- Fishing experts are looking for ways to fish in the mesopelagic zone, a layer of water that stretches from 200-1,000 meters (660-3,300 feet) beneath the surface, which has, thus far, remained relatively unexploited.
- Many challenges stand in the way of making mesopelagic fishing a reality, such as the difficulties of finding and capturing mesopelagic fish, and processing them into usable products.
- Yet experts are working to overcome these obstacles, with one suggesting that mesopelagic fishing could begin in the next few years.
- Conservation experts have expressed concern about the possible start of mesopelagic fishing, arguing that it could cause environmental problems.
Two hundred meters below the surface of the sea is a cold, faintly lit layer of water known as the mesopelagic, or twilight, zone. Here lives a menagerie of peculiar-looking creatures: blue-glowing plankton and squid; spindly fish flashing kaleidoscopes of colors, some baring rows of barbed teeth.
Many of these creatures lurk in this region’s dark depths, while others pass through it as they flit between the surface and the deep ocean, partaking in a daily migration. Due to the constant movement of these fish, they’re notoriously hard to catch, and thus far, the mesopelagic zone has remained relatively unexploited. If these animals are caught and brought to the surface, many turn into a kind of gelatinous goop.
Yet Norwegian fisher Karsten Østervold said he believes he’s found a way to handle these strange, gloppy fish. In his opinion, the key is to quickly process and preserve them, and his family business, MESO, a fishing company based in Bekkjarvik, Norway, has built a prototype system for doing so right on board the trawler.
“Mesopelagic fish contains a lot of enzymes in the belly, and when it’s harvested and comes onboard the fishing vessel, it very quickly deteriorates — the quality is very fast getting bad,” Østervold told Mongabay. “So it’s very difficult to harvest with conventional fishing vessels and to process it with conventional fishmeal factories.”
Many types of mesopelagic fish aren’t considered edible by people, so a target product for twilight-zone fish is silage, a liquid typically made from fish waste that’s used in fertilizers and aquaculture feeds. According to Østervold, working out the preservation-as-silage process will be a “key factor” that enables fishers to find a market for mesopelagic fish, which he refers to as an “unexploited natural resource.”
There are other challenges to making mesopelagic fishing a reality, such as finding and efficiently catching large quantities of these fish. Scientists also say there’s still much to learn about these organisms and the ecological role they play in the ocean.
But Østervold said he believes these issues will be quickly overcome, and that the mesopelagic fishing industry will kick off within the next few years. The EU is pumping large amounts of money and time into studying the viability of these fisheries, with Norway being a hotspot for such research. Not only that, but trial fishing for twilight species began in Norway in 2016 and is set to continue. Fishing trials have also occurred in places as diverse as the Gulf of Oman and the Southern Ocean.
The possibility of an imminent mesopelagic fishery has raised concerns among conservationists, who argue that mesopelagic species play a critical role in absorbing and sequestering carbon and in bolstering the entire marine food web. If these fish are removed in large quantities, they warn, the entire ocean ecosystem could begin to fall apart.
What is the mesopelagic zone?
The mesopelagic zone is a layer of cold water that stretches from a depth of 200 to 1,000 meters (660 to 3,300 feet) below the surface, directly under what’s known as the sunlight or epipelagic zone, where most fishing is done. Some of these oddball twilight species, such as lanternfish — wide-eyed creatures with light-producing organs from the family Myctophidae — rise to the surface at night to feed before returning to deeper waters in the day in a process known as diel vertical migration. Such species take in carbon by consuming organic matter at the surface, then dive down and excrete this matter, which transfers carbon to the deeper parts of the ocean and, in the process, helps regulate the climate. This process is part of what’s known as the “biological pump,” which is driven not only by marine organisms but also land runoff, which works together to transfer carbon from the atmosphere into the deep ocean for long-term storage.
Besides moving carbon around, mesopelagic fish are an important food source for many oceanic predators, including dolphins, tunas, marlins and sharks.
Curiously, researchers didn’t know much about the mesopelagic zone and its migratory inhabitants until the 1940s. U.S. researchers were testing a sonar device that could be used to detect German submarines at the time, but when they sent sound waves into the deep ocean, they were surprised: The seafloor was shallower than they thought, and its depth appeared to change at night. As it turned out, their sonar was hitting a high density of organisms in the twilight zone whose swim bladders were reflecting back the sound waves, creating a “false bottom.” This density of creatures is now called the deep scattering layer (DSL).
Since the discovery of the DSL, scientists have been working to understand these motile fish, and also how many fish, in total, live in the twilight zone. While trawling estimates have suggested there are about 1 billion metric tons of mesopelagic fish, a highly cited 2014 study in Nature Communications used acoustic monitoring and modeling to suggest that the truth was likely to be far higher: at least 10 billion metric tons.
“That [study] was very central for commercial interest for mesopelagic fisheries around the globe,” Dag Standal, a senior research scientist at SINTEF, an independent scientific organization headquartered in Trondheim, Norway, that has conducted research into mesopelagic fishing, told Mongabay.
Yet, thus far, trawls haven’t been able to capture vast amounts of mesopelagic fish. This has led Standal to suggest that the study may be “subject to revision,” and researchers remain divided over how to accurately estimate the amount of mesopelagic fish in the global ocean.
Fishing in the mesopelagic zone
There are some existing fisheries targeting mesopelagic species such as krill, toothfish, or squid. For the most part, however, very little mesopelagic fishing currently takes place, despite the large estimate of the mesopelagic fish biomass. For instance, there are no commercial fisheries that target the hundreds of different species of lanternfish, which one study estimates to be among the most abundant fish in the mesopelagic zone, with an estimated biomass of about 600 million metric tons.
That said, there have been many trial fisheries for mesopelagic species, some dating back to the 1970s. These have taken place in the North Atlantic, South Atlantic, Oman Sea, Arabian Sea, and the Southern Ocean, yet most proved to have “limited economic viability and poor catches,” according to one study.
Mesopelagic fishing at large might not be a reality yet, but there are many efforts to make it so. Experts say mesopelagic fish could provide an important source of fishmeal at a time when global fishmeal production is declining.
Fishmeal is heavily used in the aquaculture industry, which provides about half of the world’s fish supply, according to a report from the Food and Agriculture Organization of the United Nations (FAO). While alternative feeds are becoming available, demand for traditional fishmeal continues to soar — and, as a result, shortages are being reported. For instance, the IFFO, or Marine Ingredients Organization, found that in 2023, fishmeal production in a dozen top-producing countries was about 23% less than it was in 2022. The group attributes this decline to a fish supply shortage from Peru, which accounts for a substantial portion of the ingredients for fishmeal production around the world. Experts predict that demand for fishmeal will steadily increase as the aquaculture industry grows and wild fish populations decline due to anthropogenic stressors like overfishing and climate change.
“What we know from the Norwegian salmon feed production is that there is a very big demand for sustainable feed ingredients and especially marine feed ingredients,” Østervold said. “This is not only in Norway — it’s a worldwide requirement for sustainable marine feed ingredients. If it’s possible to preserve mesopelagic fish and to make protein and fish oil from it, this will be a very good thing for the aquafeed industry.”
At the moment, mesopelagic fish aren’t usually considered suitable for direct human consumption, due to many species being “unpalatable” and potentially containing environmental pollutants like cadmium and arsenic, according to one report. Yet Kristian Fjeld, a research assistant at SINTEF Ocean, a research group within SINTEF, said some mesopelagic fish could be fine to eat and would therefore assist with food security. For instance, in 2018, researchers enlisted a chef to prepare krill and Mueller’s pearlside (Maurolicus muelleri) experimentally, with reasonable success.
Fjeld also said scientists are studying how mesopelagic fish might be used in nutraceuticals such as omega-3 supplements and fish oils, and even in pharmaceuticals such as “antimicrobial and anti-cancer drugs from the microorganisms that live in the mesopelagic fish,” making these fish even more appealing for human uses.
Interest in mesopelagic fishing led to the establishment of two large-scale research projects in September 2019 to investigate the viability of these fisheries: the MEESO and SUMMER projects, both funded by the European Union with a budget of more than 13 million euros (about $17 million at the exchange rate at the time). MEESO finished in February 2024 and SUMMER will finish in August this year.
Tor Knutsen, a researcher at Norway’s Institute of Marine Research, the institution that coordinated the MEESO project, said both projects are “trying to address all these unknowns” about the mesopelagic domain, for which there is far less knowledge than exists for other kinds of fisheries. However, he added he believes mesopelagic research is still in the “starting phase.”
“I think things are a little bit in their infancy, at least from our side as scientists,” Knutsen told Mongabay. “We try to have, of course, an open mind about these systems, but we are very well aware that we need more knowledge to understand the functionality of these systems.”
Is mesopelagic fishing a ‘castle in the sky?’
One of the biggest obstacles to fishing, and even studying the mesopelagic zone, is finding the fish. This may seem surprising given the high biomass estimates. Yet these fish are well-equipped to avoid being caught. One study found that the noise and light from a research vessel and its equipment caused mesopelagic fish to move away “by diving downwards or by moving horizontally out of the vessel’s path.”
Knutsen also said mesopelagic fish tend to be “mixed with a range of other marine organisms,” which may stymie fishers targeting a certain species. In fact, he said he believes sorting deep-water catches at sea is “nearly impossible.”
David Reid, a scientist currently at Ireland’s Marine Institute who was involved in the MEESO program, said he and his colleagues also encountered difficulties differentiating one mesopelagic fish from another during acoustic surveys.
“It’s not just mesopelagic fish — you’ve got crustacea, you’ve got gelatinous zooplankton, you’ve got various other beasties,” Reid told Mongabay. However, he said one of the big achievements of his and his colleague’s research was being able to distinguish Mueller’s pearlside from the rest of the mesopelagic fish in the DSL. They did this by developing a “multi-frequency algorithm” during their acoustic survey process that distinguished the pearlsides in echograms. The team is now trying to do this for crustaceans, zooplankton and other mesopelagic organisms, but Reid said this research is still in its “early days.”
Catching mesopelagic fish, which tend to have small bodies, also requires special nets with smaller mesh sizes. But these nets can also scoop up large amounts of bycatch, Fjeld said.
“You can run the risk of getting bycatch of other juvenile fish because [the nets] are designed to retain the small fish, whereas a normal trawl with bigger mesh sizes are designed to let out all the small fish to avoid the bycatch,” Fjeld said. “So that’s a problem.”
A net with a smaller mesh also causes more drag, which, in turn, requires more fuel for fishing boats to drag through the twilight zone. Østervold of MESO said this creates yet another challenge to the viability of a mesopelagic fishery since “the price for fuel is very high.”
Then there’s the issue of processing mesopelagic fish into something usable that Østervold said he hopes to solve. According to him, trial fisheries operating off the coast of Iceland were successful in capturing pearlsides, but they weren’t able to immediately preserve the fish, ultimately making their catches unusable.
These many difficulties have led some researchers to suggest that mesopelagic fishing might be a “castle in a sky” or a “fairy tale,” as one study put it. Standal, who contributed research to the SUMMER program, even told Mongabay he believes interest in mesopelagic fishing has “faded away.”
In spite of the doubters, companies like MESO are working to help get a potential industry onto its feet. Not only has the company designed equipment to immediately process the “complete catch” of mesopelagic fish into silage aboard a trawler, but it’s also working to get custom-made nets and trawling equipment so it’s easier to capture mesopelagic fish, according to Østervold. The company has already done some trial fishing in the Irminger Sea and is awaiting licenses to do more in other parts of the ocean, he said.
“We believe that if we start in the correct areas where we can find big concentrations, then it should be possible to catch it and process it,” Østervold said.
“We strongly believe that it has a great future,” he said about mesopelagic fishing.
‘Catastrophic consequences’ for the ocean?
Meanwhile, conservation experts worry that if twilight-zone fishing becomes a reality, it could cause numerous environmental problems. One concern is that taking large amounts of mesopelagic fish out of the ocean will disrupt the carbon cycle.
“Mesopelagic fishes have potentially enormous importance in the global ocean, as prey for other species, many of these predators themselves, targets for valuable fisheries, and for their role in sequestering carbon into the deep sea and mitigating climate change,” write the authors of a 2020 report from the U.K.-based NGO Blue Marine Foundation.
“Based on what we already know, large-scale commercial fishing of mesopelagic fishes could have catastrophic consequences for marine life and global climate,” they write. “The ecological and environmental value of the twilight zone almost certainly far exceeds its extractive value.”
While scientists tend to agree that the mesopelagic zone is critical for the carbon cycle in the ocean, it’s still unclear how much carbon mesopelagic fish take in and sequester.
“We know that it contributes the majority of the storage from the biological pump, and then it gets a bit more fuzzy,” Jamie Wilson, a scientist at the University of Liverpool, told Mongabay. “We still don’t quite understand why and what’s driving it and what factors are important and why and how it might change in the future. So we’ve been trying to figure that out.”
Due to the many knowledge gaps, some conservation experts have called for a moratorium on fishing the mesopelagic zone until more is understood about this part of the ocean, the species that live in it and depend on it, and the potential impacts of fishing.
Duncan Currie, an environmental lawyer who acts as an adviser for the Deep Sea Conservation Coalition, a network of about 100 organizations working to protect the deep sea, compared mesopelagic fishing to deep-sea mining, another activity that has yet to begin, but that conservation experts say could be highly damaging to the ocean and requires more research to understand. He told Mongabay he has “real concerns” that the industry will “come in the back door” if legal mechanisms aren’t in place to regulate it before fishing starts.
“It could come in, for example, in a South Pacific RFMO, and based on what we’ve seen with some other fisheries, the environmental impact assessments [for those RFMOs] haven’t been very rigorous,” Currie told Mongabay, referring to intergovernmental bodies called regional fishery management organizations. “I think you’d have to be very alive to it. And once one player starts, other players may start as well.”
He added he believes the fishing industry may well find a way to make mesopelagic fishing viable, especially as global fish stocks continue to decline and fishing companies need to move “further and further offshore” to find fish.
“Once they put their mind to it and create new technology to find the fish,” he said, “I don’t think that’d be a problem.”
Banner image: A type of ophidiiform — an order of ray-finned fish that includes the cusk-eels, pearlfishes, viviparous brotulas, and others. Image by Schmidt Ocean Institute (CC BY-NC-SA 4.0).
Elizabeth Claire Alberts is a senior staff writer for Mongabay’s Ocean Desk. Follow her on Twitter @ECAlberts.
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Citations:
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