maru (since replaced by the newly built Kaijin Maru) and conducted seismic imaging as well as sediment sampling around the Kikai caldera, off the south coast of Japan's Ky?sh?

island. The outstanding detail of the seismic reflection data revealed the sedimentary structure with a vertical resolution of 3 meters and down to a depth of several hundred meters below the seafloor.

Shimizu explains: "Due to the fact that volcanic ejecta deposited in the sea preserve well, they record a lot of information at the time of eruption. By using seismic reflection surveys optimized for this target and by identifying the collected sediments, we were able to obtain important information on the distribution, volume, and transport mechanisms of the ejecta."

In their article published in the Journal of Volcanology and Geothermal Research, the geoscientists report that an eruption that happened 7,300 years ago ejected a large amount of volcanic products (ash, pumice, etc.) that settled in an area measuring more than 4,500 square kilometers around the eruption site.

With a dense-rock equivalent volume of between 133 and 183 cubic kilometers, the event was the largest volcanic eruption to have taken place within the Holocene (the most recent 11,700 years of Earth's history following the end of the last ice age) known to science.

In the process of their analysis, the research team confirmed that the sedimentations on the ocean floor and those deposited on nearby islands have the same origin and from their distribution around the eruption site they could clarify the interaction between the pyroclastic flow and water.

They noticed that the underwater portion of the flow could travel vast distances even uphill.

Their findings yield new insights into the elusive dynamics of volcanic mega events that may prove useful in identifying the remains of other events as well as in estimating their size.

Seama explains, "Large volcanic eruptions such as those yet to be experienced by modern civilization rely on sedimentary records, but it has been difficult to estimate eruptive volumes with high precision because many of the volcanic ejecta deposited on land have been lost due to erosion. But giant caldera eruptions are an important phenomenon in geoscience, and because we also know that they influenced the global climate and thus human history in the past, understanding this phenomenon has also social significance." In this light, it is fascinating to think that the event that created a caldera about the size of a modern capital city was in fact the largest volcanic event since humans have spread all over the globe.

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Mysterious underwater acoustic world of British ponds revealed in new study

The previously hidden and diverse underwater acoustic world in British ponds has been uncovered by a team of researchers at the University of Bristol.

Ponds are magnets for life and a lot of that life is very noisy. Water beetles, bugs, fish, frogs, and even aquatic plants all produce sound creating a diverse underwater orchestra that scientists are only just starting to understand.

Acoustic monitoring has been shown to effectively survey birds and monkeys in rainforests, and marine mammals in the oceans. However, freshwater environments have remained largely unexplored despite their diverse soundscapes.

"Ponds are packed full of bizarre and mysterious sounds made by scratching aquatic insects, booming fish, and popping plants. It's like an underwater disco!" explained lead author Dr Jack Greenhalgh from Bristol's School of Biological Sciences.

To better understand these mysterious soundscapes, the team collected 840 hours of underwater sound recordings from five ponds in the southwest of England using an underwater microphone (a hydrophone).

And in findings published in the journal Freshwater Biology, analysis of the audio files revealed clear daily acoustic activity cycles in each pond.

Typically, a nocturnal chorus is made by aquatic insects that compete to attract mates by producing strange scratching sounds as they rub their genitals against their abdomens. During the daytime, however, aquatic plants dominate the underwater orchestra with rhythmic whining and ticking sounds produced as tiny oxygen bubbles are released by plants respiring in the hot sun.

Prof. Gareth Jones said: "Recording animal sounds has provided great advances for monitoring and surveying terrestrial animals remotely. Given the rich diversity of underwater sounds that is only now being revealed, the potential for assessing the health of freshwater ecosystems is great, especially with low-cost monitoring devices now becoming available."

Using this acoustic method, the presence of species, and a determination of ecological health, can be inferred simply by listening to the natural world without disturbing the environment or harming the plants and animals within it. This research is the first to provide a detailed description of pond soundscapes in the UK and will help inform the acoustic monitoring of freshwater ecosystems to help prevent irreversible species loss due to climate change and habitat loss.

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Underwater gardens boost coral diversity to stave off ‘biodiversity meltdown’

Corals are the foundation species of tropical reefs worldwide, but stresses ranging from overfishing to pollution to warming oceans are killing corals and degrading the critical ecosystem services they provide. Because corals build structures that make living space for many other species, scientists have known that losses of corals result in losses of other reef species. But the importance of coral species diversity for corals themselves was less understood.

A new study from two researchers at the Georgia Institute of Technology provides both hope and a potentially grim future for damaged coral reefs. In the study, published October 13 in Science Advances, Cody Clements and Mark Hay found that increasing coral richness by 'outplanting' a diverse group of coral species together improves coral growth and survivorship. This finding may be especially important in the early stages of reef recovery following large-scale coral loss -- and in supporting healthy reefs that in turn support fisheries, tourism, and coastal protection from storm surges.

The scientists also call for additional research to better understand and harness the mechanisms producing these positive species interactions, with dual aims to improve reef conservation and promote more rapid and efficient recovery of degraded reefs.

But the ecological pendulum swings the other way, too. If more coral species are lost, the synergistic effects could threaten other species in what Clements and Hay term a "biodiversity meltdown."

"Yes, corals are the foundation species of these ecosystems -- providing habitat and food for numerous other reef species," said Clements, a Teasley Postdoctoral Fellow in the School of Biological Sciences. "Negative effects on corals often have cascading impacts on other species that call coral reefs home. If biodiversity is important for coral performance and resilience, then a 'biodiversity meltdown' could exacerbate the decline of reef ecosystems that we're observing worldwide."

Clements and Hay traveled to Mo'orea, French Polynesia, in the tropical Pacific Ocean, where they planted coral gardens differing in coral species diversity to evaluate the relative importance of mutualistic versus competitive interactions among corals as they grew and interacted through time.

"We've done the manipulations, and the corals should be competing with each other, but in fact they do better together than they do on their own," said Hay, Regents Professor and Teasley Chair in the School of Biological Sciences. Hay is also co-director of the Ocean Science and Engineering graduate program at Georgia Tech. "We are still investigating the mechanisms causing this surprising result, but our experiments consistently demonstrate that the positive interactions are overwhelming negative interactions in the reef settings where we conduct these experiments. That means when you take species out of the system, you're taking out some of those positive interactions, and if you take out critical ones, it may make a big difference."

Under the sea, in a coral-growing garden, in the shade

Coral reefs are under threat worldwide. Hay notes that according to the EPA, the Caribbean has lost 80 to 90 percent of its coral cover. The Indo-Pacific region has lost half of all its corals over the last 30 years. During the bleaching event of 2015-2016 alone, nearly half of the remaining corals along the Great Barrier Reef bleached and died.

"The frequency of these big bleaching and heating events that are killing off corals has increased fairly dramatically over the last 20 to 30 years," he said. "There are hot spots here and there where coral reefs are still good, but they're small and isolated in general."

In their coral gardens in French Polynesia, Hay and Clements manipulated the diversity of the coral species that they planted on platforms resembling underwater chess tables, to try and see if species richness and density affected coral productivity and survival.

Hay noted many previous, similar experiments involved bringing corals into a lab to "pit species against each other." But he points out, "We do all of our experiments in the real world. We're not as interested in whether it can happen, but whether it does happen."

An experimental setup suggested by Clements involving Coke bottles helped the scientists arrange their garden. The end tables "have Coca-Cola bottlecaps embedded in the top of them," Hay said. "We can then cut off the necks of Coke bottles, glue corals into the upside-down necks of these things, and then screw them in and out of these plots. This allows us to not only arrange what species we want where, but every couple of months we can unscrew and weigh them so we can get accurate growth rates."

The researchers found that corals benefitted from increased biodiversity, "but only up to a point," Clements noted. "Corals planted in gardens with an intermediate number of species -- three to six species in most cases -- performed better than gardens with low, or one, species, or high, as in nine, species. However, we still do not fully understand the processes that contributed to these observations."

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