By Shi En Kim for Sentient.
Broadcast version by Suzanne Potter for California News Service reporting for the Sentient-Public News Service Collaboration

Deep below the surface of the ocean are the parts left mostly unexplored and where sunlight barely touches. One of these is called the mesopelagic layer, so-named for its spot in the middle of the pelagic zone. This "twilight zone" of the ocean contains more fish than at any other depth: estimates range from 1 to 10 billion metric tons. Until recently, the fish that dwell there remained mostly untouched by fishing vessels. But growing demand for farmed fish spurred the industry to explore new depths, urgently looking for more tiny fish to use as feed.

The "twilight zone" contains 90 percent of the ocean's total biomass in fish. These strange-looking creatures are well-adapted to the constant pall at 660 to 3,300 feet below. They often sport googly eyes, translucent bodies and bioluminescent veneers. Appearance and color aren't important considerations for citizens of a world shrouded in near-darkness.

What the dwellers lack in looks, they make up for in numbers. Since that remains a driving argument for fishing there, researchers have called for a moratorium to prevent future exploitation.

There Are Plenty of Mesopelagic Fish (For Now)

You can't order mesopelagic seafood in a restaurant or find it at the seafood counter. But there is plenty of seafood industry interest in mesopelagic fish - for turning them into fish oil supplements or aquafeed.

Aquaculture has long been touted as a way to relieve the burden on wild fish operations. Yet the industry itself needs wild fish to operate. Most aquaculture operations use small wild-caught fish, such as anchovies, as fishmeal for larger predatory fish that humans normally put on their plates. While the UN's Food and Agriculture Organization researchers say data on exact numbers is scarce, research suggests at least some fishmeal stocks are teetering towards depletion. In West Africa, for example, feedfish have been over-harvested by foreign companies. And illegal operators are devastating marine biodiversity, and threatening local livelihoods.

As a substitute for wild-caught feed, some aquaculture operators raise their fish on a vegetarian diet such as soy. But many experts are quick to point out the inanity of this alternative: it's like forcing a lion into an herbivore lifestyle, Xabier Irigoien, a researcher at AZTI and King Abdullah University of Science and Technology tells Sentient. "We are actually using land and freshwater to grow soy - to grow a fish that really doesn't need freshwater or land. So we are doing really odd things in terms of food."

At the same time, our global appetite for fish, especially farmed fish, continues to rise, and the industry is exploring new options. In late 2023, the Norwegian firm Nofima conducted a three-month trial of feeding farmed salmon mesopelagic catch. The researchers observed their salmon growing faster than their counterparts fed a typical protein diet. "It's very promising," Ingunn Marie Holmen, a research manager in fisheries technology at the Norwegian research institute SINTEF Ocean, tells Sentient. Nofima "had to actually stop the trial one week early because they ran out of feed."

Not everyone is on board, however. Since 2020, researchers have called for a global moratorium on mesopelagic fishing, concerned that plumbing these new depths for fish could take a toll on climate action and biodiversity.

Fish Capture and Store Carbon in the Sea

All fish, including mesopelagic fish, play a vital role in sequestering atmospheric carbon, slowing the pace of climate change by taking emissions out of the atmosphere. Deep-dwelling mesopelagic fish are even more vital to that process, simply because of where they live. Many of these fish undertake a daily vertical migration, surfacing at night to feed, then returning during the day to hide from predators. Mesopelagic fish exhale carbon dioxide, poop and themselves get eaten by other mesopelagic hunters. And all of these activities help ferry carbon from surface waters to the deeper ocean, where it can be stored more effectively than on land.

Another ecological downside of mesopelagic fishing is the damage to ocean biodiversity. The role of mesopelagic fish in the marine food web is still largely unknown. But researchers know that several epipelagic species, such as dolphins, swordfish and tuna, feed on their submarine neighbors. Trawling in the mesopelagic zone might risk disrupting fisheries at the ocean surface, including those the industry thinks of as the most valuable at market.

"We don't know the system well enough to start harvesting mesopelagic fish," says Middle East Technical University oceanographer, Deniz Dişa. Last June, Dişa and her team published a study modeling the impact of mesopelagic fishing in the mid-Atlantic Ocean. The researchers found that even removing a conservative fraction of the mesopelagic stock would lead to cascading effects of carbon cycle disruption, with a decrease of 14 percent in total carbon movement, and food web chaos. According to the team, fishing in mesopelagic waters would only have a 20 percent chance of being sustainable.

The bottom line, Dişa says: the consequences are "not that simple." And a growing number of ocean researchers agree, arguing that fishing deeper isn't a solution to dwindling marine stocks - it only punts the problem from one corner of the world to another. Given their unique ecological roles in biodiversity and carbon sequestration, mesopelagic fish are just as worthy of protection as those near the surface, if not more.

"This is a group which is unlike most of the species that we exploit at the moment," says Callum Roberts, a University of Exeter marine biologist who is among those calling for a mesopelagic fishing ban. "If there's any part of the ocean that you really don't want to mess with, it's the mesopelagic fish."

Technological Hurdles Add Another Layer of Caution

For now, there are a number of challenges keeping mesopelagic fishing at bay. Experts say this type of fishing requires high fuel consumption, for one. The fish are also very small, some no larger than a finger pad, so fishers need to deploy nets with fine mesh and contend with high levels of drag.

Mesopelagic fish also spoil easily because they're full of digestive enzymes, so they have to be processed or frozen promptly. "You almost can't touch it before it's all porridge," Holmen says. According to marine scientist Stein Kaartvedt, one South African ship's mesopelagic bycatch apparently caught fire because it was so oily.

The fish that hang out in the twilight zone can also be tough to catch. Most are stragglers - they don't congregate while swimming to make for an easy one-scoop kind of bounty - and they are also notorious for dodging nets.

"They are everywhere," says Kaarvedt of the University of Oslo, but that doesn't mean they are easy to catch. "The main thing with fishing is not whether we have 1 billion or 10 billion, but whether you have sufficient concentrations to make fishing feasible."

Mesopelagic Fish Ended Up a Red Herring

The Norwegian fishing industry seems to have come to the same conclusion. In the late 2010s, a group of companies conducted a trial of mesopelagic fishing. The vessels brought in abundant catch in their first attempt, but the next two tries yielded near-empty nets. Since then, the fishing industry there has moved on to more lucrative targets, such as cod and herring. Mesopelagic fishing interest has also watered out in Iceland and South Africa, in favor of mackerel and sardines.

Across the world, more of the fish we eat comes from farms rather than wild-caught fisheries these days. But the proportion of fish stocks that have been classified as overfished has also been growing in tandem. Today, that fraction is 30 percent; and fish populations that are at capacity come in at 60 percent.

Although practically unnecessary at the moment, a moratorium on mesopelagic fisheries would help thwart future exploitation, says University of Exeter's Roberts. Nevertheless, all that research on the mesopelagic zone is far from wasted, industry proponents say. This is a rare instance of a marine industry sinking time and money into conducting a full cost-benefit analysis before diving in head first. "It could be an example of how to approach expansion of any kind," says another researcher, Irigoien, for fish as well as other ecosystems and natural resources.


Shi En Kim wrote this article for Sentient.


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As Halloween approaches, a spooky campaign aims to clear up the common misunderstandings about some of nature's most feared creatures.

In collaboration with other organizations, Defenders of Wildlife is launching its "Real Scary Movies" campaign, to highlight how animals often vilified in horror stories - like sharks, bats, and wolves - are vital to the health of their ecosystems.

Jane Davenport, senior attorney with Defenders of Wildlife, said she hopes to dispel these myths - many of which she said have been fueled by Hollywood blockbusters, like the 1975 movie Jaws.

"Obviously, a shark attack on a human is a terrifying and, unfortunately, sometimes tragic event," said Davenport. "There's only about 70 unprovoked shark attacks on people every year, all around the world. But on the other hand, humans are killing northwards of 80 million sharks every year."

Davenport said she believes the shark's frightening reputation has contributed to a lack of focus on their preservation, leading some species toward extinction.

The shark-fishing industry defends its practices by claiming U.S. shark fisheries are among the most sustainable, with laws that prevent overfishing and enhance economic value.

The campaign urges the public to make more sustainable choices of seafood and consumer products, and to look beyond the scary imagery - even if it's just for Halloween fun.

For Davenport, the public should recognize that the real "spooky and scary" story is how human activities pose a significant threat to wildlife.

"Species like sharks and bats and wolves are not out there trying to do anything other than than live their lives," said Davenport. "And so, we really need to bring awareness and do some myth-busting, to try to dispel the image that these are creepy, scary creatures."

In the spirit of the season, the campaign will put a unique twist on popular scary movies to highlight the harmful impacts of issues like bycatch - when non-target marine animals are inadvertently hooked or netted by fishermen targeting other species.

It's a different perspective to consider while binging the Jaws series.

California has set a goal to protect 30% of its land and waters by the year 2030, so the Ocean Protection Council is working on which waters should count as protected and is identifying new conservation measures.

A public comment period on its draft framework just ended.

Drew Talley, professor of environmental and ocean sciences at the University of San Diego, said the proposal is on the right track but needs a little more work when it comes to estuaries, where the rivers meet the sea.

"It doesn't look at some of the main threats those systems are facing," Talley explained. "Including sea level rise, degraded water quality and ongoing effects of historical alterations like dams and diking and urban runoff."

The Ocean Protection Council is expected to release its final proposal Dec. 9. Estuaries are crucial habitat for many species. They also buffer communities from floods during a storm, reduce ocean acidification and help fight climate change, since marine plants store a great deal of carbon.

Kerstin Wasson, research coordinator for the Elkhorn Slough National Estuarine Research Reserve near Monterey, said the contours of protected areas need to be widened, to account for the effects of climate change.

"With accelerated sea level rise, they're going to need a lot of space to migrate upwards," Wasson urged. "If we were to draw a circle around an estuary to say what's protected, it needs to be 10 feet above where the marsh is today."

More than 90% of California's historic wetlands have been lost to development.

Support for this reporting was provided by The Pew Charitable Trusts.


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Anglers along the Atlantic coast want stricter regulations to help improve stocks of Atlantic herring, a forage fish vital to a healthy ocean ecosystem.

The population was once depleted from decades of overfishing and has struggled to recover, affecting the health of larger, prized fish up the food chain.

Rich Hittinger, first vice president of the Rhode Island Saltwater Anglers Association, said it hurts local fishermen and the small coastal businesses that support them.

"We see the striped bass and the other fish not showing up in an area," Hittinger observed. "Because the forage fish aren't there."

Anglers have asked the New England Fishery Management Council to reestablish a 12-mile offshore buffer zone for large herring trawlers. Nearly all respondents to a recent public comment period backed further actions to protect the Atlantic herring and other forage fish.

A recent assessment of Atlantic herring stock showed little progress has been made in rebuilding the population. Despite increased management, federal regulators say the likelihood of meeting stock targets for next year is less than 1%.

Jaclyn Higgins, forage fish program manager for the Theodore Roosevelt Conservation Partnership, said she is concerned the science driving harvest restrictions of forage fish has not caught up with what is happening in the ocean.

"We really want to make sure that we use all of the information that we have to move this ecosystem-based management framework forward," Higgins urged. "Improve the models, improve what we have, and not stay stagnant with the status quo."

Higgins noted the Atlantic herring stock currently sits at just 26% of its biomass target. She is hopeful more data collection and research funding will lead to regionally specific and precautionary measures to better protect fisheries and the communities they serve.