The World's Largest Animal Migration Happens Every Night

During the Second World War, sonar operators kept finding the seabed in the wrong place. Their instruments returned a solid echo at two or three hundred metres in waters charted as kilometres deep, and the false bottom moved: rising toward the surface after sunset, sinking again before dawn. It took years to establish that the echo was not geology but biology, a layer of living animals so dense it reflected sound like rock. They named it the deep scattering layer, and in doing so accidentally discovered the largest animal migration on Earth.

Every night, across every ocean, an immense quantity of marine life swims upward from the twilight zone to feed in surface waters, and every morning it descends again. The animals involved are mostly small, mostly unfamiliar, and almost entirely unseen. Collectively they may outweigh every other fish in the sea, they help regulate the composition of the atmosphere, and until very recently almost nobody was arguing about who owns them. That has now changed, which is why this hidden migration has become a policy question as well as a biological one.

 

An Entire Ecosystem Moves After Dark

 

The mesopelagic zone, the ocean between roughly 200 and 1,000 metres, makes up about a fifth of the ocean's volume. It is too deep for photosynthesis and too shallow to be fully dark, lit by a faint blue residue of surface light that fades to nothing at the bottom. This is the twilight zone, and it is where the migrants spend their days.

The cast is enormous and mostly anonymous. Copepods and other zooplankton at the base. Krill and shrimp. Squid. And above all lanternfish, the family Myctophidae, which are probably the most abundant vertebrates on the planet and which most people have never heard of. A lanternfish is typically a few centimetres long, silvery, with rows of light-producing photophores along its body.

The distances involved deserve a moment of attention. An animal five centimetres long that climbs four hundred metres and returns the same night has travelled roughly sixteen thousand times its own body length in a day. Scaled to a human, that is something close to a thousand kilometres. These animals do it every twenty-four hours, for their entire lives, without rest and without navigation aids beyond the fading of the light.

How much biomass is moving is a genuinely open question, and the spread of estimates is the most honest indicator of how little is known. Figures cited in the current policy debate range from around 600 million tonnes of mesopelagic fish at the conservative end to as much as 10 billion tonnes at the upper end, with the higher figures implying that roughly 90 percent of all fish in the sea by weight live in this layer. That is a span of more than an order of magnitude for one of the largest animal populations on the planet. Part of the reason is that these fish actively avoid trawls, so net surveys systematically undercount them, and acoustic estimates depend on assumptions about swim bladders and body composition that are themselves uncertain.

 

Why They Wait Until Night

 

The explanation is a trade-off, and it is one of the cleanest illustrations of predation risk shaping behaviour anywhere in nature.

Food is at the surface. Photosynthesis happens only where there is light, so the phytoplankton that feed the zooplankton, which feed everything else, are concentrated in the upper layers. An animal that stayed in the twilight zone permanently would be safe and hungry.

Predators are also at the surface, and most of them hunt by sight. Tuna, billfish, seabirds and marine mammals rely heavily on vision, and a small silvery fish in sunlit water is extremely visible. An animal that fed at the surface during the day would be well fed and eaten.

Vertical migration resolves the conflict by separating the two in time rather than space. The animals ascend when darkness neutralises visual predators, feed through the night, and retreat below the photic zone before dawn. Light is both the cue and the constraint: the migration tracks changing light intensity so precisely that it shifts with moon phase, with cloud cover, and even during solar eclipses. There is a real cost to it, in energy spent swimming and in metabolic rate, and the fact that so many unrelated lineages independently pay that cost is strong evidence of how severe the predation pressure must be.

 

This Migration Helps Move Carbon

 

The biological carbon pump is the process by which carbon fixed at the surface ends up in the deep ocean, where it is isolated from the atmosphere for centuries. Most descriptions of it focus on gravity: dead plankton, faecal pellets and aggregates sinking slowly through the water column. That passive flux is estimated to export somewhere in the range of four to nine petagrams of carbon a year globally.

Vertical migration adds a second, active mechanism, and it works differently in a way that matters. A migrating animal feeds at the surface and then physically swims the carbon downward inside its own body. At depth it respires, defecates and sometimes dies, releasing that carbon several hundred metres below where it was consumed. The crucial point is that this bypasses the water column in between. A sinking particle is intercepted, eaten and remineralised repeatedly on its way down, which is why only a small fraction of surface production ever reaches depth. A migrating lanternfish carries its cargo through that gauntlet in a few hours and delivers it intact.

This is sometimes called the migrant pump, and estimates of its contribution range from roughly 0.9 to 3.6 petagrams of carbon a year, against the four to nine petagrams moved by gravitational sinking. Studies attributing export specifically to mesopelagic fishes put their share of total organic carbon export at anywhere from above 10 percent to more than 40 percent, with strong regional variation: under 10 percent in the California Current, above 40 percent in the North Pacific Subtropical Gyre.

A caution is warranted on one widely repeated claim. It is sometimes stated that without mesopelagic fish, atmospheric carbon dioxide would be 50 percent higher and global temperatures several degrees warmer. That figure rests on a single estimate and should be treated as illustrative rather than established. The defensible version of the claim is less dramatic and still significant: a substantial share of the ocean's ability to move carbon into long-term storage depends on animals swimming it there, and the ocean's carbon uptake is not simply a matter of physics and chemistry.

Recent modelling work underlines how much remains unresolved. A 2025 study in Biogeosciences built a trait-based model of micronekton migration, covering fishes, crustaceans and cephalopods between roughly 10 and 80 millimetres, using environmental data from a long-term station in the northeast Atlantic. It found active carbon transport reaching almost 40 milligrams of carbon per square metre per year in temperate conditions, peaking in summer, with respiration rates and prey capture efficiency explaining most of the variability. The authors' conclusion is the one worth carrying forward: micronekton are an important but under-represented component of the biological pump, and their behaviour under a warming ocean is uncertain.

 

Why This Suddenly Became a Policy Question

 

For most of the twentieth century the twilight zone was interesting and commercially irrelevant. The fish are small, bony, oily and widely dispersed, and catching them costs more than they are worth as food. That calculation is shifting.

Aquaculture needs fishmeal and fish oil, demand for both is rising, and the forage fisheries that currently supply them are fully exploited. A biomass measured in hundreds of millions or billions of tonnes, sitting outside anyone's quota system, is an obvious place to look. Norway and Iceland have run trial fisheries. The European Union has invested on the order of 17 million dollars in feasibility research. The United States moved early in the other direction, prohibiting directed commercial mesopelagic fishing off its West Coast in 2016 and its East Coast in 2017.

Trial results have been inconsistent in a pattern that should be familiar to anyone who has watched a new fishery open: large initial hauls followed by rapid decline, which is the classic signature of fishing down aggregations rather than harvesting a stock. The blue whiting fishery's boom and bust is cited as the cautionary precedent. Modelling also suggests that depleting mesopelagic fish would propagate upward, reducing populations of tuna, sharks, marine mammals and deep-sea species that depend on them, which means a twilight zone fishery would compete directly with existing surface fisheries for the same production.

The governance position is close to a blank page. The high seas account for 61 percent of the ocean, no regulations specifically protect twilight zone fish there, and mesopelagic species were largely absent from the negotiations that produced the high seas biodiversity treaty. There is no stock assessment, no quota framework, no bycatch reporting and, for most of the zone, no competent management authority.

 

What Happened at the IUCN Congress

 

On 15 October 2025, delegates at the IUCN World Conservation Congress in Abu Dhabi adopted a motion on protecting the mesopelagic zone, by 713 votes to 46.

The motion is non-binding, and it is deliberately narrower than it might have been. It calls for activities that could harm the zone, including fishing, geoengineering and deep-sea mining, to proceed cautiously and be guided by the best available science, for catch and bycatch to be reported, for marine protected areas, monitoring and strategic environmental assessment to be used, and for policymakers to recognise the zone's role in climate regulation. What it does not do is call for a pause, unlike the IUCN's 2021 motion on deep-sea mining. Its logic is preventive rather than restorative: get the rules written before the fishery exists rather than after.

The disagreement it papers over is real and worth stating fairly. Chris Dorsett of Ocean Conservancy framed the vote as a step toward awareness and summarised the position as looking before leaping. Esben Sverdrup-Jensen of the European Association of Fish Producers Organisations took the other view, arguing that if mesopelagic biomass is sustainably managed, the potential of the resource is limitless. Blue Marine Foundation has gone further than the motion, calling for a global moratorium on new mesopelagic fisheries while research continues and for marine protected areas spanning the full water column rather than just the seabed.

What makes this argument unusual is that both sides are reasoning from the same gap in knowledge. Nobody can currently say with confidence whether the twilight zone holds 600 million tonnes of fish or ten billion, how quickly they reproduce, how much carbon they move, or what a fishery would do to any of it. One side reads that uncertainty as a reason to wait. The other reads it as a reason not to foreclose an opportunity.

 

A Hidden Migration Shapes Ocean Life

 

Diel vertical migration is the mechanism that connects two ocean worlds that would otherwise barely interact. Surface production feeds the deep, deep-dwelling animals subsidise surface predators, and carbon moves downward faster and further than physics alone would carry it. It happens every night, everywhere, and has done so for far longer than there have been humans to miss it.

The practical point for anyone thinking about ocean policy is that the twilight zone is now simultaneously the least understood major ecosystem on the planet and one of the few remaining unclaimed biological resources of global scale. Those two facts are arriving at the same moment, which is precisely the combination that has gone badly before. The IUCN vote suggests a reasonable number of people have noticed.

 

Note: This article reflects the state of research and policy as of October 2026, drawing on sources including the IUCN World Conservation Congress motion on the mesopelagic zone adopted in October 2025, Blue Marine Foundation's 2025 report on the twilight zone, Thibault et al. in Biogeosciences (2025) on micronekton contributions to carbon export, and published estimates of mesopelagic biomass and carbon flux. Biomass and carbon figures for this zone carry exceptionally wide uncertainty and are presented as ranges for that reason.