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Seascape: the state of our oceansMarine life



IN THE OCEAN’S TWILIGHT ZONE, A FISH THAT COULD FEED THE WORLD – OR DESTROY IT

Lanternfish, so-called because of their bioluminescent photophores, perform a
vital role by helping transfer carbon from the sea’s surface to the deep.
Photograph: Morgan Trimble/Alamy
Lanternfish, so-called because of their bioluminescent photophores, perform a
vital role by helping transfer carbon from the sea’s surface to the deep.
Photograph: Morgan Trimble/Alamy

Lanternfish, the Earth’s most abundant vertebrates, may be the ultimate food
source. But will catching them ruin the climate?


Seascape: the state of our oceans is supported by

About this content
Helen Scales
Thu 29 Sep 2022 11.01 BSTLast modified on Thu 29 Sep 2022 11.03 BST
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In 1789, the explorers Alessandro Malaspina and José de Bustamante set sail from
Cádiz on Spain’s first scientific expedition around the world. For five years,
Malaspina and Bustamante studied and collected animals and plants across the
Spanish empire, which stretched along the North, Central and South American
Pacific coasts, and westwards to the Philippines.

In 2010, another Spanish expedition set off from Cádiz, tracing much of the
original route and studying what the oceans are like today.



The team measured pollutants, plastics and chemicals that were not there in
Malaspina and Bustamante’s time. They collected samples of seawater and
plankton. And all the way through the 31,000-mile voyage, the ship’s sonar was
switched on, listening for echoes from below. Their chief targets? Small silver
fish that look like sardines or anchovies – only with bigger eyes and rows of
spots that glow in the dark.


A school of lanternfish. Photograph: Oar/National Undersea Research Program

They are lanternfish: there are about 250 species and they are not only the most
common fish in the oceans’ twilight zone but the most abundant vertebrates on
the planet. Huge numbers were first noticed during the second world war, when
naval sonar operators saw echoes from what appeared to be a solid seabed, one
that rose to the surface at night and fell back down at daybreak. In fact, the
pulses of sound were echoing off the swim bladders – the internal gas-filled
bubbles – of billions of lanternfish, as they congregated in dense layers hiding
in the deep, then at sunset swam up thousands of metres to feed at the surface.
Every night, along with other animals, such as the squid that prey on them,
lanternfish undergo the greatest animal migration on the planet.



Before the 2010 Malaspina expedition, studies based on trawl surveys estimated
that the twilight zone contains about a gigatonne (1bn tonnes) of fish. But this
was most likely an underestimate, it turns out, because lanternfish avoid being
caught by swimming away from the open nets. The Malaspina acoustic survey did
not rely on nets, and in 2014 its research led to new estimates of the abundance
of twilight-zone fish, ranging between 10 and 20 gigatonnes.

> One study estimated that deep-dwelling fish capture and store the equivalent
> of 1m tonnes of carbon dioxide every year



The prospect of such a colossal harvest raised an old question: could fish from
the twilight zone help to feed a growing human population?


TOO TEMPTING TO IGNORE

Lanternfish are unlikely to appear directly on anybody’s plate – they are far
too oily and full of bones. However, their high oil content means they could be
mashed down for animal feed, mostly for fish farms. After the Malaspina
discovery, it has been suggested that if just half of the lower estimated mass
of twilight-zone fish were caught – still a massive 5 gigatonnes – it could
theoretically be turned into enough fishmeal to yield 1.25 gigatonnes of farmed
seafood, which is considerably more than the current annual 0.1 gigatonne catch
of wild fish.



However, even if harvesting lanternfish were to begin, and setting aside other
environmental impacts of many types of fish farming, such as pollution from
pharmaceuticals and waste, many question whether it would achieve the virtuous
goal of securing food for everyone to eat.


Lanternfish come in a variety of shapes and sizes, all of which have
bioluminescent organs. Photograph: Paul Caiger/Woods Hole Oceanographic
Institution

A lot of fishmeal gets fed to salmon and prawns for food-rich, developed
countries, and a growing volume is increasingly being sold as a supplement in
pet food. Moreover, previous attempts to establish lanternfish fisheries,
including by Russian and Icelandic fleets, have been a commercial failure.
Fishing these deep waters has so far proven too expensive, and fishmeal too
cheap.



More recently, however, prompted in part by the high estimates of lanternfish
populations, plans are under way to investigate how to make twilight-zone
fisheries profitable. The EU has funded a five-year research project to
investigate such opportunities. In 2017, Norway issued 46 exploratory fishing
licences for the twilight zone. These fisheries will probably seek to become
profitable, not by producing low-cost fishmeal, but by supplying the more
lucrative “nutraceuticals” industry supplying products such as omega-3
supplements and the fish-oil pills more people are taking despite little
evidence of their benefits.

These and other initiatives to develop a “twilight fishery” reflect an
overwhelming imperative to hunt for wild fish. Amid talk of sustainability – and
of the need to “feed the world” – is the counter-assumption that to leave those
fish unfished would somehow be a waste. The term “underexploited” is often used,
as if the only purpose of those animals is for human benefit. The idea of a
thousand trillion shining fish cascading through the twilight zone is too
tempting for many to ignore.


A lanternfish, which has light organs on its tail to confuse predators, is
pursued by a viperfish in the deep ocean. Photograph: Minden Pictures/Alamy


To catch enough lanternfish and make it worth the effort, these fisheries will
probably need to use huge midwater trawl nets and target the fish during the
day, as they cluster together in large shoals that are easy to find with sonar.
The nets will not touch the bottom or smash through 1,000-year-old corals, but
as they sieve and strain the open water, they will catch other animals – sharks,
dolphins, turtles – that already have troubles enough.


CLIMATE CONSEQUENCES

In contrast to extremely slow-growing deep-sea species such as orange roughy,
lanternfish are more likely to withstand substantial hunting pressure; they are
much faster growing, and their lives are measured in months, some living for
less than two years. Nevertheless, fishing in the twilight zone could trigger a
different kind of catastrophe by disrupting the way lanternfish and similar
species help regulate the climate.

Their daily routine of swimming up and down forms vital connections between the
surface and the deep by boosting the “particle injection pumps”. This is the
process of little fish feeding in the shallows, then plunging downwards, where
they are eaten by bigger fish that remain in the deep, thereby “pumping” carbon
dioxide from the atmosphere into the deep ocean where it can be stored. If
particles sink below 1,000 metres their carbon can be stored for up to 1,000
years before returning to the surface. A study of the continental slope off
western Ireland estimated that deep-dwelling fish capture and store the
equivalent of 1m tonnes of CO2 a year.


A hula skirt siphonophore, common in the deep ocean, has reflective, gas-filled
bubbles inside its body. Photograph: imageBroker/Alamy

No one can be sure how quickly or critically this biological carbon pump might
weaken if twilight-zone fisheries were to damage that link between the surface
and the deep. But there is a risk that lanternfish are a part of the global
climate system that needs to be left alone.

Alarmingly, not everyone agrees with the new elevated figure for the numbers of
twilight-zone fish. Even the 2010 Malaspina study states its uncertainty and the
limitations of the methods used. But the headline – that the twilight zone
contains at least 10 times the amount of fish as previously thought – grabbed
people’s attention.

Subsequent studies have looked more critically at those figures and the
assumptions that underpin them. Crucially, the Malaspina study assumed that the
acoustic “backscatter” – the measure of sound reflected from the deep and
received by the sonar – came entirely from fish. But they are not the only
animals in the twilight zone with reflective, gas-filled bubbles inside their
bodies. They are also found in many siphonophores– intricate jellies that the
19th-century German naturalist Ernst Haeckel identified and illustrated. And
some twilight-zone fish lack swim bladders, so are not detected by sonar.

Offshore fish farms: a new wave of food production … or the ‘wild west’ of ocean
pollution?
Read more


A 2019 study reinterpreted the acoustic data from the Malaspina expedition,
taking these uncertainties into account. The resulting estimates of
twilight-zone fish ranged from 1.8 to 16 gigatonnes. It is too soon to say where
on this scale the true value lies, which means it is surely too soon to start
catching lanternfish based on the risky premise that there might be 20
gigatonnes out there.

Recent history tells us that when industrial fisheries sweep into new regions to
catch new species there are always devastating environmental effects. Can the
same mistake be avoided in the twilight zone?

 * An edited extract from The Brilliant Abyss: True Tales of Exploring the Deep
   Sea, Discovering Hidden Life and Selling the Seabed by Helen Scales,
   published by Bloomsbury Sigma
   
   
   

Topics
 * Marine life
 * Seascape: the state of our oceans

 * Fishing
 * Conservation
 * Climate crisis
 * Greenhouse gas emissions
 * Food
 * features

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