FEW have heard of the mesopelagic. It is a layer of the ocean, a few hundred metres below the surface, where little light penetrates, so algae do not live. But it is home to animals in abundance. There are bristlemouths: finger-sized fish with gaping maws that sport arrays of needle-like teeth. They number in the quadrillions, and may be the most numerous vertebrates on Earth. There are appendicularians: free-swimming relatives of sea-squirts a few millimetres across. They build gelatinous houses several times their body-size, to filter food from the water. There are dragonfish (pictured). They have luminescent spotlamps which project beams of red light that they can see, but their prey cannot. There are even squid and swordfish—creatures at least familiar from the fishmonger’s slab.

And soon there will be nets. Having pillaged shallower waters, the world’s fishing powers are looking to the mesopelagic as a new frontier. The UN’s Food and Agriculture Organisation reported in 2002 that the fish-meal and fish-oil industries would need to exploit this part of the ocean in order to feed fish farms. In the past nine months Norway has issued 46 new licences for vessels to fish there. In September the government of Sindh, a province that is home to most of Pakistan’s fishing fleet, issued a draft policy on licensing mesopelagic fishing in its waters. And at the North Atlantic Seafood Forum, held in March, in Bergen, Norway’s principal port, the session about fishing the mesopelagic was entitled “the Big Apple”.

On the face of things, biting that apple seems a good idea. The mesopelagic is home to 10bn tonnes of animals. Cropping a mere 1% of this each year would double the landed catch of the ocean’s fisheries. Most of this catch would probably not appeal to human palates. But fish farmers and meal merchants would lap it up.

The mesopelagic also, however, acts as a carbon pump. Every year it pulls between 5bn and 12bn tonnes of that element out of the surface waters and into the depths, where there is a vast reservoir of the stuff. Though currents which well up from that reservoir return a similar amount of carbon to the shallows, this cycle still plays an important role as a counterbalance to man-made global warming.

Deep waters

To try to understand the mesopelagic better, the Woods Hole Oceanographic Institution (WHOI) in Massachusetts, NASA, America’s space agency, and Norway’s Institute of Marine Research are all embarking on projects to study it. As ecosystems go, it is an odd one. Its inhabitants are in a state of perpetual migration, rising to the surface at night to feed, then returning to depths of between 200 metres and 1km at dawn, to escape predation. It is this migration, the biggest in the world, that drives the carbon pump. The nocturnal feasting consumes prodigious amounts of that climate-changing element in the form of small, planktonic creatures. Then, during the day, the feasters release part of what they have consumed as faeces. Some of them also die. These faeces and bodies fall through the water column as what is known as marine snow, and accumulate at the bottom. Without mesopelagic predators, far more plankton would die in the surface waters, their bodily carbon returned rapidly to the atmosphere. The vast harmless reservoir of carbon in the depths would thus be a little smaller; the damaging burden of atmospheric carbon a good bit greater.

Until now, the only sensible way to probe mesopelagic activity has been by sonar. This is, indeed, how the zone was discovered, in 1942, by an American anti-submarine research project. From their earliest days such soundings suggested a lot of creatures live in the mesopelagic. They are sufficiently abundant that the equipment then available saw the zone as a “false bottom”, beneath which sonar could not penetrate and submarines might thus hide. But it was subsequent probing by a Spanish expedition, the Malaspina circumnavigation in 2010, which came up with the current 10bn-tonne estimate and showed just how big a part of Earth’s biosphere the mesopelagic actually is.

Sonar is still important for investigating the zone. Norway, which has long paid attention to the sustainability of its fishing operations, will launch the third incarnation of Dr. Fridtjof Nansen, the flagship of its marine-research fleet, in May, with an explicit focus on mapping and understanding mesopelagic life using the most advanced civilian sonar available. But sonar can see only so much. WHOI’s goal is to study the zone using robots, which the institute’s engineers are now constructing.

The largest of these planned devices is called Deep See. It is a sensor-packed underwater sled weighing about 700kg. One of WHOI’s research vessels will tow Deep See through the mesopelagic, gathering wide-angle camera footage and environmental data. When the probe spots something, a second robot will swim down from the research vessel to explore.

This second device, Mesobot, weighs 75kg and is shaped like a bar of soap. Unlike Deep See, Mesobot will run untethered. It is designed to hang in the water column and observe mesopelagic life for extended periods, in particular by using high-definition cameras to track animals up and down during their daily migration. WHOI’s roboticists are paying special attention to Mesobot’s thrusters, ensuring that they do not disturb the life the probe is trying to video. It will be the first time that the behaviour of mesopelagic animals has been recorded in a natural setting. Mesobot will also have a special sieve for capturing organisms in a way that preserves them from the disruptive pressure change associated with surfacing.

WHOI’s third type of mesopelagic robot will be disposable probes called Snowclops. These will sink through the water column, measuring the amount of marine snow at various depths. On its way down, snow is a potential source of food for other animals. Recording its fate at different levels is thus crucial to understanding how the carbon pump works.

Combining data from these three types of robots will paint a more accurate picture of life in the mesopelagic, and thus of its importance to matters climatic. In collaboration with NASA, WHOI also hopes to find variables that are observable by satellite and that correlate with the health of the mesopelagic and the size of its carbon flux. The principal satellite involved here, if it can survive the Trump administration’s budget proposal to cut its funding to zero, will be PACE (short for Plankton, Aerosol, Cloud, ocean Ecosystem). This is scheduled for launch in 2022. Though PACE will not be able to see directly into the mesopelagic, it will be able to measure, from the spectrum of light reflected from the ocean, things like rates of plankton consumption.

The forthcoming decade should, then, serve to start answering the question of how much fishing of the mesopelagic can be undertaken without disrupting it—and with it, its role in climate regulation. Once the fleets start hauling in their catches, the temptation will be to collect more and more. History shows that such piscatorial free-for-alls usually end badly. In the case of the mesopelagic, though, regulators will start with a clean slate, and thus a rare opportunity to agree in advance a way of stopping that happening. Whether they will take it is another matter.