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The Ocean’s ‘Twilight Zone’ Faces Fishing Threat

The mesopelagic area of the ocean plays a crucial role in marine ecosystems, but it has remained largely untouched and unexplored But now some nations are eyeing the deep sea as potentially rich new fishing grounds.

Written by Alastair Bland Published on Read time Approx. 7 minutes
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The fangtooth lives in the deep ocean, where it preys upon fish and crustaceans.Courtesy of Danté Fenolio/DEEPEND

Virtually no sunlight reaches the deep waters of the mesopelagic zone of the ocean, which ranges from about 650 to 3,000ft (200–900m) in depth. And until now, few fishing nets have either.

But that soon may change.

Fishing nations are exploring the possibilities of trawling the ocean’s “twilight zone,” a vast mid-water world left largely untouched by fishing, where light-generating fish and pelagic shrimp swarm in immense masses. Norway has led the push, driven largely by a coastal aquaculture industry in need of plentiful feed for farm-raised fish.

Now scientists and international fishery managers are scrambling to learn more about the creatures that live in the twilight zone and the potential to fish them into depletion.

“There’s a real lack of data to help us determine if these fisheries are at all sustainable, or if it will be a one-time, go-and-grab-it-and-then-look-for-another type of fishery,” said Tracey Sutton, a marine scientist at Nova Southeastern University in Florida who has extensively studied the mesopelagic reaches of the ocean.

Sutton recently participated in a series of presentations at the 2018 Ocean Sciences Meeting in Portland, Oregon, that focused on this unique ecological region. He and several other researchers discussed scientific efforts to study mesopelagic ecosystems, the gaps in scientific knowledge that could impede successful management of future fisheries and how intensively fishing this depth layer could affect marine ecosystems worldwide.

The latter question could be of particular importance. That’s because many of the fish and invertebrates of the mesopelagic depths – otherworldly creatures like lanternfish, sabretooth fish, pearlsides and a variety of shrimp and cephalopods – provide a unique ecological service that scientists call the “carbon pump” effect. That is, these animals ascend into shallow waters, usually during the night, to eat plankton before returning to the depths. In doing so they transport energy in the form of carbon toward the seabed. There, the carbon moves through the food web – a nutrition source that might not reach such depths without the mesopelagic’s widely ranging inhabitants.


“These animals are a critical link in facilitating carbon transport to the deep ocean – that’s one of the major ecosystem services they provide,” said Kevin Boswell, a marine ecologist with Florida International University who participated in the recent discussions in Oregon. He noted that by removing the linkage between surface and deepwater zones, “you may very well be playing with the balance of carbon in the world’s oceans.”

The mesopelagic zone has been fished in the past, though not extensively.

Manuel Barange, the fisheries and aquaculture director of the United Nations Food and Agriculture Organization (FAO), says fishing this zone has proven rather unprofitable for several reasons. For starters, mesopelagic fish have very little economic value, as the only large-scale market for them is fishmeal production. (“I don’t think you could convince many people to have a plate of mesopelagic fish for dinner,” Barange quipped.)

Coupled with the fact that accessing the mesopelagic’s depths often requires very long boat journeys past the continental shelf, this can mean limited rewards for fishers willing to make the effort. Another issue is that processing the very oily flesh of mesopelagic fish into meal or paste can be a challenge for existing machinery.

“These fish are so high in oil content that they can actually jam up the processing plants,” Barange said.

Still, interest in fishing the mesopelagic is growing.

In 2017, an official with Norway’s Directorate of Fisheries gave a presentation at the North Atlantic Seafood Forum calling for research into the feasibility of exploiting the mesopelagic’s various fish and invertebrates, which the presentation described as “an important, almost untapped source for marine feedstuffs” to support fish farming. (Norway is one of the world’s largest exporters of farmed fish.) At least one more similar discussion will take place at this year’s conference in March.

The renewal of interest has come partly as a result of research published in 2014 that indicated the global quantity of mesopelagic fish and invertebrates could be 10 times greater than was previously believed. The authors of that study used acoustic sampling methods to gauge the size and density of the teeming masses of animals 700ft (200m) and more beneath their boat during a 32,000-mile (50,000-km) cruise in 2010.

Their results suggested a global mesopelagic biomass of as much as 10 billion metric tons. Such volumes are naturally attractive to fishing industries, despite the potential logistical challenges of catching and marketing them.

Lanternfish are an important part of the “deep scattering layer” or DSL. The DSL is a community of deep-water wildlife that migrates, every day, to shallower waters to feed on plankton at night. They return to the depths by day. It is the largest migration of wildlife on the planet, and it happens every 24 hours. (Courtesy of Danté Fenolio/DEEPEND)

However, Barange thinks the estimate is high, since acoustic sampling can provide rough – or even entirely inaccurate – data. He said that how the echoed signals return to the vessel can be affected by the depth from which the sound signals are generated, the angle at which they reflect off animals or debris, what that material is made of and whether detected fish have gas-filled swim bladders.

“And because the measure of reflectivity is logarithmic, any error is multiplied very quickly,” Barange said.

Though uncertainty remains in terms of how much biomass exists in the mesopelagic, it seems generally agreed that diversity of species here is tremendous.

“We’re talking upwards of thousands of species,” said Boswell. “This isn’t like 20 species, or even hundreds, and because it’s so diverse, it must mean it’s pretty special, and we should be careful with how we proceed if large-scale harvest is a goal.”

In spite of the richness and the remoteness of this ecosystem, the potential for overfishing it could be greater than for other ocean environments. For one thing, fishing at such depths involves the use of enormous nets more than 100ft (30m) wide and as long as a football field. The volume of fish taken with such gear is massive. Fishers also have little control over what fish they catch when using such wide-mouthed trawl nets, which inevitably means non-targeted marine life will be killed as bycatch.

“It’s really hard to target certain species with this type of fishing, so the bycatch could be incredible in this situation,” Boswell said.

Moreover, he explains, because the animals of this region frequently move up and down in the water column, fishing gear set to target a certain species or group of species may routinely encounter unwanted creatures as they make their daily vertical migrations.

Yet another factor that could make mesopelagic species more vulnerable and that scientists say deserves more research is the life cycles of the animals. Slow-growing fish tend to reproduce more slowly than fast-growing species, which makes them more vulnerable to depletion. Though researchers know little about the life histories of mesopelagic species, Boswell said it’s reasonable to assume that at least some mesopelagic fish grow, and reproduce, more slowly than many commercially valued fish of shallow water zones.

“It would be irresponsible in my view to open a mesopelagics fishery unless and until we understand the ecological consequences,” Geoff Shester, California campaign director at environmental group Oceana, said in an email. “It is truly the last untapped protein source in the largest habitat on Earth. In fact, we spent the last 10 years preventing mesopelagic fisheries off the U.S. West Coast.”

Though few regulations currently exist for fishing in the mesopelagic zone, Barange says creating some wouldn’t be terribly complicated. Since much of the mesopelagic zone lies within national or territorial waters, individual governments would be responsible for setting regulations. In international waters, Barange said groups of countries with a common interest in the resource would need to come together and set fishing limits – rather the way that they do to manage midocean tuna stocks. Barange says an FAO directive requiring governments to consider both target species and the entire ecosystem when setting fishing regulations would determine how exactly laws are drafted and enforced.

And in September, negotiations begin at the United Nations to draft an international treaty to protect the biodiversity of the high seas – the nearly 60 percent of the ocean beyond national jurisdiction.

Though any coordinated and large-scale efforts to fish this zone are probably years away, Sutton says Norway’s interest in fishing the mesopelagic is worth keeping an eye on.

“The alarming part of this is that we don’t have enough information now to predict what the effects would be of large-scale removals of what is basically the middle of the food chain,” he said.

Barange says he isn’t surprised that fishing nations would be curious about mining the potentially huge protein reserves of the deep ocean.

“If the world’s population goes up to nine or 10 billion people, where will the food come from to feed them?” he asked.

However, he expects the technical and economic challenges of fishing so far from shore for such unpalatable fish will dissuade most fishery ambitions for years to come.

“I think we’re a long way from developing a fishery, and I am glad that’s the case, to be honest,” he said.

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