The East China Sea: Kelp, Raft Farms and the Zhoushan Fishing Ground

East of Shanghai, the coast of Zhejiang dissolves into islands — about 1,390 of them by the usual count — and the sea turns the colour of tea with milk. This is the Zhoushan archipelago, gateway to what was for centuries the most productive fishing ground in China: the Zhoushan fishing ground, where the Yangtze's immense summer flood meets the warm currents arriving from the south. It is not a classical kelp-forest coast — the water is too turbid, the bottom too soft, the summers too hot — but it is one of the most instructive pieces of ocean on the planet for anyone who wants to understand where world seaweed farming came from, what overfishing does to a ground, and how a megafishery is now trying to put its habitat back. This is that sea, from the river plume down.

Where a river makes a sea

Stand at the estuary of the Yangtze in June and you are watching one of the planet's great freshwater discharges turn marine. The plume of low-salinity, nutrient-laden water — the Changjiang Diluted Water — spreads east across the shelf in summer, thin and fertile, before veering northeast towards Jeju under the monsoon winds. Beneath and around it, the Taiwan Warm Current pushes subtropical water up the coast, while further offshore the Kuroshio — the Pacific's great western boundary current — runs black and fast past the shelf edge.

The Zhoushan archipelago sits exactly in the confluence. For fish, this is perfect country: the river delivers nutrients that feed plankton blooms; the islands and banks create fronts where that productivity concentrates; the shallow banks and turbid water give juvenile fish somewhere to hide. For seaweed, harder. Wild kelp needs light and rock, and the estuary-adjacent sea offers little of either — the light gives out within metres on a turbid day, and vast stretches of the shelf are mud. The rocky island shores do carry weed — Sargassum and small browns in the winter-spring window, green Ulva in summer — and the archipelago's clearer outer islands hold real seaweed beds. But the genuine kelp forests belong at the latitude's cooler margins: north into the Yellow Sea's farmed offshore longline realm, and east to the wild reefs of Korea's East Sea coast.

The fishing ground: rise, crash, recovery

Zhoushan's fishing ground is where the Chinese phrase for "overfished" earned its dictionary definition. For centuries the grounds around the archipelago supplied what the locals called the four great fish: large yellow croaker, small yellow croaker, hairtail and cuttlefish. Large yellow croaker — a golden, grunting member of the drum family — was once so abundant here that fishers claimed to hear the shoals arrive.

The twentieth century was less gentle. Fleet capacity climbed steeply from the 1950s; by the 1970s the croaker shoals were fished into commercial collapse, and by the 1990s the ground was landing mostly smaller, faster, replacement species. The state response, eventually, was sweeping and worth studying. Since the mid-1990s the East China Sea and neighbouring waters have been covered by an annual summer moratorium — a blanket seasonal closure running, in modern form, roughly from May to late summer, one of the largest regular fishery closures on Earth. Alongside it: buy-back and scrapping schemes for vessels, total catch quotas, and swimming-pool-scale stock enhancement — hundreds of millions of juvenile croakers and other species released into the ground most years to rebuild the breeding shoals. The croaker is genuinely back at catchable levels now, albeit with debates about how much of the recovery owes to released fish and how much is wild rebound. It is a landmark case in what happens when a fishing nation, having exhausted a ground, spends thirty years and a fortune trying to repair it.

Marine ranching: raising habitat, not just fish

The newer chapter is subtler and, for our purposes, more interesting. Beginning in the 2000s, the Zhoushan area and the Zhejiang coast more broadly became a proving ground for "marine ranching" (haiyang muchang): designated areas of sea treated as pasture to be improved rather than solely extracted from. In practice that has meant sinking artificial reef structures to add hard habitat, creating seaweed and algae beds — the term moba travels from Japanese straight into Chinese practice — over the new substrate, and managing release-and-catch fisheries over the improved ground.

The kelp link runs straight through the middle of it. A seaweed bed is the cheapest urchin-free nursery that exists, and a rocky artificial reef sown with Sargassum or kelp fronds gathers grazing snails, juvenile crabs and young croaker within a season. Marine ranching, in other words, is restoration with a fishing rod attached — the same harvest-positive logic that Japan's satoumi programmes spent the last decades perfecting, scaled to a continental coastline. Our science piece on the kelp restoration toolkit sets out what the evidence says about how reliably these rebuilds work; the Zhejiang projects are among the largest live tests of that evidence in the world.

Farming the arc south

The seaweed industry proper starts a little further south. Zhejiang's seaweed farms concentrate on laver (Pyropia, the southern sister of the classic nori species, farmed as zicai), the red alga Gracilaria for agar, and kelp in its sheltered pockets, but the great kelp belt of the East China Sea coast begins across the boundary in Fujian. Xiapu county, on the East China Sea's southern shore, has turned itself into a seaweed-farming landscape of a kind seen almost nowhere else: tens of square kilometres of rafts and pole-and-rack grids, kelp harvested and dried onto terraces each early summer, laver racks filling the bays from autumn — the southern, warm-water end of the same farmed-kelp arc that runs north through the Yellow Sea and ends on a boat deck off Hokkaido's kombu coast. How a species with brown-algal cold-water taste came to accept semitropical latitudes has a simple local answer: "on a line, in the lit water, and for one winter season only."

The nutrient ledger: blooms, hypoxia, and the argument for seaweed

No honest account of this sea can skip the runoff. The Yangtze basin drains the most intensively farmed and industrialised land on Earth, and the river's discharge carries landward consequence seaward: the East China Sea shelf experiences some of the world's most studied coastal hypoxia — bottom water drawn down to dangerously low oxygen in warm-stratified summers — and notorious harmful algae blooms. Our explainer on river plumes and kelp covers the mechanism.

Here is where the seaweed farms complicate the story, usefully. A kelp or laver crop is, among other things, a nutrient-removal machine: it harvests dissolved nitrogen from the water column and lands it as biomass. Researchers working this coast have measured substantial nitrogen drawdown by the raft-farmed kelp floats of Fujian and Zhejiang, and have proposed, in echo of the Yellow Sea work, that strategically sited seaweed cultivation is a legitimate policy tool against bloom pressure. It is not a silver bullet — the scale of the river input dwarfs any conceivable farm footprint, and the farms themselves generate fouling and plastic-rope waste that have to be managed — but the reasoning is exactly the logic we examine in integrated multi-trophic aquaculture: use the farms as the sea's kidneys, not just its crop.

What the East China Sea teaches

Every coast in this Atlas has a lesson, and Zhoushan's is the least rhetorical of them. First: a superproductive fishing ground can still be emptied, and quickly, if capacity outruns the fish. Second: capacity can be walked back — moratoria, quotas, scrapping and restocking are expensive, imperfect and real. Third: the habitat is the thing. Moratoria protect a crashed stock; putting weed beds and reefs back is how you give the stock somewhere to come home to. When Tidewright's project teams talk about Holdfast nurseries and Drifter-seeded reefs as infrastructure, the Zhejiang archipelago's marine ranches are among the working precedents we study.

Key takeaways

• The Zhoushan fishing ground was historically China's greatest, built on the confluence of the Yangtze's nutrient plume, the Taiwan Warm Current and island-bank fronts.
• It was fished hard into collapse by the 1990s, and has since been a continental-scale experiment in moratoria, vessel buy-backs and mass stock enhancement — with the large yellow croaker genuinely returning.
• Newer "marine ranching" projects sink artificial reef and seed seaweed beds over them — harvest-positive restoration at scale, and the kelp-restoration logic done in waters too turbid for natural canopy.
• The East China Sea's intensive laver, agar-weed and kelp farming belt extends south into Fujian's extraordinary Xiapu — the warm-water end of the farmed-kelp arc.
• The Yangtze's nutrient load feeds shelf hypoxia and algal blooms, and the raft-farm seaweed that strips nitrogen from those waters is part of the proposed answer — as well as part of the disease pressures to manage.

FAQ

Are there wild kelp forests around Zhoushan?

Not in the classic sense. The coastal water is too turbid and warm for large canopy kelps like the Laminaria forests of cooler coasts; the wild vegetation is smaller Sargassum and seasonal browns on the clearer outer island rocks. The great kelp resources of China's coasts are mostly farmed — in the Yellow Sea's offshore rafts and the Fujian–Zhejiang seaweed belt.

What happened to the large yellow croaker?

It was one of Asia's most abundant food fish, native to the Zhoushan ground, and it was fished into commercial collapse by the 1970s. It remains listed as critically endangered in the wild. Since the 1990s, the seasonal moratorium and enormous stock-enhancement releases have rebuilt catchable numbers — though what fraction of the recovery is truly wild breeding is a live scientific question.

What is marine ranching?

The practice of treating a piece of sea floor as pasture: building habitat (artificial reefs, seaweed beds), releasing or enhancing stock over it, and managing the harvest. It sits halfway between aquaculture and restoration — China's answer to depleted fishing grounds, piloted most prominently around the Zhoushan archipelago.

Why is the water around the islands brown?

The Yangtze and Qiantang rivers discharge tremendous loads of fine sediment and dissolved nutrients into a shallow shelf sea, and the tide keeps it all suspended. It is not dirty water in the simple sense — it is turbid, and it is the same turbidity that fertilises the fishing ground and tightens the light budget for seaweed.

Can seaweed farming help the East China Sea's water quality?

Locally, measurably — seaweed strips dissolved nitrogen from the water it grows in, and studies on this coast have quantified substantial uptake by the raft farms. At the scale of the Yangtze's total nutrient load, cultivation alone cannot fix shelf hypoxia; it is best understood as one meaningful tool beside river-basin nutrient management, with fouling and marine-litter pressure of its own that must be governed.