Explain how fisheries and aquaculture feed the world, why stocks collapse, how MSY and the yield–effort curve guide quotas, and evaluate management from MPAs to eco-labels, with HL law, monitoring and ethics.
Each is in the ESS glossary, with every other term in the subject.
A chemical coating, often containing copper or other biocides, applied to nets, cages and boat hulls to stop algae, barnacles and other organisms growing on them.
The farming of aquatic organisms, including fish, molluscs, crustaceans and aquatic plants, with intervention in the rearing process such as stocking, feeding and protection from predators.
The use of explosives to stun or kill schools of fish so they can be collected, a destructive and widely illegal method that shatters coral reefs.
A fishing method that drags a large cone-shaped net, weighted to stay in contact with the seabed, to catch fish and shellfish living on or near the bottom.
The unwanted animals caught unintentionally while fishing for a target species, such as juvenile fish, turtles, dolphins and seabirds, which are often discarded dead.
The catch obtained for a standard amount of fishing effort, such as kilograms per hour of trawling, used as an indirect index of the abundance of a fish stock.
A period during which fishing for a species is banned, usually timed to protect it while it spawns or while its young grow.
Outline the role of phytoplankton and macrophytes as the producers of freshwater and marine food webs.
Describe how and why human demand for aquatic foods has changed, using data.
Explain how bottom trawling, ghost fishing and the use of poisons and explosives damage aquatic ecosystems.
Explain how overexploitation can lead to fishery collapse, using the northern cod of Newfoundland as an example.
Interpret a yield–effort curve and explain why maximum sustainable yield should cap fishing quotas.
Explain how climate change and ocean acidification are affecting aquatic ecosystems and food production.
Evaluate policy, legislation and consumer action, including marine protected areas, for making fisheries sustainable.
Evaluate the environmental impacts of aquaculture and one technique that reduces them.
Explain how stratification, upwelling and nutrient loading control aquatic productivity, and how fish stocks and catches are measured. (HL)
Discuss the risks of harvesting at maximum sustainable yield and how cooperation helps overexploited stocks recover. (HL)
Evaluate the governance of exclusive economic zones and the high seas, and the ethics of hunting seals, whales and dolphins. (HL)
Aquatic food production systems are the ways in which people obtain food from oceans, seas, lakes and rivers, either by catching wild organisms or by farming them. Fish and other aquatic foods supplied about 15% of the animal protein eaten worldwide in 2022, and around 62 million people worked in fishing and fish farming, so the health of aquatic ecosystems is closely tied to
The official deck for this part of the syllabus.
What are phytoplankton?
What is plankton?
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The whitening of corals when heat or other stress causes them to expel the symbiotic algae living in their tissues, leaving them without their main food source and at risk of death.
The use of sodium cyanide sprayed onto coral reefs to stun fish for capture, mostly for the live reef-fish food and aquarium trades, which kills coral and non-target animals.
A label on a product showing that it meets an independent standard for environmental sustainability, such as seafood from a fishery certified as well managed.
A farmed aquatic animal that has escaped from its cage or pond into the wild, where it may compete with, breed with or spread disease to wild populations.
A zone of sea extending up to 200 nautical miles (370 km) from a country's coast, in which that country has sovereign rights over fishing and other natural resources under UNCLOS.
A population of one fish species in a defined area that breeds largely within itself and is assessed and managed as a single unit.
An activity or industry that catches or farms a particular aquatic species in a particular area, together with the people, vessels and methods involved.
A dramatic and lasting decrease in a fish stock to a level at which it can no longer support a commercial fishery and cannot recover quickly, even when fishing stops.
The amount of fishing carried out over a period, measured for example as the number of vessels, days at sea, hours of trawling or hooks set.
A legal limit on the amount of a species, usually in tonnes, that a country, fleet or vessel may catch in a period, often set from a total allowable catch.
The continued catching and killing of marine animals by fishing gear, such as nets, lines and traps, that has been lost or abandoned at sea.
The parts of the ocean beyond any country's exclusive economic zone, open to all states and governed only by international agreements and regional fisheries bodies.
The UN agreement on marine biodiversity of areas beyond national jurisdiction, adopted in 2023 and in force from January 2026, which allows marine protected areas and impact assessments on the high seas.
A form of aquaculture that farms species from different trophic levels together, so that wastes from fed species such as fish become food or nutrients for shellfish, deposit feeders and seaweeds.
The international body set up under the 1946 International Convention for the Regulation of Whaling that sets catch limits and has kept a moratorium on commercial whaling since 1986.
An aquatic plant large enough to be seen with the naked eye, growing emergent, submerged or floating in fresh water or shallow coastal water.
A clearly defined area of sea or coast managed by law or other effective means to conserve nature, with restrictions on activities such as fishing, which range from partial limits to full protection.
The size of the holes in a fishing net, which can be regulated so that young, small fish escape and reach breeding age.
An area of sea or fresh water in which all fishing and other removal of living or non-living resources is banned.
The total amount of nutrients, mainly nitrates and phosphates, entering a body of water over a period from natural sources and human activities such as farming and sewage.
The removal of fish from a stock faster than the stock can replace them by reproduction and growth, so that the population declines over time.
Microscopic photosynthetic organisms, such as diatoms and cyanobacteria, that drift in the surface waters of oceans, seas and fresh water and form the base of most aquatic food webs.
Organisms that live in water and drift with currents because they cannot swim strongly against them, including photosynthetic phytoplankton and consuming zooplankton.
Any aquatic animal or plant from the sea or from fresh water that people eat, including fish, molluscs, crustaceans and seaweeds.
The movement of adult fish, and of eggs and larvae, from a protected area into neighbouring fished waters, which raises catches outside the protected area.
The scientific estimation of the size, age structure and productivity of a fish stock, and of how fishing affects it, used to advise on safe catch limits.
The United Nations Convention on the Law of the Sea (1982), the international treaty defining the rights and responsibilities of states in the ocean, including territorial seas and exclusive economic zones.
A satellite system that automatically reports the position, course and speed of fishing vessels to the authorities, used to enforce closed areas and other rules.
The hunting of whales for meat, oil, blubber and other products, whether commercially, for scientific permits or for the subsistence of Indigenous communities.
A model graph showing how the sustainable catch from a fish stock changes as fishing effort increases, rising to a peak at maximum sustainable yield and falling at higher effort.
Small, mostly microscopic animals and animal-like organisms that drift in water, many of which feed on phytoplankton and are in turn eaten by fish and other consumers.
A fishery can be studied as a system: fish enter a stock through reproduction and growth and leave it through natural death and catches. Whether a fishery is sustainable depends on keeping the catch within what the stock can replace, and on how governments, fishing communities and consumers with different perspectives share a resource that often belongs to no one in particular. Fisheries connect directly to SDG 14 (Life below water) and SDG 2 (Zero hunger).
Sources: FAO, The State of World Fisheries and Aquaculture (2024).
Every aquatic food chain starts with organisms that capture light energy by photosynthesis. In the open ocean and in the middle of large lakes, these producers are almost all microscopic. Close to the shore and in shallow fresh water, larger plants and seaweeds rooted in the sediment or floating at the surface also become important.
Phytoplankton are part of the plankton, the community of organisms that drift with the currents because they cannot swim strongly against them. The animal part of the plankton, the zooplankton, includes copepods, krill and the larvae of fish and crabs; many of them graze on phytoplankton. Although phytoplankton make up only about 1% of the world’s plant biomass, they carry out about half of all photosynthesis on Earth, because they reproduce quickly and are eaten almost as fast as they grow.
Macrophytes dominate shallow lakes, rivers, marshes and coastal seagrass meadows. Seaweeds such as kelp are large algae rather than true plants, but they play the same role on rocky coasts. Besides producing food, macrophytes and seaweeds give shelter to young fish, add oxygen to the water and trap sediment, so their loss affects fisheries even when no one eats them.
Aquatic food chains are often longer than those on land. A tuna may be four or five feeding steps away from the phytoplankton, and at each step most of the energy is lost as heat in respiration, so only about 10% or less passes to the next trophic level. For this reason a sea that produces a great deal of phytoplankton supports a far smaller mass of large predatory fish, and fishing for top predators is less efficient than harvesting animals lower in the food web.
People harvest a wide range of organisms from fresh and salt water: finfish (such as cod, tuna, salmon, carp and tilapia), molluscs (mussels, oysters, clams and squid), crustaceans (shrimp, crabs and lobsters), echinoderms (sea urchins and sea cucumbers) and aquatic plants and algae (seaweeds such as nori and kelp, and freshwater plants such as water spinach). The general term for these foods is seafood, although many come from rivers, lakes and fish farms rather than the sea.
Some aquatic foods are eaten close to where they are caught or grown; others travel around the world. A useful contrast is between a local food, caught or farmed and eaten in one region, and a global commodity traded across continents.
Aquatic food | Where it comes from | Who eats it | Scale |
|---|---|---|---|
Atlantic salmon (Salmo salar) | Farmed in sea cages, mostly in Norway and Chile | Consumers in Europe, North America and East Asia | Global |
Nile perch (Lates niloticus) | Caught in Lake Victoria (Kenya, Uganda, Tanzania) | Fillets largely exported to Europe; local people eat less valuable parts and smaller fish | Caught locally, eaten globally |
Peruvian anchoveta (Engraulis ringens) | Caught off Peru and Chile | Mostly turned into fishmeal and fish oil to feed farmed fish and livestock elsewhere | Global (indirectly) |
Seaweeds (e.g. Kappaphycus, Eucheuma) | Farmed on lines in shallow water in Indonesia, the Philippines and Tanzania | Eaten in East Asia; processed into carrageenan for food worldwide | Local and global |
Tilapia and carp | Farmed in ponds in China, Bangladesh, Egypt and many other countries | Mostly eaten in the country where they are grown | Local |
The Nile perch shows how a local catch can become a global product. The fish was introduced to Lake Victoria in the 1950s and is now caught commercially by lakeside communities, but much of the valuable fillet is exported, and sales to the European Union reached €169 million in 2003. Who benefits from an aquatic food therefore depends on trade and markets as well as on the ecosystem that produces it.
Sources: Wikipedia, Nile perch; Peruvian anchoveta; Seaweed farming (2026).
Global demand for aquatic foods has risen much faster than the human population. World fish and seafood supply per person more than doubled between 1961 and 2023, from 9.0 kg to about 20 kg a year, while the world population itself rose from about 3 billion to 8 billion. Total demand therefore grew by roughly six times over the same period.
Population growth: more people need more food, especially in coastal Asia and Africa, where fish is a traditional source of protein.
Rising incomes and urbanisation: as households become wealthier and move to cities, they buy more animal protein, including fish, and have access to refrigerated supply chains and supermarkets.
Changing diets and health messages: fish is promoted as a healthy, low-fat source of protein and omega-3 fatty acids, and some people choose it in place of red meat for health or climate reasons.
Globalisation and technology: freezing, air freight and processing allow fish caught in one ocean to be eaten on another continent, and the growth of fish farming has made products such as salmon and shrimp cheaper and available all year.
Consumption varies enormously between countries. In 2022, supply was about 85 kg per person in Iceland, 80 kg in the Maldives, 45 kg in Japan and 42 kg in China, but only about 9 kg in India, 3 kg in Kenya and 0.5 kg in Ethiopia. These differences reflect geography (access to a coast or large lakes), culture and religion, income, and the availability of other protein sources.
Sources: FAO food balances via Our World in Data (2026).
What people choose to eat is shaped by their values. A diet based on fish may be seen as healthy and traditional in Japan or Norway, essential for survival in an Arctic community, a luxury in an inland city, or ethically unacceptable to someone who rejects killing animals. Some consumers avoid species they see as threatened or check eco-labels, while others choose purely on price. These choices add up to the global demand that drives fishing and fish farming.
Perspective | How it views rising demand for seafood | Typical response |
|---|---|---|
Technocentrism | Demand can be met by better technology | Expand aquaculture, breed faster-growing fish, develop plant- or insect-based fish feed |
Anthropocentrism | Fish stocks are a resource to be managed wisely for people now and in future | Set catch limits based on science, protect nursery areas, certify sustainable fisheries |
Ecocentrism | Marine life has value in itself and demand must be reduced | Eat less seafood or none, large no-take areas, end destructive methods |
When demand rises, the pressure on wild fish stocks rises with it. A fish stock is a population of one species in a defined area that breeds largely within itself, such as the North Sea herring or the northern cod off Newfoundland. A stock can be harvested sustainably only if the catch each year is no larger than the number of new fish added by reproduction and growth.
Overfishing is one form of overexploitation. Its early signs are falling catches for the same effort, smaller and younger fish in the catch, and fleets moving to more distant waters or to new species. The share of the world’s assessed marine stocks that are overexploited rose from 10% in 1974 to 35.5% in 2021, so roughly one stock in three is now fished beyond what it can sustain.
Overexploitation has both ecological and social causes. Modern vessels use sonar and satellite navigation to find fish, freezer trawlers can stay at sea for weeks, and government subsidies for fuel and boats keep fleets fishing even when stocks are low. Many fish stocks are also a common-pool resource: no one owns the fish until they are caught, so each fisher gains by catching more today, even though everyone loses when the stock declines. This is the tragedy of the commons applied to the sea.
Some of the catch is taken illegally or never reported. Illegal, unreported and unregulated fishing includes boats fishing without licences, exceeding their quotas, using banned gear or landing fish in ports with weak controls. It undermines management because scientists do not know how many fish are really being removed.
Some fishing methods harm the ecosystem far beyond the target species. They damage habitats that fish need for feeding and breeding, kill large numbers of non-target animals, or continue killing after the fishers have gone.
Bottom trawling drags a large cone-shaped net, held open by heavy metal doors and weighted chains, across the seabed to catch fish and shellfish that live on or near the bottom, such as cod, flatfish and shrimp. It was the largest single source of the world’s reported wild catch in 2018, landing about 27 million tonnes. The gear flattens seabed features, crushes slow-growing corals and sponges, and stirs up sediment. On deep-sea seamounts, where cold-water corals may be centuries old, the damage can last for generations.
Trawls are also unselective, so they take a large bycatch: the unwanted animals caught along with the target species, including juvenile fish, sharks, rays, turtles and seabirds. Much of the bycatch is thrown back dead, which wastes food and removes young fish before they can breed.
Ghost fishing is the continued catching of animals by nets, lines and traps that have been lost or abandoned at sea. Modern nets are made of durable plastic, so a lost net can drift or snag on a reef for years, trapping fish, turtles, seals and whales. The animals that die attract scavengers, which are then caught in turn.
An estimated 48,000–100,000 tonnes of fishing gear is lost every year, and fishing-related plastics make up an estimated 46% of the mass of the floating plastic in the Great Pacific Garbage Patch.
Blast fishing uses home-made bombs to stun or kill a whole school of fish, which float up or sink and are then collected. Each blast shatters the coral where the fish live, and a single blast leaves a reef that takes 5–10 years to recover. It has been common in South-East Asia and East Africa: a 1987 study estimated that blast fishing supplied a quarter of municipal fish landings in the Philippines, and 441 blasts were recorded in two months at Mnazi Bay in Tanzania in 1996.
Cyanide fishing squirts a solution of sodium cyanide onto coral heads to stun fish so that they can be caught alive for the live reef-fish restaurant trade and for aquariums. The poison also kills coral polyps and many non-target animals. Both methods are illegal in most countries, but they persist where poverty is high, enforcement is weak and live reef fish sell for high prices.
Sources: Sea Around Us via Our World in Data (2026); Wikipedia, Ghost net; Blast fishing (2026).
YouTubeIf overfishing continues for long enough, a stock may fall so low that it can no longer support a commercial fishery. At that point a fishery collapse has occurred. A collapse differs from an ordinary decline because the stock does not bounce back quickly once fishing stops.
Several processes keep a collapsed stock low. Very few adults are left to breed, and the largest, oldest females, which produce by far the most eggs, are usually the first to be caught. The ecosystem may also change in the fish’s absence: prey species, competitors or predators of the young fish increase and occupy its place. The system can then settle into a new state, a tipping point having been crossed, and the old fishery may not return for decades.
The best-known example is the northern cod of Newfoundland and Labrador, which supported a fishery for about 500 years before it collapsed in the early 1990s. Its history shows every stage of a collapse, from rising catches and optimistic quotas to a fishing ban that lasted more than 30 years. Another example is the Peruvian anchoveta, whose catch fell sharply after 1972 when heavy fishing coincided with a natural fall in the food supply during an El Niño event.
A fish stock grows by adding new young fish (recruitment) and by the growth in body mass of the fish already there, and it shrinks through natural deaths and catches. The surplus it produces each year, over and above what is needed to keep it the same size, is its net productivity. That surplus can be harvested without shrinking the stock, which makes it a sustainable yield.
The maximum sustainable yield is therefore a ceiling, not a target to exceed. A quota above MSY takes more than the stock produces, so the stock shrinks and next year’s surplus is smaller still.
Fishing effort is the amount of fishing carried out: the number of boats, the days they spend at sea, the hours a trawl is towed or the number of hooks set. The yield–effort curve is a model that shows how the sustainable catch changes as effort increases, once the stock has had time to adjust to each level of fishing.
At low effort, the stock stays close to its natural size and each extra boat adds to the catch. As effort increases, the stock is fished down to a smaller size, where it grows faster because there is less competition for food and space. The curve peaks at MSY, with a stock size roughly half of what it would be without fishing. To the right of the peak, the stock is too small to produce a large surplus: more effort means a smaller sustainable catch, and at very high effort the stock can collapse.
Two fisheries with the same catch can therefore be in very different states. At point A, effort is low, the stock is large and fishing is profitable. At point B, the same catch needs about four times the effort, so costs are far higher and the stock is close to collapse. Many real fisheries operate to the right of the peak: reducing effort there would increase the long-term catch as well as the stock.
Using MSY to cap fishing quotas requires the quota to be set at or below the surplus the stock can produce. In practice, this usually means much lower fishing rates than many fleets are used to, because fleets were often built up when stocks were large and are now bigger than the stock can support.
Fishing is not the only pressure on aquatic food systems. Climate change is warming the oceans, changing where fish can live and how productive the sea is, and the extra carbon dioxide dissolving in seawater is making it more acidic. Together these changes may push some populations past the point where they can recover.
Most fish and invertebrates are ectotherms, so the temperature of the water controls their growth, breeding and survival. As the sea warms, many species shift towards the poles or into deeper, cooler water, so fishing communities may lose the species they have always caught and gain others. Warm water holds less dissolved oxygen, and a warmer surface layer mixes less with the nutrient-rich water below, which can reduce the growth of phytoplankton at the base of the food web. Warmer conditions also favour some diseases and harmful algal blooms.
Tropical coral reefs cover a tiny fraction of the sea floor but shelter an estimated quarter of all marine species, and they supply fish and income to millions of people in small coastal fisheries. Reef-building corals depend on microscopic algae living in their tissues, which photosynthesise and pass sugars to the coral.
Coral bleaching usually begins when the water stays about 1 °C or more above the normal summer maximum for several weeks. The Great Barrier Reef off Queensland, Australia, has suffered mass bleaching in 2016, 2017, 2020, 2022, 2024 and 2025. In 2016 about two-thirds of the corals died in the worst-hit northern section. Reef tourism generates over AU$3 billion a year for the region, and commercial and recreational fishers depend on reef species such as coral trout, so repeated bleaching threatens livelihoods as well as biodiversity. Worldwide, a fourth global bleaching event that began in 2023 had exposed about 84% of the world’s reef area to bleaching-level heat by 2025.
Sources: Wikipedia, Coral bleaching; Great Barrier Reef (2026).
The ocean absorbs roughly a quarter of the carbon dioxide released by human activities. The dissolved CO₂ forms carbonic acid, which releases hydrogen ions and lowers the pH; this is ocean acidification. The average pH of the ocean surface fell from about 8.15 to 8.05 between 1950 and 2020. Because the pH scale is logarithmic, a fall of 0.1 means about 26% more hydrogen ions.
The extra hydrogen ions combine with carbonate ions, leaving fewer carbonate ions for animals that build shells and skeletons of calcium carbonate. Corals, oysters, mussels, sea urchins and planktonic snails called pteropods all have to spend more energy to build and maintain their shells, and young stages are the most sensitive. Pteropods and other calcifying plankton are food for fish such as salmon and herring, so the effects can pass up the food web to fisheries. Acidification and warming act together: a reef that is both bleached and slower to calcify is less able to recover from storms.
Unsustainable exploitation can be reduced by rules on how, when, where and how much is caught, and by changing what consumers buy. Because fish move across borders and many stocks are shared, action is needed at every scale, from international agreements to the choices of a single shopper.
Tool | How it works | Strengths | Limitations |
|---|---|---|---|
Permits and licences | Only licensed vessels may fish; the number of licences limits effort | Controls the size of the fleet; identifies who is fishing | Each licensed boat may simply fish harder; illegal boats ignore it |
Quotas (total allowable catch) | A maximum mass of fish may be landed each year, shared between countries or vessels | Directly limits the catch; can be based on scientific advice | Only works if set at or below MSY and enforced; may cause discarding of over-quota fish |
Closed seasons | Fishing is banned at certain times, such as during spawning | Protects breeding adults and young fish; easy to understand | Effort may simply increase in the open season |
Mesh size | Nets must have holes large enough for young fish to escape | Lets fish reach breeding age before capture | Hard to check at sea; does not help species caught as bycatch |
Closed areas and zones | Fishing is banned or limited in nursery or spawning areas, or certain gear is banned there | Protects habitat and key life stages | Needs patrols or satellite tracking to enforce |
Gear rules | Bans on blast fishing, driftnets or trawling on sensitive habitats; turtle excluder devices | Reduces habitat damage and bycatch | Enforcement is difficult in remote and poor regions |
Food labelling and eco-labels | Labels show the species, where and how it was caught, and whether the fishery is certified | Lets consumers reward sustainable fisheries | Only some consumers pay attention; certification can be costly |
Mesh size rules and closed seasons work on the same principle: they protect the fish that will produce the next generation. Quotas work on the total catch, licences on effort, and zones on the places where fish feed and breed. Most well-managed fisheries combine several tools.
International: neighbouring countries agree how to share stocks that cross borders, and regional fisheries management organisations set quotas for migratory species such as tuna in international waters. Shared rules on gear and catches need wide agreement, or boats simply move to waters where the rules do not apply.
National: governments set and enforce quotas, licences, seasons, closed areas and mesh sizes in their own waters, inspect landings at ports, and decide whether to subsidise or reduce their fleets.
Local and individual: fishing communities can manage nearby waters themselves, for example by closing a reef for a season; consumers can choose certified or less threatened species, eat smaller fish lower in the food chain, and avoid species they know to be overfished.
The best-known eco-label for wild fish is the blue label of the Marine Stewardship Council (MSC), founded in 1996 by WWF and the food company Unilever and independent since 1999. A fishery is checked against standards for the health of the stock, the impact on the ecosystem and the quality of its management. Supporters see certification as a market reward for good practice. Critics point out that the fishing industry pays for assessments, that some certified fisheries have been controversial, and that the cost of assessment can exclude small-scale fisheries in poorer countries.
Sources: Wikipedia, Marine Stewardship Council (2026).
A marine protected area (MPA) is a defined area of sea or coast managed to conserve nature, with limits on activities such as fishing, mining and anchoring. Protection ranges from partial limits on some gear to a no-take zone in which all fishing is banned. The more complete the protection and the longer it lasts, the larger the benefits tend to be.
Inside a no-take zone, fish live longer and grow larger. Large, old females produce many more eggs than young ones, and seabed habitats such as reefs and seagrass recover from trawling and anchoring. As the population inside grows, some adults swim out into the surrounding waters, and eggs and larvae drift out with the currents. This spillover effect means that an MPA can support catches outside its boundary as well as the food chains within it.
Cabo Pulmo National Park, on the Gulf of California coast of Baja California Sur, Mexico, shows how strong the effect can be. The 71 km² park was created in 1995 after local families who had fished the reef supported a ban on fishing, and it became a largely no-take reserve. Between 1999 and 2009 the total fish biomass on its reefs rose by 463%, and the biomass of top predators such as groupers and sharks rose elevenfold, while other protected and unprotected areas in the region showed no significant change. Researchers linked the recovery to strong community leadership and effective enforcement, and many former fishers now earn their living from dive tourism.
Sources: Aburto-Oropeza et al., PLoS ONE (2011); Wikipedia, Cabo Pulmo National Park (2026).
Strengths: they protect whole ecosystems rather than single species, allow habitats and food chains to recover, provide spawning grounds and nurseries, support spillover to nearby fisheries, and act as reference areas that show scientists what an unfished ecosystem looks like.
Limitations: many MPAs exist only on paper, with no patrols or penalties; fishing effort may simply move to the edges of the area; fishers lose access in the short term and may oppose the area; benefits take years to appear; and a single MPA cannot protect species that migrate across oceans or escape warming and acidification.
Aquaculture is the farming of aquatic organisms. It ranges from small ponds of carp on family farms in Asia to industrial sea cages holding hundreds of thousands of salmon, and it includes shellfish grown on ropes and seaweeds grown on lines in shallow water.
Aquaculture has grown faster than any other major food sector. Wild capture has stayed at roughly 90–95 million tonnes a year since the mid-1990s, while aquaculture, including seaweeds, rose from about 17 million tonnes in 1990 to 141 million tonnes in 2024. In 2022, farmed aquatic animals made up 51% of all aquatic animal production for the first time. Fish farming provides food, export income and jobs, which is why many governments promote it as part of economic development.
An open-net cage in the sea can be modelled as an open system. It receives inputs of young fish, feed, medicines and chemicals, holds a storage of growing salmon, and releases outputs of harvested fish and of wastes, chemicals, parasites and escaped fish into the surrounding water. Because the cage is a net, the boundary between the farm and the wild ecosystem is open, and many of the outputs cannot be controlled.
In practice, a farm is a cluster of such cages moored in a sheltered bay or sea loch, served by a feed barge and work boats. The sheltered sites chosen to protect the cages from storms are also the places where water exchange is slowest, so wastes can accumulate.
Habitat loss: mangroves, salt marshes and seagrass beds are cleared for shrimp ponds and seaweed farms, removing the nurseries of wild fish and natural coastal protection.
Pollution from feed and faeces: uneaten feed and fish waste add organic matter and nutrients, which raise oxygen demand and can cause algal blooms and eutrophication in sheltered bays.
Chemicals: anti-fouling agents (often containing copper) painted on nets to stop seaweed and barnacles growing on them leak into the water and are toxic to marine invertebrates; pesticides are used against parasites.
Antibiotics and other medicines: drugs given in feed can pass into the environment and encourage the spread of antibiotic-resistant bacteria.
Spread of disease and parasites: crowded fish are easily infected, and diseases and parasites such as sea lice spread to wild fish swimming past the cages.
Escapees: farmed fish that escape compete with wild fish, can breed with them and weaken the genetic adaptations of wild populations, and may spread disease. Where genetically modified fish are farmed, escapees could carry modified genes into the wild.
Feed from wild fish: carnivorous species such as salmon and shrimp are fed partly on fishmeal and fish oil made from wild-caught small fish such as anchoveta. Estimates from the late 1990s put this at 2–4 kg of wild fish for each kilogram of farmed salmon. Plant ingredients in feed have since lowered the ratio, but in 2008 farmed salmon and trout still received 50–80% of the world’s fish oil, so farming carnivores can add to the pressure on wild stocks rather than relieving it.
The last point follows from energy flow. Farming a carnivorous fish means feeding it at a high trophic level, so most of the energy in its feed is lost in respiration. Farming filter-feeding mussels or seaweeds, which need no added feed at all, is far more efficient.
Chile became one of the world’s two largest producers of farmed salmon, with most farms in the sheltered fjords and channels of the Los Lagos and Aysén regions in the south. In 2007 an outbreak of infectious salmon anaemia (ISA), a viral disease, spread through densely stocked farms, and Chile’s production of Atlantic salmon fell from about 400,000 tonnes in 2005 to about 100,000 tonnes in 2010. The government responded with stricter sanitary rules.
In October 2018 a storm damaged a farm at Punta Redonda and about 690,000 salmon escaped into the wild; they had been treated with the antibiotic florfenicol, so they were not fit to eat. Studies have also linked nutrient enrichment from farms to harmful algal blooms in some Chilean fjords, although the industry disputes a direct link.
Sources: Wikipedia, Aquaculture in Chile; Aquaculture of salmonids (2026).
Integrated multi-trophic aquaculture copies the way nutrients cycle in a natural ecosystem. Fish such as salmon are fed; mussels and oysters nearby filter out fine particles of waste and plankton; deposit feeders such as sea cucumbers eat waste that settles on the bottom; and seaweeds such as kelp take up the dissolved nitrogen and phosphorus released by the fish.
In the Bay of Fundy, Canada, Atlantic salmon, blue mussels and kelp have been grown together in this way. The farmer gains extra crops to sell and the nutrient load on the bay falls. The method needs careful siting and is more complex to run, and how much waste the seaweeds and shellfish really remove varies from site to site.
Technique | Impact it reduces | Limitation |
|---|---|---|
IMTA (fish with shellfish and seaweed) | Nutrient pollution and eutrophication | Complex to manage; nutrient removal varies by site |
Lower stocking density and fallowing (leaving sites empty between harvests) | Disease, parasites and waste build-up under cages | Less fish produced per site, so higher costs |
Closed containment or recirculating tanks on land | Escapees, parasites, waste reaching the sea | High energy use and cost |
Cleaner fish (e.g. ballan wrasse) and vaccines | Sea lice and disease, so fewer chemicals and antibiotics | Cleaner fish are often taken from the wild; welfare concerns |
Plant- or insect-based feeds | Pressure on wild forage fish | May change fish quality; crops need land and water |
Farming shellfish and seaweeds instead of carnivorous fish | Need for feed; nutrient pollution | Different products that not all consumers want |
YouTubeSupporters argue that aquaculture is the only realistic way to meet rising demand without draining wild stocks, and that seaweed and shellfish farming can even improve water quality. Seaweed farming, for example, employs about one million people in Indonesia and is an important source of income for women in Tanzania. Critics reply that intensive farming of carnivorous fish and shrimp moves environmental costs elsewhere: to the anchoveta fisheries that supply feed, to mangroves cleared for ponds, and to wild salmon exposed to lice and disease. The sustainability of aquaculture depends on what is farmed, where and how, rather than on aquaculture as a whole.
Sources: Wikipedia, Integrated multi-trophic aquaculture; Seaweed farming; Aquaculture of salmonids (2026); FAO (2024).
Atlantic cod (Gadus morhua) is a large bottom-dwelling predator of the cold North Atlantic. The stock known as “northern cod” lives on the continental shelf off eastern Newfoundland and southern Labrador, Canada, including the northern part of the Grand Banks, a shallow plateau where cold Labrador Current water meets warmer Atlantic water and plankton production is high. Cod feed heavily on capelin (Mallotus villosus), a small schooling fish, and on shrimp and crabs.
European fleets fished these waters for about 500 years, and whole coastal communities in Newfoundland depended on cod. Until the 1950s most of the catch was taken by small inshore boats using hooks, traps and nets. An estimated 8 million tonnes of cod were caught in the century from 1647 to 1750.
From the 1950s, large factory trawlers from Europe and elsewhere arrived on the Grand Banks. Equipped with radar, sonar and electronic navigation, they could find dense schools, trawl the seabed in deep water through the winter, and freeze and process the catch on board. They took as much cod in 15 years as had been caught in the 103 years from 1647 to 1750. The catch of northern cod peaked at 810,000 tonnes in 1968, about three times the largest catches before the trawlers arrived, and then fell steeply through the 1970s.
On 1 January 1977 Canada’s 200-nautical-mile fisheries zone came into force, which pushed most foreign fleets off the shelf. This could have allowed recovery. Instead, the Department of Fisheries and Oceans (DFO) set quotas based on optimistic estimates of the stock, and a Canadian offshore trawler fleet expanded to fill the gap. Catches rose again in the 1980s. In 1986 scientists advised that the total allowable catch should be halved, but the advice was not followed. As late as 1992 the quota was set at 187,969 tonnes, although only 129,033 tonnes had been caught in the previous year because the fish could not be found.
Several factors combined in the final collapse. Trawlers had removed most of the large, old spawning fish and caught many juveniles and capelin as bycatch. Stock estimates relied partly on commercial catch rates, which stayed high because trawlers could still find the remaining dense schools even as the stock shrank. In the late 1980s and early 1990s the ocean off Newfoundland was unusually cold, capelin collapsed in 1991, and natural mortality of adult cod rose sharply. By 1992 northern cod were at about 1% of their historic level, and the spawning biomass had fallen by 99%.
Year | Indicator (northern cod) | Value |
|---|---|---|
1968 | Peak catch | 810,000 t |
1991 | Catch actually taken | 129,033 t |
1992 | Quota set before the moratorium | 187,969 t |
1992 | Stock relative to historic level | about 1% |
2024 | Total allowable catch when commercial fishing reopened | 18,947 t |
2024 | Reported catch, including bycatch | 15,661 t |
2025 | Spawning stock biomass (95% confidence interval) | 524 kt (404–678 kt) |
2025 | Spawning stock relative to the limit reference point | 2.0 times (1.2–3.3) |
Sources: DFO Science Advisory Report 2025/043; Wikipedia, Collapse of the Atlantic northwest cod fishery (2026).
The data show the classic pattern of a fishery collapse: a long rise in catches as technology improved, a peak far above anything sustainable, a partial recovery that encouraged high quotas, and then a crash from which the stock took decades to recover even with very little fishing.
In July 1992 the federal government announced a two-year moratorium on fishing for northern cod, the largest industrial closure in Canadian history. In 1993, as other cod stocks also collapsed, the ban was extended indefinitely. About 35,000–37,000 fishers and fish plant workers from more than 400 coastal communities lost their jobs. Federal programmes, first the Northern Cod Adjustment and Recovery Program and then the Atlantic Groundfish Strategy, paid income support and retraining, and many young people left Newfoundland to find work.
The ecosystem did not return to its old state. With cod scarce, snow crab and northern shrimp increased and became the basis of a new shellfish industry of roughly equal economic value. Cod also changed genetically: in the decade before the collapse, intense fishing of large fish had favoured cod that matured younger and at a smaller size, a form of fisheries-induced evolution that persisted for years afterwards.
Small inshore and recreational fisheries continued during the moratorium, and the stock grew slowly from its low point in the mid-1990s, especially between 2010 and 2016. In 2024 DFO adopted a new assessment model that reached back to 1954 and set a new limit reference point at 40% of the stock size that gives MSY. Under this framework the stock moved from the “critical” to the “cautious” zone, and the commercial fishery reopened in 2024 for the first time since 1992, with a total allowable catch of 18,947 tonnes.
Stakeholder | Interest and perspective | Role in the collapse or the recovery |
|---|---|---|
Federal government and DFO | Jobs and votes in Atlantic Canada; economic use of a national resource (anthropocentric) | Set quotas above scientific advice; later imposed the moratorium and funded adjustment |
Offshore trawler companies | Profit from a large, efficient fleet (technocentric) | Took a growing share of the catch after 1977; lost most when the fishery closed |
Inshore fishers and communities | Livelihoods and a way of life based on cod | Complained about trawlers from the late 1960s; bore most of the job losses |
Foreign fleets | Access to rich grounds outside Canada’s EEZ | Fished heavily before 1977; parts of the Grand Banks remain outside the EEZ |
Scientists | Accurate assessment of the stock | Overestimated the stock in the 1980s; advice to halve catches was ignored |
Conservation view (ecocentric) | Cod and the ecosystem have value beyond their catch | Sees the collapse as the loss of a whole ecosystem and argues for very cautious reopening |
The collapse is widely judged a failure of management rather than of knowledge alone. Warning signs were available, but short-term economic and political pressures kept quotas high, and stock estimates that relied on catch rates gave a false impression of abundance. The cold conditions and the loss of capelin made the crash deeper, yet they acted on a stock that fishing had already reduced and made less resilient by removing its oldest spawners.
The 2024 reopening is debated. Supporters point to a spawning stock about twice the new limit reference point and to more than 30 years of sacrifice by fishing communities. More cautious voices point to DFO’s own assessment: the stock has not grown since 2017, its growth is limited by scarce capelin, and even with no fishing at all there is a 56% chance of decline over the following three years. Part of the change in status also came from revising the model and the reference point, not only from more fish. The case shows how the same data can support different decisions depending on how risk and livelihoods are weighed.
The northern cod illustrates almost every idea in aquatic food production: rising demand and technology driving overexploitation; destructive bottom trawling and bycatch; quotas set above the maximum sustainable yield; a fishery collapse in which the stock and the ecosystem shifted to a new state; the social costs of a moratorium; and the difficulty of measuring a stock accurately enough to manage it. It also shows that a fishery is a socio-ecological system, in which political choices and ocean conditions act together.
Sources: DFO Science Advisory Report 2025/043 (2025); Sea Around Us (2026); Wikipedia, Collapse of the Atlantic northwest cod fishery (2026).
Phytoplankton need both light, which is available only near the surface, and nutrients such as nitrate, phosphate, silicate and iron, which sink into deep water with dead organisms. In a water body with stratification, a thermocline separates the warm surface layer from the cold, nutrient-rich deep layer and blocks mixing between them; upwelling and strong mixing bring the deep water back to the light. The primary productivity of a sea or lake, and therefore the fish yield it can support, depends on how often this happens.
Upwelling zones: along the west coasts of the Americas and Africa, winds blowing along the coast push surface water offshore and cold, nutrient-rich water rises to replace it. This upwelling supports huge phytoplankton blooms, short food chains and very large catches of small pelagic fish. The five major coastal upwelling systems (the Humboldt, California, Canary, Benguela and Somali currents) cover only about 5% of the ocean but supply about 25% of the world’s marine fish catch.
Shallow seas and continental shelves: in water less than about 200 m deep, waves, tides and storms mix the whole water column, and rivers bring nutrients from the land. The North Sea, the Grand Banks and the shelves of East Asia are examples.
Temperate and polar seas in spring: winter storms and cooling mix the water and bring nutrients up; when light increases in spring, a large bloom occurs, followed by stratification and lower productivity in summer.
Estuaries, coral reefs, mangroves and salt marshes: nutrients from rivers and rapid recycling support very high productivity in a small area.
In contrast, the centres of the great subtropical gyres, such as the Sargasso Sea and the South Pacific Gyre, are permanently stratified and receive few nutrients, so their productivity is very low. These clear blue waters are sometimes called ocean deserts.
The Humboldt Current upwelling off Peru and Chile supports the Peruvian anchoveta, which has often been the largest single-species fishery in the world. The catch reached 13.1 million tonnes in 1971, when the fleet was taking close to everything the stock could produce. In 1972 heavy fishing coincided with an El Niño event, which cut the supply of nutrients to the surface, and the stock collapsed; later El Niños in 1982–1983 and 1997–1998 also cut it sharply. A catch that is sustainable in a normal year can therefore be far too large in a poor one, which is why a fishery run close to its limit in a variable environment is at high risk.
Peru now sets a catch limit for each fishing season after a scientific survey of the stock, and since 2008 has assigned each vessel a share of the catch, so that the total can be cut quickly when the survey finds few young fish.
Sources: Wikipedia, Upwelling; Peruvian anchoveta; Phytoplankton (2026).
Nutrient loading is the total amount of nutrients, mainly nitrates and phosphates, entering a water body over a period, from natural sources and from farming, sewage and industry. A moderate increase can raise productivity and fish catches, which is why some coastal fisheries near river mouths are rich. A large increase in a stratified or poorly mixed water body causes eutrophication.
In eutrophication, blooms of algae die and decompose, decomposers use up the dissolved oxygen below the thermocline, and fish and shellfish die or move away, forming dead zones. Stratification, mixing and nutrient loading are therefore connected: the same nutrients that raise productivity in a well-mixed sea can remove oxygen from a stratified one.
Fish cannot be counted directly: they live underwater, move constantly and are spread over huge areas. Managers therefore depend on stock assessment, which combines several kinds of data in a population model to estimate the size, age structure and growth of a stock and to advise on safe catches.
Method | What it measures | Strengths | Limitations |
|---|---|---|---|
Research vessel trawl surveys | Numbers, sizes and ages of fish caught with standard gear at random or stratified stations each year | Independent of the fishing industry; same method every year gives a reliable trend | Expensive; covers only part of the stock’s range; fish may avoid the net |
Acoustic (echo-sounder) surveys | Biomass of schooling fish from sound pulses reflected by their swim bladders | Covers large areas quickly; good for pelagic fish such as anchoveta and capelin | Must identify the species and size of each echo; poor near the seabed |
Tagging (capture–mark–recapture) | Population size, movement and the fraction of the stock caught each year | Shows migration and fishing mortality directly | Tags may be lost or not reported; assumes tagged fish mix randomly |
Age structure from otoliths (ear bones) | Age of each fish from growth rings, giving growth rates and the age composition of the catch | Shows whether young fish are joining the stock and whether old fish are being removed | Slow and skilled work; needs large samples |
Catch per unit effort (CPUE) | Catch per standard amount of fishing, as an index of abundance | Cheap; uses data the fleet already produces | Can stay high while the stock falls, because fishers find the remaining schools and gear improves |
The DFO assessment of northern cod combines many of these: research vessel trawl surveys, an inshore sentinel survey by fishers using standard gear, acoustic estimates, juvenile surveys, tagging, the age composition of landings since 1954, and data on capelin, the cod’s main prey. Each source has its own errors, so using several gives a more reliable picture than any single one.
Knowing how many fish are caught is as important as knowing how many there are. Harvest rates are monitored through:
Logbooks, in which skippers record where, when and how much they caught, and landing records and port inspections that check the catch when it is unloaded and sold.
Onboard observers, independent staff who record the catch, bycatch and discards at sea, and electronic monitoring with cameras on deck where observers are too costly.
Vessel monitoring systems (VMS), which report each licensed vessel’s position to the authorities by satellite, so that boats fishing in closed areas or out of season can be detected.
Public tracking with the Automatic Identification System (AIS): Global Fishing Watch, launched in 2016 by Google, Oceana and SkyTruth, maps vessels that broadcast their positions. Its data helped Indonesian authorities to detain the vessel MV NIKA for illegal fishing in 2019.
A vessel monitoring system only works for boats that carry and switch on the equipment, so monitoring is weakest where it is most needed: among illegal vessels and in poorly patrolled waters off West Africa and in parts of Asia.
Sources: DFO Science Advisory Report 2025/043; Wikipedia, Catch per unit effort; Global Fishing Watch (2026).
The idea of MSY comes from the S-curve of population growth. A population that starts small grows slowly at first, because there are few breeders, then faster, and finally slows as it approaches the carrying capacity (K), when competition for food and space increases. The population adds the most new individuals each year when it is at about half the carrying capacity, K/2. In theory, if a stock is held at K/2 and each year’s surplus is removed, the catch can be taken for ever.
MSY is only an estimate. It is calculated from models that depend on uncertain values, such as natural mortality and how many young fish each adult produces. DFO notes that its estimates of the stock size at MSY for northern cod are highly sensitive to these values and have been revised with each update.
The real surplus changes every year. Recruitment depends on temperature, food and predators, so a catch equal to the average surplus is too large in poor years, as the anchoveta showed during El Niño events.
Harvesting exactly at MSY is impossible. Catches are never measured perfectly, some fish are taken illegally or discarded, and quotas are often set above scientific advice. Any error is more likely to make the catch too large than too small.
MSY sits at the edge of the curve. On the yield–effort curve, effort only slightly above E(MSY) moves the fishery to the falling side. A stock below K/2 produces a smaller surplus, so the same catch is now too large.
It ignores the rest of the ecosystem. Taking the maximum from one species reduces the food available to its predators and changes competition with other species.
Once the catch exceeds the surplus, a positive feedback loop can begin. A smaller stock has fewer breeding adults, especially the large, old females that produce the most eggs, so fewer young fish are recruited. If the quota is fixed, the same catch now removes a larger share of a smaller stock. The decline accelerates rather than correcting itself, and the stock can fall past a point from which it cannot recover.
Modern fisheries management therefore treats MSY as a limit to stay well below, not a target. Following the precautionary principle, managers set a target catch below MSY, define a limit reference point below which fishing must be cut sharply, and include buffers for uncertainty. Canada’s limit reference point for northern cod, for example, is a spawning stock of 40% of the stock size that would give MSY.
An overexploited stock can recover if fishing pressure falls far enough and for long enough. The main restoring measures are temporary fishing bans or moratoria, a limited number of licences, measures to prevent bycatch, catch limits set below MSY, and information that lets consumers choose sustainably harvested species. None of these works if one group ignores it, so recovery depends on cooperation among stakeholders who want different things.
Stakeholder | Main interest | Possible contribution to recovery | Typical conflict |
|---|---|---|---|
National governments and regional fisheries bodies | Food supply, jobs, export income, international relations | Set and enforce quotas, bans and licences; reduce subsidies | Pressure from fishing communities to keep quotas high |
Fishing industry (large fleets and small-scale fishers) | Income now and a fishery in future | Accept cuts, use selective gear, report catches honestly | Short-term losses; fear that others will keep fishing |
Consumers | Price, taste, health, ethics | Avoid overfished species; buy certified products | Limited information; price usually matters most |
NGOs and scientists | Conservation and sound science | Campaigns, stock assessments, certification schemes | May be seen by fishers as outsiders |
Wholesale markets and supermarkets | Reliable supply and brand reputation | Refuse to buy from overfished stocks; require traceability | Lower margins; supply gaps |
Atlantic bluefin tuna (Thunnus thynnus) are large, slow-maturing fish worth very high prices, especially in Japan, where about 80% of the bluefin caught worldwide is eaten. The eastern stock spawns in the Mediterranean, where purse-seine fleets, helped by spotter aircraft, targeted the dense spawning schools. In October 2009 the International Commission for the Conservation of Atlantic Tunas (ICCAT), the regional body that manages the stock, reported that it had fallen by 72% over 40 years.
In 2007 ICCAT’s scientists had advised a catch of 15,000 t to maintain the stock, or 10,000 t to allow recovery. ICCAT first set a quota of 36,000 t, while surveys suggested that up to 60,000 t were actually being taken.
Pressure grew: conservation groups placed bluefin on red lists, and in 2010 Monaco proposed banning international trade under CITES. Japan, the largest consumer, led opposition, arguing that ICCAT was the proper body, and the proposal was rejected by 68 votes to 20. Faced with the threat of a trade ban, ICCAT members agreed much tighter controls, and the quota for 2010 was cut to 13,500 t. As the stock recovered, the quota was raised step by step, to 16,142 t in 2015 and 23,155 t in 2017, and in 2021 the IUCN moved Atlantic bluefin tuna from Endangered to Least Concern on its Red List.
The recovery shows each stakeholder playing a part: scientists providing the evidence, NGOs and the threat of a trade ban creating pressure, governments in ICCAT agreeing lower quotas and stricter enforcement, and the industry accepting short-term losses for a larger future catch. It also shows the limits: quotas were set far above advice for years, illegal catches were large, and some regional populations remain depleted.
Sources: Wikipedia, Atlantic bluefin tuna (2026).
UNCLOS gives every coastal state a territorial sea of 12 nautical miles, where it has nearly full sovereignty, and an exclusive economic zone (EEZ) extending up to 200 nautical miles (370 km) from its coast. Within its EEZ a state has sovereign rights over fishing, oil and gas, and seabed minerals, and it decides who may fish and how much. The idea grew from earlier claims: Chile and Peru claimed 200-nautical-mile zones in 1947, and the Cod Wars between Iceland and the United Kingdom over fishing rights ended with an agreement in 1976.
Beyond the EEZs lie the high seas, which are open to all states. Estimates of their extent range from about 60% of the ocean to more than two-thirds, depending on how they are measured. Under the principle of freedom of the high seas, any state’s vessels may fish there, and each vessel is governed by the laws of the country whose flag it flies. Regional fisheries management organisations, such as ICCAT for tunas and the Northwest Atlantic Fisheries Organization for the parts of the Grand Banks outside Canada’s EEZ, set quotas for some species, but they cover only their member states and selected stocks, and enforcement is weak. The high seas are therefore a commons open to overexploitation by the states with the largest distant-water fleets.
Some coastal states, especially in West Africa and the Pacific, earn income by selling foreign fleets the right to fish in their EEZs rather than fishing the stocks themselves. The European Union’s protocol with Mauritania for 2021–2026, for example, pays €60.8 million a year, of which about €3.3 million a year supports Mauritania’s own fisheries policy. In return, EU vessels may catch up to 280,050 tonnes a year, including up to 225,000 tonnes of small pelagic fish taken by 19 freezer trawlers. The EU’s protocol with Senegal expired in November 2024 and has not been renewed.
Supporters argue that such agreements bring governments money, set rules for foreign fleets that might otherwise fish illegally, and fund local monitoring. Critics see an issue of environmental justice: small pelagic fish such as sardinella are an important, cheap source of protein in West Africa, local artisanal fishers in canoes compete with large industrial trawlers, and the benefits of the payment may not reach fishing communities. The question is whether a state should manage its EEZ for the food security of its own people or sell access for revenue.
Sources: European Commission, Sustainable fisheries partnership agreements: Mauritania; Senegal (2026).
For decades there was no general way to protect biodiversity on the high seas. The High Seas Treaty, formally the Agreement under UNCLOS on the Conservation and Sustainable Use of Marine Biological Diversity of Areas beyond National Jurisdiction (BBNJ), fills part of this gap. Its text was agreed in March 2023, it was adopted on 19 June 2023, and it reached its 60th ratification on 19 September 2025, so it entered into force on 17 January 2026.
Marine protected areas on the high seas: for the first time, states can create MPAs in international waters, which is essential if 30% of the ocean is to be protected by 2030.
Environmental impact assessments: activities that may harm high-seas ecosystems must be assessed first.
Marine genetic resources: benefits from genetic material found on the high seas, for example for medicines, must be shared fairly.
Capacity building and transfer of marine technology to help lower-income countries take part.
The treaty is an important step in international environmental law, with more than 100 parties by 2026, but its effect depends on whether major fishing nations join, and how decisions on new MPAs are taken at its meetings. Fishing itself is still managed mainly by the regional fisheries bodies, so the treaty complements rather than replaces them.
Sources: Wikipedia, High Seas Treaty; Exclusive economic zone; International waters (2026).
Seals, whales and dolphins are hunted for meat, blubber, skins and ivory in a few places, mainly in the Arctic and North Atlantic and in Japan. The hunts raise questions that cannot be answered by population data alone. Even when a hunt is sustainable in numbers, people disagree about whether it is right to kill intelligent, social mammals that can feel pain, and about who should decide.
Commercial whaling reduced many great whale species to a fraction of their former numbers in the 20th century. The International Whaling Commission (IWC), set up under the International Convention for the Regulation of Whaling of 1946, voted in 1982 by 25 votes to 7, with 5 abstentions, to set commercial catch limits at zero from the 1985–86 season. This moratorium still applies, but hunts by Indigenous communities for subsistence are allowed under separate aboriginal subsistence quotas. Norway objected to the moratorium and continues commercial whaling; Iceland left the IWC in 1992, rejoined in 2002 with a reservation, and resumed commercial whaling in 2006. Japan left the IWC and resumed commercial whaling in its own waters in July 2019.
For seals, the European Union banned the import of seal products in 2009, but exempted products from hunts by Inuit and other Indigenous communities. Inuit hunting is only about 3% of the total seal hunt, yet Inuit communities suffered economic harm because the market for sealskin collapsed: pelts that sold for over CA$100 in 2006 fetched only about $8–15 after the ban. The case shows how a rule based on animal welfare can harm Indigenous livelihoods even when it tries to exempt them.
Sources: Wikipedia, International Whaling Commission; Seal hunting (2026).
In the grindadráp of the Faroe Islands, a self-governing territory of Denmark in the North Atlantic, boats drive pods of long-finned pilot whales (Globicephala melas), and sometimes dolphins, into shallow bays, where the animals are killed on the beach with a knife that cuts the spinal cord. Written records of the hunts go back to 1584. About 670 pilot whales and 298 Atlantic white-sided dolphins have been killed on average each year since 2000. The hunt needs the approval of the local sheriff, is supervised by the police, and participants must be trained.
Perspective of Faroese supporters | Perspective of animal rights campaigners | |
|---|---|---|
Core value | Cultural tradition, self-reliance and the right of a community to use local resources | Whales and dolphins are sentient, intelligent animals with a right not to be killed |
Sustainability | The IUCN estimates the Faroese pilot whale subpopulation at about 100,000 and considers the catch likely sustainable | Sustainability in numbers does not make killing individual animals acceptable |
Welfare | The spinal knife kills quickly; laws have been tightened to make the hunt more humane | The drive and beaching cause stress and suffering to whole family groups |
Food | Local food with a low carbon footprint compared with imported meat | Pilot whale meat is contaminated; the islands have other food |
Who decides | The Faroese, through their own parliament and laws | An international moral community, since whales migrate across borders |
Opinion within the Faroes is itself divided. In September 2021, 1,428 Atlantic white-sided dolphins were killed in a single hunt at Skálabotnur, far more than usual, and the event drew criticism inside the islands as well as abroad. A survey that year found 83% of islanders supporting the pilot whale hunt but 53% opposing the killing of dolphins, and in 2022 the government set a provisional quota of 500 dolphins a year.
Health is a further issue: pilot whales are high in a marine food chain, so mercury and other pollutants biomagnify in their meat, and in 2008 the Faroese chief medical officer advised that pilot whale meat was no longer fit for human consumption; the official advice since 2011 is to eat no more than one portion a month.
Sources: Wikipedia, Grindadráp (2026).
For Inuit communities in Arctic Canada and Greenland, hunting narwhals (Monodon monoceros) is a source of food, culture and income. Almost every part of the animal is used, and muktuk, the raw skin with its layer of blubber, is a valued food and a rare source of vitamin C in the Arctic diet. About 1,000 narwhals are killed each year, roughly 600 in Canada and 400 in Greenland, from a world population of about 170,000; the IUCN lists the species as Least Concern, and international trade in its tusks is controlled under CITES Appendix II.
The hunt is defended as an exercise of Indigenous rights and self-determination, and as part of a long relationship with the sea. Others argue that the shrinking sea ice is already making narwhals more vulnerable, so any hunting should be very cautious.
Sources: Wikipedia, Narwhal (2026).
Rights-based ethics: if whales and dolphins have rights because they are sentient, killing them is wrong whatever the benefits. If Indigenous peoples have a right to their culture and food, a ban that removes that right is also wrong. The two rights conflict.
Consequentialist ethics: the hunt is judged by its outcomes: the suffering of the animals, the effect on the population, the food and cultural value to the community, the health risks from contamination and the cost of imported replacements.
Virtue ethics: the question becomes what a respectful, responsible person or community would do: whether the hunt is carried out with skill, restraint and gratitude, or wastefully, as critics said of the 2021 dolphin kill.
These perspectives explain why the debate rarely ends. Conservation science can show whether a hunt is sustainable, but whether it is ethical depends on values about animal rights, cultural rights and who has the authority to decide.
Aquatic food production appears in all parts of the external assessment. Paper 1 may give you data on an unfamiliar fishery or fish farm (catches, effort, a yield–effort curve, a map of protected areas) and ask you to calculate, interpret and evaluate. Paper 2 Section A favours short explanations of MSY, destructive methods, management tools and aquaculture impacts. Section B essays often ask you to evaluate strategies for sustainable fisheries or aquaculture, where named examples and several perspectives are essential.
The next question tests understanding of the yield–effort curve rather than recall of its shape.
Data-based items in Section A often combine a calculation with an explanation.
The resource is invented for practice. A lake fishery for tilapia is managed by a district council. Table 1 gives its records (hypothetical data). In 2023 the council proposed closing 20% of the lake, including the main spawning beds, as a no-take zone.
Year | Number of licensed boats | Average days fished per boat | Total catch (t) |
|---|---|---|---|
2005 | 120 | 150 | 2,700 |
2010 | 180 | 160 | 3,456 |
2015 | 260 | 160 | 3,328 |
2020 | 340 | 170 | 2,890 |
Phytoplankton and macrophytes are the producers of aquatic food webs; energy passes up several trophic levels to the large fish people catch.
People eat finfish, molluscs, crustaceans and aquatic plants; seafood supply per person rose from 9.0 kg (1961) to about 20 kg, driven by population growth, rising incomes and changing diets.
Overfishing and destructive methods such as bottom trawling, ghost fishing and blast and cyanide fishing damage stocks and habitats; about 35% of assessed stocks were overexploited in 2021.
Overexploitation can cause fishery collapse, as with the northern cod of Newfoundland, which peaked at 810,000 t in 1968, collapsed by 1992 and reopened only in 2024.
MSY is the largest catch that can be repeated every year; the yield–effort curve shows yield falling when effort exceeds E(MSY), so MSY should cap quotas.
Warming, coral bleaching and ocean acidification are stressing aquatic ecosystems and the fisheries that depend on them.
Fisheries are managed by quotas, licences, seasons, mesh sizes, zones, gear rules and eco-labels at international, national and local scales; MPAs protect habitats and support catches through spillover.
Aquaculture now exceeds wild capture but can cause habitat loss, pollution, disease, chemical residues and escapees; IMTA and farming low-trophic species reduce the impacts.
Productivity is highest where mixing or upwelling brings nutrients to the sunlit surface, and lowest in permanently stratified open oceans; nutrient loading can raise productivity or cause eutrophication. (HL)
Stock size is estimated by surveys, acoustics, tagging, ageing and CPUE; catches are monitored by logbooks, observers and VMS; harvesting at MSY is risky because of uncertainty and positive feedback. (HL)
UNCLOS gives coastal states 200-nautical-mile EEZs; the high seas are hard to regulate, and the High Seas Treaty came into force in January 2026. (HL)
Hunting seals, whales and dolphins sets animal rights against Indigenous and cultural rights, as in the Faroese grindadráp and Inuit narwhal hunting. (HL)
I can outline the role of phytoplankton and macrophytes in aquatic food webs.
I can describe, with data, how demand for aquatic food has changed and explain why.
I can explain how bottom trawling, ghost fishing, blast fishing and cyanide fishing are unsustainable.
I can explain the collapse of the northern cod fishery using specific facts and figures.
I can draw and interpret a yield–effort curve and explain why MSY should cap quotas.
I can explain the effects of climate change and ocean acidification on aquatic ecosystems and design an acidification experiment.
I can evaluate fisheries management tools and marine protected areas at different scales.
I can evaluate the impacts of aquaculture and one technique that reduces them.
I can explain how stratification, upwelling and nutrient loading control productivity. (HL)
I can describe methods of stock assessment and harvest monitoring and calculate CPUE. (HL)
I can explain the risks of harvesting at MSY and the role of cooperation in stock recovery. (HL)
I can evaluate EEZs, the high seas and the High Seas Treaty, and discuss a marine mammal hunt from two perspectives. (HL)
Step 3: find the matching lower effort. The same yield occurs the same distance below the peak as 14 is above it: . Check: thousand tonnes per year.
Step 4: comment. An effort of 6 thousand vessel-days gives the same catch as 14 thousand, with less than half the cost and a much larger fish stock left in the sea. The fishery at 14 is overexploited; cutting effort to 10 would raise the catch to 200 thousand tonnes, and cutting it below 10 would make the fishery safer still.