Marine Resources, Overfishing, and Aquaculture

Marine resources are the living and non-living materials taken from the sea, including fish, shellfish, oil, gas, minerals and energy; overfishing is catching fish faster than their populations can replace themselves; and aquaculture is the farming of fish, shellfish and aquatic plants under controlled conditions. In brief: fish provide around 3 billion people with a significant share of their animal protein. Capture fisheries have stopped growing since the late 1980s, and more than a third of the assessed fish stocks are overfished according to the Food and Agriculture Organization (FAO). Aquaculture, which has grown rapidly, now supplies about half of the aquatic animals that people eat, but it also has environmental problems.

The sea feeds hundreds of millions of people and supports livelihoods around the world, but its resources are under pressure. This guide explains the main marine resources, how overfishing happens and how it is managed, how aquaculture works and what it costs, compares capture fishing and aquaculture in a table, works through a simple stock model, outlines the history and corrects common mistakes.

What marine resources are there?

• Living resources: fish, crustaceans (shrimp, crab, lobster), mollusks (squid, oysters, mussels), seaweed and marine mammals. Fish are a major source of protein and micronutrients such as omega-3 fatty acids, iodine and vitamin D, and fisheries and aquaculture employ tens of millions of people, mostly in small-scale operations in developing countries.
• Energy resources: offshore oil and gas (around a quarter to a third of world oil production comes from offshore fields), and increasingly wind (offshore wind farms), tidal and wave energy.
• Minerals: sand and gravel for construction, salt, and, in the deep sea, polymetallic nodules, hydrothermal sulfide deposits and cobalt-rich crusts, whose mining is still at an early stage and is controversial for its environmental risks.
• Other uses: shipping lanes, tourism, cables and pharmaceuticals from marine organisms.

What is overfishing and what are its causes and effects?

Fish populations (stocks) renew themselves through reproduction and growth. Overfishing happens when fish are caught faster than they can be replaced, so the stock declines below the level that gives the highest long-term catch. The maximum sustainable yield (MSY) is the largest catch that can be taken year after year without reducing the stock. According to FAO, around 37% of the assessed stocks were fished at unsustainable levels in the early 2020s, while about 60% were at the maximum sustainable level and the rest underfished. Most of the catch (about three quarters) comes from biologically sustainable stocks, because well-managed large stocks account for a big share of landings.

Causes include too many boats and too much gear (fleet overcapacity), government subsidies that support fishing, weak enforcement and illegal, unreported and unregulated (IUU) fishing, which may account for around one in five fish caught, the “tragedy of the commons” in open-access waters, destructive methods such as bottom trawling and the use of explosives, bycatch (the unwanted capture of non-target species such as turtles, sharks, seabirds and dolphins), and growing demand.

Effects are the collapse of stocks and the loss of jobs and income, changes in food webs (removing predators or prey), damage to the seabed and the loss of biodiversity. A famous example is the Atlantic cod off Newfoundland, where catches fell dramatically and Canada imposed a moratorium in 1992, which put about 30,000 people out of work, and the stock has recovered only slowly. The Peruvian anchoveta fishery, the largest single-species fishery in the world, collapsed in the early 1970s because of overfishing and El Niño.

How can fisheries be managed?

• Catch limits: total allowable catches set by scientists based on stock assessments, and divided among countries or fishers as quotas, including individual transferable quotas (ITQs), which give fishers a share of the catch.
• Effort and gear controls: limits on the number of boats and days at sea, mesh-size rules to let young fish escape, and bans on destructive gear.
• Closed seasons and areas: to protect spawning fish, and marine reserves, where fishing is banned, which allow stocks to recover and “spill over” into neighboring waters.
• Reducing bycatch: turtle excluder devices, circle hooks and bird-scaring lines.
• Monitoring and enforcement: observers, satellite tracking of vessels and port controls against IUU fishing, and the removal of harmful subsidies, as agreed in part by the WTO in 2022.
• Certification and consumer choices: labels such as that of the Marine Stewardship Council (founded in 1997).
• Community-based management: giving small-scale fishing communities a role in rules and rights, which works well in many places.

What is aquaculture?

Aquaculture includes the farming of finfish (salmon, tilapia, carp, catfish), shellfish (oysters, mussels, shrimp) and seaweed, in ponds, tanks, net pens at sea and on ropes. It is the fastest growing food production sector, and it surpassed wild capture as the main source of aquatic animals for human consumption in recent years. China is by far the largest producer, followed by India, Indonesia, Vietnam and Bangladesh, and Norway and Chile are major producers of salmon.

Benefits: it can supply fish without depleting wild stocks, creates jobs and income, and some types, such as mussels, oysters and seaweed, require no feed and may even improve water quality. Problems: habitat loss, as when mangroves are cleared for shrimp ponds, pollution from waste, uneaten feed and chemicals, disease and parasites (such as sea lice in salmon farms) that may spread to wild fish, escapes of farmed fish that may breed with wild stocks, use of wild fish for feed (fishmeal and fish oil), and antibiotic use. Improvements include better feeds with less fishmeal, integrated multi-trophic systems (growing different species together so that the waste of one feeds another), recirculating tanks on land, better site selection and certification.

Capture fisheries and aquaculture compared

Worked example: sustainable yield and feed

The values are hypothetical and illustrate typical calculations.

1. Logistic stock model: a fish stock grows according to dB/dt = r × B × (1 − B ÷ K), where B is the biomass, r is the intrinsic growth rate and K is the carrying capacity. For r = 0.4 per year and K = 1,000,000 tonnes, the stock grows fastest when B = K ÷ 2 = 500,000 tonnes.
2. Maximum sustainable yield: at this point, the growth is 0.4 × 500,000 × (1 − 0.5) = 100,000 tonnes a year. This is the MSY, which equals r × K ÷ 4 = 0.4 × 1,000,000 ÷ 4 = 100,000 tonnes.
3. Overfishing: if the annual catch is 150,000 tonnes, which is more than the maximum growth of 100,000 tonnes, the stock cannot sustain it and declines each year, so it will eventually collapse unless the catch is reduced.
4. A cautious target: managers might set the catch at 80% of MSY, 0.8 × 100,000 = 80,000 tonnes, to allow for uncertainty and keep the stock above the level that gives MSY.
5. Feed conversion ratio: farmed salmon needs about 1.2 kg of feed to gain 1 kg. To produce 50,000 tonnes of salmon, 50,000 × 1.2 = 60,000 tonnes of feed are needed. If 15% of the feed is fishmeal and fish oil from wild fish, and it takes 4.5 kg of wild fish to make 1 kg of fishmeal and oil, the wild fish needed is 60,000 × 0.15 × 4.5 = 40,500 tonnes, which gives a “fish-in, fish-out” ratio of 40,500 ÷ 50,000 = 0.81.
6. Share of production: if global aquatic animal production is 185 million tonnes and aquaculture produces 94 million tonnes, aquaculture accounts for 94 ÷ 185 = 51%.

How have fisheries developed?

People have fished for thousands of years, and early aquaculture was practiced in ancient China and Egypt. The introduction of steam trawlers in the late 19th century, followed by diesel engines, sonar, refrigeration and factory ships in the 20th century, raised the catch dramatically: world marine catches grew from about 20 million tonnes in 1950 to a peak of more than 80 million tonnes in the mid-1990s. In 1982, UNCLOS gave coastal states control of the resources in 200-nautical-mile exclusive economic zones, and the collapse of the Newfoundland cod in 1992 showed the risks of poor management. The FAO’s Code of Conduct for Responsible Fisheries was adopted in 1995, and the Sustainable Development Goals include a target to end overfishing (SDG 14.4). Aquaculture has grown by about 5–6% a year since the 1990s. A WTO agreement in 2022 on limits to harmful fisheries subsidies marked a step forward.

Which mistakes are common?

Mistake 1: Thinking that overfishing is only a problem of the high seas

Most overfishing occurs in coastal waters, including within national zones, where weak management and many boats put pressure on the stocks.

Mistake 2: Assuming that aquaculture always protects wild fish

Feeding carnivorous farmed fish with fishmeal can increase pressure on wild stocks, and farms can pollute and spread disease. Impact depends on species, feed and system.

Mistake 3: Believing that the maximum sustainable yield is a safe target in all cases

MSY estimates are uncertain, and fishing at MSY leaves little margin for error, so many managers aim for a lower level.

Mistake 4: Focusing only on the fish caught and ignoring bycatch

Large numbers of non-target animals are caught and discarded, which affects the whole ecosystem.