The fish on Kerala's coast are fed by a wind. Change the wind and the ocean stops delivering

The sea looks uniform from a boat, and it is nothing of the sort. Most of the open ocean is close to a desert — clear, blue, and blue precisely because there is very little living in it. The productive parts are small, localised, and depend on machinery that delivers nutrients from deep water up into sunlight.

Along India's south-west coast, that machinery runs on the monsoon. For a few months each year the water off Kerala and Karnataka becomes one of the more productive stretches of ocean anywhere, and the fishery that has fed the coast for centuries is the direct consequence. Understanding how it works makes it clear why relatively modest changes in wind and temperature matter so much more than they sound.

Why the open ocean is mostly empty

Photosynthesis needs light and nutrients, and in the ocean they are found in opposite places.

Light is only available near the surface, in roughly the top hundred metres. Nutrients — nitrate, phosphate, silicate, iron — are concentrated at depth, because everything that dies in the sunlit layer sinks, and decomposes as it falls. The ocean therefore continuously strips nutrients from the surface and stores them below.

Warm surface water sits on top of cold deep water and, being less dense, has no tendency to mix downwards. That density boundary — the stratification — is a lid. Where the lid is firmly in place, the surface layer runs out of nutrients and productivity collapses, which is why the clearest tropical water is the emptiest.

Everything interesting in ocean biology therefore happens where something breaks that lid.

What the monsoon actually does to the water

Along the south-west coast of India, the southwest monsoon does it with wind.

When wind blows along a coast, the water it pushes does not travel in the direction of the wind. The Earth's rotation deflects the wind-driven surface flow — the effect responsible for what oceanographers call Ekman transport — so that the net movement of the surface layer is at an angle to the wind, and with the right coastal orientation, away from the shore.

Surface water moving offshore has to be replaced, and the only available supply is from below. Cold, dense, nutrient-rich water rises along the coast to take its place. That is coastal upwelling, and it is the single most important process in the fishery.

The result is dramatic and seasonal. Sea surface temperature off the coast drops noticeably. Nutrients arrive in the sunlit layer. Phytoplankton bloom, zooplankton graze on them, and small pelagic fish — above all the Indian oil sardine — feed on the zooplankton in enormous numbers. A few weeks of the right wind converts an ordinary stretch of coast into one of the densest fisheries in the Indian Ocean.

When the monsoon retreats, the upwelling shuts down, the lid re-forms and the system quietens. The coast has a productive season for the same reason a farm has one.

Nutrients are not scarce in the ocean. They are simply in the wrong place, and the only question that matters for a fishery is what brings them up.

The layer with no oxygen

The Arabian Sea has a second feature that follows directly from its productivity, and it is extreme enough to be globally significant.

All that phytoplankton eventually dies and sinks, and bacteria decompose it as it falls, consuming oxygen. In a well-ventilated ocean, currents resupply that oxygen. The Arabian Sea is poorly ventilated — it is landlocked to the north, its deeper water is not efficiently renewed, and the organic rain from above is heavy. The consequence is one of the world's most intense oxygen minimum zones: a thick layer, beginning a couple of hundred metres down, in which oxygen is almost absent.

This matters well beyond fish. Most larger marine animals cannot live in it, so it compresses their usable habitat into the thin oxygenated layer above — which concentrates them, and makes them easier to catch, and also more vulnerable. And in the absence of oxygen, bacteria turn to nitrate for respiration, converting biologically available nitrogen into nitrogen gas. The Arabian Sea is therefore one of the planet's significant sites of nitrogen loss from the ocean, which links a regional feature to the global nitrogen cycle.

Oxygen minimum zones worldwide have been expanding as the ocean warms, because warm water holds less dissolved oxygen and stronger stratification slows resupply.

Something changed in the north

In the northern Arabian Sea, winter used to belong to diatoms — the armoured, silica-shelled phytoplankton that sit at the base of a productive food chain and feed zooplankton efficiently.

Over the past two decades, large winter blooms of Noctiluca scintillans have become prominent there instead. Noctiluca is a large dinoflagellate that in this region carries a photosynthetic symbiont inside it, so it both photosynthesises and eats. It tolerates low-oxygen conditions that diatoms do not, and the blooms appear where winter cooling mixes oxygen-poor subsurface water upward.

The significance is not the bloom's appearance but its position in the food web. Noctiluca is poorly grazed by the copepods that usually convert phytoplankton into fish food; it is more readily eaten by jellyfish and salps. A shift from diatoms to Noctiluca is therefore a change at the base of the chain, in a direction that does not obviously lead to fish. It is one of the clearer examples anywhere of an ecosystem reorganising from the bottom up rather than the top down.

Warming attacks the delivery system

The Indian Ocean has been warming rapidly, and the Arabian Sea has seen a marked increase in marine heatwaves and in the occurrence of intense cyclones, which historically were far more a Bay of Bengal phenomenon.

The most consequential effect on the fishery, though, is subtler than heat itself. Warmer surface water strengthens stratification — it makes the lid harder to break. Research on the western Indian Ocean has reported declines in phytoplankton over recent decades, attributed to exactly this mechanism: a warmer, more stable surface layer resists the mixing that brings nutrients up. The water is not missing anything; the delivery system is being throttled.

The fishery response has been visible on the Kerala coast within living memory. The Indian oil sardine, which dominates landings in good years, has gone through severe boom-and-collapse cycles, with abundance tracking upwelling strength and ocean conditions, and with the population's centre of distribution shifting. Sardines are naturally volatile everywhere in the world, which makes attribution genuinely difficult and is the honest caveat here — but a fishery whose foundation is a wind-driven nutrient pulse is not one to assume will stay where it is.

Fishing pressure sits on top of all this. Landings of juveniles before they have spawned, bottom trawling that damages seabed habitat, and the diversion of low-value catch to fishmeal all reduce the stock's capacity to recover from a bad environmental year. India's monsoon trawling ban along the west coast exists precisely to protect the spawning season, and minimum legal size rules exist to stop fish being caught before they breed. Their enforcement is the variable that a coastal community can actually control, which is not true of the wind.

Why it matters for students and researchers

Marine science in India has an unusual combination: a coastline of over seven thousand kilometres, a fishery supporting millions of livelihoods, an ocean system driven by a monsoon found nowhere else at this scale, and a research output far smaller than any of that warrants.

The tractable questions are plentiful. Long-term monitoring of upwelling indices against landings in specific districts. Taxonomic and ecological work on species whose basic biology is still thin, which is most of them. Documentation of the oxygen minimum zone's upper boundary, whose movement directly compresses fish habitat. Studies of what is actually landed, including the juvenile fraction and the composition of low-value catch, which is frequently unrecorded in any usable form. And straightforward, careful surveys of species distribution, which is how a shifting population is detected at all — and which requires baselines that in many places simply do not exist yet.

That scope — marine organisms, ecosystems and biogeochemical processes, from taxonomy and physiology through ecology to fisheries and aquaculture — is the stated remit of the International Journal of Marine Life (ISSN 3048-8885), a peer-reviewed journal launched in 2024. For life sciences, fisheries and environmental science students, the Arabian Sea is a good place to notice that an ecosystem can be rich not because of what it contains but because of a process that delivers, and that processes are easier to disrupt than reservoirs.

Frequently asked questions

Why is most of the open ocean unproductive?

Because light is available only near the surface while nutrients accumulate at depth, and warm surface water forms a stable layer that resists mixing. Without a process that brings deep water up, the sunlit layer runs out of nutrients.

What is coastal upwelling?

Wind blowing along a coast drives surface water offshore because the Earth's rotation deflects wind-driven flow. That water is replaced from below by cold, nutrient-rich water rising along the coast, which triggers plankton growth and supports a fishery.

Why does the fishery off Kerala have a season?

Because the upwelling that supplies its nutrients is driven by the southwest monsoon. When the monsoon winds blow, nutrients reach sunlit water and productivity surges; when they retreat, stratification re-forms and the system quietens.

What is an oxygen minimum zone?

A subsurface layer where oxygen is nearly absent, formed when sinking organic matter is decomposed by oxygen-consuming bacteria in water that is poorly ventilated. The Arabian Sea holds one of the most intense examples, which restricts where larger marine animals can live.

Why do Noctiluca blooms matter?

Because Noctiluca has largely replaced diatoms in winter blooms in the northern Arabian Sea, and it is poorly eaten by the copepods that normally convert plankton into fish food. That is a change at the base of the food web rather than at the top.

How does ocean warming reduce productivity?

Warmer surface water is less dense, which strengthens stratification and makes it harder for mixing to bring nutrients up from depth. The nutrients remain present but are not delivered to the sunlit layer where photosynthesis occurs.