Marine Aquaculture

Overfishing: catching more than the stock can replace

Overfishing happens when a species is caught faster than it can breed and replace the numbers lost, so the breeding stock shrinks year after year. Overharvesting is the same pressure applied to any marine species, not only to fish: shellfish beds can be stripped, seaweeds can be cut faster than they regrow, and slow-breeding species such as sharks and groupers are especially vulnerable because they produce very few young. Fishing normally removes the largest and oldest individuals first, and those are the fish that produce the most eggs, so the stock becomes less productive as well as smaller. Technology makes it easier: sonar to find a shoal, GPS and spotter aircraft to return to it, bigger boats with stronger engines, and refrigeration that lets a vessel stay at sea for weeks instead of days.

A single haul contains two kinds of catch. The target species is what the fishery is aiming at, such as cod or tuna. Bycatch is everything else caught by accident: young fish of the target species, other fish, seabirds, turtles, dolphins, rays and corals. The less selective the gear, the more bycatch is taken. A trawl dragged over the seabed catches everything in its path and also damages the habitat the fish depend on; a gill net holds whatever swims into it until it drowns; a longline hooks seabirds as the baited hooks sink. Most of the bycatch is dead or dying when it is thrown back, so the stock loses animals that were never wanted. Small mesh makes this worse, because it traps juveniles before they have had a chance to breed.

Taking a species out of the sea changes the food chain in both directions. Remove the predators - large tuna, cod, sharks - and the smaller fish and invertebrates they fed on increase, often at the expense of other species. Remove the small plankton feeders - anchovies, sardines, krill - and the seabirds, dolphins, seals and larger fish above them go hungry, because the energy that supported the whole chain has been taken away. When a fleet can no longer fill its quota on the big, valuable species it switches to smaller species lower down the chain, so the community shifts towards a short, simple food web of fast-breeding fish and jellyfish. This is what fishing down the food web means, and the result is an ecosystem that is less stable and more easily damaged by the next shock.

• Overfishing: the stock is harvested faster than it can breed, so both the breeding population and the future catch fall.
• Overharvesting: the same pressure applied to any marine species, including shellfish and seaweeds.
• Target species: the species the fishery wants. Bycatch: everything else caught by accident and usually discarded dead.
• Food chains: removing predators lets their prey increase; removing prey starves the predators above them.
• Fishing down the food web: as large species are lost, effort shifts to smaller species lower in the chain.

Marine aquaculture: the farming of marine species in captivity

Marine aquaculture is the farming of marine species in captivity. Farming means that people control the stock: they choose the species, hatch or collect the young, keep the animals or plants together, feed and protect them, and harvest them when they reach the size the market wants. The three groups are fish, such as salmon, sea bass, sea bream, groupers and milkfish; crustaceans, such as prawns, shrimps and crabs; and seaweeds, such as the red seaweeds grown for carrageenan and the brown kelps grown for food. No particular species is needed for the examination - you need the three groups and what farming involves.

Farming now supplies about half of the fish and shellfish that people eat, and its share is rising, because the wild catch has been almost level since the late 1980s while the human population and the demand for protein have grown. Farming also suits the producer: the stock is protected from predators, the harvest can be graded and timed to the market, losses are lower than in the open sea, and the farm can be placed near the processing plant and the port that exports the crop.

One distinction is easy to miss. True aquaculture begins with young produced in a hatchery, so the farm does not depend on the wild stock for its seed. Some enterprises instead catch wild juveniles and grow them on in cages, as is done with tuna and eels; this is really a form of fishing, because animals are still removed from the wild, and it tends to hold the pressure on wild stocks steady rather than relieve it. Marine aquaculture is also not the same as freshwater fish farming of carp, tilapia or trout in ponds and rivers, and it is not sea ranching, where hatchery-reared juveniles are released into the sea to be caught later by fishermen.

• Marine aquaculture: the farming of marine species in captivity.
• The three farmed groups: fish, crustaceans and seaweeds.
• About half of the seafood eaten worldwide is now farmed, and the share is growing.
• True aquaculture starts with hatchery-produced young; growing on wild-caught juveniles is still a form of fishing.
• Do not confuse marine aquaculture with freshwater fish farming or with sea ranching.

The farm systems: pens, ponds, raceways, recirculating and integrated farms

Open-net pens are cages of netting hung from a floating frame and moored in a sheltered bay, loch or fjord. Water flows through the net, so the farm needs no pumps and the waste is carried away by the tide; that makes pens the cheapest way to farm salmon, sea bass and sea bream. The weakness comes from the same design. Nothing is controlled, so disease, parasites and nutrients pass freely between the farm and the open sea in both directions, and a torn net, a storm or an attack by seals or sea birds lets part of the stock escape.

Ponds are dug enclosures on the coast or on a delta, filled with brackish water and topped up by the tide or by pumps. Most of the world's farmed shrimp, milkfish and crabs come from ponds, which can be built cheaply and can be run by a family, but they are often cut into mangrove forest or saltmarsh, which removes the nursery grounds and the storm protection that the coast depended on, and they need large volumes of water exchanged every day. Raceways are long channels with a continuous flow of water through them, used where a large clean flow is available; water quality is good, but the waste simply leaves in the outflow, so the river or the coast has to absorb it.

Recirculating aquaculture systems (RAS) are land-based tanks in which the water is filtered, disinfected, aerated and pumped back round, with only a small fraction replaced each day. A RAS separates the farm from the sea: waste is captured as sludge that can be treated, escapes are almost impossible and the temperature and salinity can be controlled, so a tropical species can be grown in a temperate country. The costs are real, though. Pumps, chillers, filters and oxygen equipment use a great deal of electricity, the capital cost is high, so only high-value species pay for it, and if the power fails the whole stock can die within hours.

Integrated systems farm species that help one another. Integrated multi-trophic aquaculture (IMTA) grows a fed species, such as salmon, next to a seaweed that takes up the dissolved nitrogen and phosphorus the fish release and a shellfish such as mussels that filters the organic particles out of the water, so what leaves the farm as pollution becomes a second and third crop. In the tropics the same idea appears as integrated mangrove and shrimp ponds, or as a rotation of shrimp, rice and crabs, in which the waste from one crop feeds the next. Integration reduces nutrient release and can raise income from the same site, but the species have to be balanced and sold together, and the farm still occupies coastal space.

What aquaculture gives back

Reduced exploitation of natural fisheries. Every tonne of farmed seafood eaten is a tonne that does not have to be taken from a wild stock, so farming can take pressure off species that are already overfished. This gain is largest where the farmed product substitutes for an overfished wild fishery, and it is partly cancelled out where the farmed species is fed on wild-caught fish, which is the central limitation of the industry and is covered below.

Increased food supply for humans. Farming multiplies the harvest from the same area of sea, because the farmer controls survival, feeding and predation. Seaweed needs no feed at all and shellfish filter their food out of the water, so both can be grown with almost no input, while finfish can be reared at densities far above anything a wild coast would support. Farmed seafood is a source of protein, omega-3 oils and micronutrients, and in much of Asia, where most of the world's aquaculture is, it is the affordable animal protein of everyday diets.

The social and economic benefits are as important as the food. Fish farms, hatcheries, feed mills, processing plants, ice suppliers and transport firms all create employment, much of it in coastal districts where the wild fishery has declined or where quotas have cut the days at sea; the crop is often exported, so it earns foreign currency. Seaweed farming in particular is low-cost: it needs a few lines or stakes and no boat, it can be done at low tide close to the shore, and it fits around other work, which is why it has become a significant source of income for women on the coast of East Africa and Indonesia.

• Reduced exploitation of natural fisheries: farmed seafood substitutes for wild catch that would otherwise be taken from overfished stocks.
• Increased food supply: the harvest from a given area of sea is multiplied, because survival and feeding are controlled.
• Low-input crops: seaweeds need no feed and shellfish filter their food from the water.
• Jobs and income: hatcheries, feed mills, farms, processing, ice and transport, often in districts where the wild fishery has declined.
• Export earnings for coastal countries, and work that fits around household tasks, especially in seaweed farming.

The environmental costs: escapes, disease, waste and the feed problem

Risk of escape. Farmed fish leave the pens through storm damage, torn or fouled nets, attacks by seals that tear the net, and simple human error at handling and transport. Escaped farmed salmon interbreed with wild salmon, and because farmed stock has been selected for fast growth in cages rather than for survival in a river, the offspring are less well adapted to the wild: their local genetic adaptations, such as the timing of migration and resistance to disease, are diluted. Escapees also compete with wild fish for food and for spawning sites, and where the farmed species is not native to the coast at all, the escapees can establish themselves as an invasive species.

Risk of disease and parasites. Thousands of fish held together in one place, in one body of water, is exactly the condition in which infection spreads quickly. Sea lice, small parasites that feed on the skin of salmon, multiply on farmed fish and are then carried by the tide onto wild salmon smolts as they leave the rivers, so a farm can raise the parasite burden of a wild population that has no protection. Other diseases, such as infectious salmon anaemia, can sweep through a site and through neighbouring farms, which is why whole coastal regions have suffered production collapses. Where disease is treated with antibiotics, the residues can enter the sediment under the farm and help to build up antibiotic resistance, and the movement of eggs, juveniles and equipment between sites spreads pathogens further.

Nutrient enrichment from waste. Uneaten feed, faeces and the ammonia the fish excrete pass out of the pens. Nitrogen and phosphorus dissolve and fertilise the water, encouraging phytoplankton and algae to grow, and the organic carbon sinks to the seabed under the cages, where it smothers the bottom community and is broken down by bacteria that use up the oxygen. The result is a patch of black, oxygen-poor mud beneath the farm, and an enriched plume extending down-current that can change the local food web. Integrated farming, which uses seaweeds and shellfish to take up that waste, and lower stocking densities are the main answers, but neither removes the nutrients completely.

The source of food for farmed species. Carnivorous fish such as salmon and sea bass are fed pellets containing fishmeal and fish oil made from wild-caught small fish such as anchoveta, sardines and sand eels, together with plant proteins, usually soya. Catching wild fish to feed farmed fish can therefore add to the pressure on wild stocks instead of relieving it, and it takes fish that people in some regions eat directly. The industry has steadily cut the wild-fish share of the feed and uses more trimmings from processing and more plant protein, but soya needs farmland and fertiliser, and replacing fish oil lowers the amount of omega-3 in the farmed fish. Energy usage runs through the whole system too: pumping and aeration at the farm, manufacture and transport of the feed, service boats, ice and refrigeration, and the processing plant. Feed production is usually the largest share, which is why a RAS that saves on transport can still be an energy-hungry way to produce a fish.

• Escapes: storms, torn nets and seal damage; escapees interbreed with wild relatives, compete for food and spawning sites and can become invasive.
• Disease and parasites: crowding spreads infection; sea lice from farmed salmon infect wild smolts; antibiotics can build resistance in the sediment and in the stock.
• Nutrient enrichment: uneaten feed and faeces fertilise the water and sink as organic sludge, producing an oxygen-poor seabed and a plume of enriched water down-current.
• Feed: fishmeal and fish oil from wild-caught small fish link the farm back to wild stocks and compete with food for people.
• Energy: pumping, aeration, feed manufacture, boats, ice and processing, with feed manufacture usually the largest share.
• Habitat: ponds and pens take coastal space, and ponds are often cut into mangrove or saltmarsh.

Managing the harvesting of wild marine species

Management of a wild fishery means controlling how much is taken, of what size, by which gear and in which places, and then finding a way to make the rules stick. Quotas set a total allowable catch, usually by species and by area, and closed seasons protect the stock while it is spawning or growing. Limits on the number of fishing days and on the size of boat and of the overall net reduce the total effort that can be applied, and an increased mesh size lets juveniles slip through the net before they are caught, so they can breed at least once before they are taken. Pole and line fishing takes one fish at a time and produces almost no bycatch.

Spatial protection works differently: a marine protected area (MPA) closes a stretch of sea to some or all fishing, so the stock inside can recover and breed and the young spill out into the surrounding fishery. Conservation laws set the legal gear, seasons and sizes, and because fish do not stop at national boundaries, international agreements are needed for shared and migrating stocks.

Each of these has a limitation, and that is where the marks are. A quota is only as good as the science behind it; it is often set above the advice of the scientists because ministers do not want to lose jobs, and the catch is then recorded by the industry itself. Rules on gear and mesh are hard to police at sea, and where enforcement is weak they are simply ignored. Closed seasons push effort into the open season and can concentrate the fishing into a short, damaging period. Limits on boat size encourage more, smaller boats, so the total capacity does not fall. An MPA protects the stock inside it but the fleet that loses access simply moves elsewhere, so the total catch may not fall at all, and a reserve with no staff, no patrol boat and no income for the local community is protected on paper only.

International agreements add a further problem: implementation and monitoring. An agreement on paper has to be signed, adopted into national law, funded, patrolled and reported on before fish benefit. The recovery of Atlantic bluefin tuna, after catch limits were cut sharply and the trade was monitored, shows that it can work when the will and the money are there. The failure to stop the collapse of the northern cod off Canada shows the cost of quotas that outlived the stock, and the reason the fleet is now managed as a whole rather than species by species.

Managing marine aquaculture so it does less damage

Site selection is the first control. A farm is best placed where the water is deep and well flushed by currents, away from seagrass beds, coral reefs and mangrove creeks, away from the mouths of rivers used by wild salmon, and far enough from other farms that a disease or a parasite cannot jump from one site to the next. Regulators now also assess the carrying capacity of a bay, which is the total amount of farm waste it can absorb without the ecosystem changing. The limitation is that each site is judged on its own, while the damage depends on the combined output of every farm in the bay, and currents carry the waste beyond the boundary of the site that produced it.

Limits on stocking density, expressed as kilograms of fish per cubic metre of water, are set in law in most farming countries. A lower density means less waste per site, less stress, and slower spread of disease, and it improves the feed conversion. It also lowers the tonnage a pen can produce, so it cuts profit, and farmers may respond by adding more pens or moving to a country with looser rules unless the rules are applied across the region.

Feed conversion ratio (FCR) is the mass of feed used divided by the mass of fish produced, so a ratio near 1.5 means 1.5 kg of feed for every 1 kg of growth. Improving it - through better pellet design, careful feeding to match appetite, and healthier stock - cuts the cost, the wild fish in the feed and the nutrients that leave the farm. Reducing the wild-fish content of the feed, using trimmings and plant protein, addresses the feed problem directly, but plant protein needs land and fertiliser, and it changes the nutritional quality of the fish.

Fallowing leaves a site, or part of a bay, empty for a period after each crop, so the waste on the seabed is broken down and the parasites such as sea lice, which need a host fish, die off. It works well where all the farms in a bay fallow together, but if one farm restocks while its neighbour is empty, the parasite survives and the benefit is lost. Closed and recirculating systems prevent escapes and capture the waste, and semi-closed pens with an impermeable barrier allow the same control in the sea, but they cost more, use more energy and need a reliable power supply. Certification, such as the standards set for responsible aquaculture, audits a farm on feed, escapes, disease, waste and social conditions, and gives buyers a way to choose; its limitations are that small farmers may not be able to afford the audit, the standards differ between schemes, and a certified farm is still a farm - the certificate reduces the impact at that site, it does not remove it.

• Site selection: well-flushed deep water, away from sensitive habitats, wild salmon rivers and other farms; limited because it ignores the combined effect of all the farms in a bay.
• Stocking density limits: fewer fish per cubic metre means less waste, less disease and better feed conversion, but also lower output and profit.
• Feed conversion ratio: feed used divided by fish produced; improving it cuts cost, wild-fish use and nutrient release.
• Fallowing: an empty period rests the seabed and breaks the parasite cycle, but only if neighbouring farms fallow at the same time.
• Closed and recirculating systems: no escapes and captured waste, at the cost of high capital and high energy use.
• Certification: independent standards give buyers a choice, but audits cost money and reduce rather than remove the impact.