Notes · Environmental ScienceUK · A-Levels

Aquatic food production systems

This chapter examines how food is obtained from the sea and fresh water, and how to do so sustainably. It covers wild fisheries and the problem of over-exploitation, the concept of maximum sustainable yield, the methods of fishing and their impacts, the ways wild fisheries can be managed, and aquaculture with its growing role and its own environmental impacts, throughout weighing the need for food against the health of aquatic ecosystems.

4 sections~13 min reading time3 competenciesLevel Standard 3 · Advanced 1

T·131313 / 16

Exam profile

AO1 · Describe fisheries, over-fishing, fisheries management and aquacultureAO2 · Apply the concept of maximum sustainable yield to stock and catch dataAO3 · Evaluate the sustainability of fishing methods, quotas and aquaculture

Operators:describeexplainanalyseevaluateapplyassess

basic level

AS-Level expects you to describe over-fishing, the main management measures and the impacts of fish farming.

higher level

The full A-Level requires you to apply maximum sustainable yield to data and to evaluate the sustainability of fishing methods and aquaculture.

Depth

Text

Contents · 4 sections

Aquatic food production systems

Fisheries and the problem of over-exploitation#

StandardAQA 7447 3.5.2

Maximum sustainable yield

Fig. 1Sustainable catch rises with effort to a peak (the maximum sustainable yield) then falls as over-fishing depletes the stock.

Wild fish are a major source of protein for much of the world's population, and because fish reproduce, a fishery is in principle a renewable resource that can be harvested indefinitely, so long as the catch does not exceed the rate at which the stock renews itself. The danger is that fishing removes fish faster than they can breed and grow, so the stock shrinks; this is over-fishing, and it has driven many important fisheries into decline or collapse.

The key management concept is the maximum sustainable yield, the largest catch that can be taken year after year without reducing the stock, because it is balanced by the stock's growth. As fishing effort increases from zero, the catch at first rises, because more fish are taken; but beyond a point the stock is being depleted faster than it recovers, so the sustainable catch falls, and if effort continues to rise the stock can collapse and the catch fall towards zero. The relationship between effort and sustainable yield therefore has a peak, the maximum sustainable yield, and fishing beyond it is self-defeating.

A collapsing fishery shows characteristic warning signs: catches fall despite greater effort, the fish caught are smaller and younger because the older breeding fish have been removed, and the catch per unit of effort declines. Because larger, older fish produce far more eggs, removing them undermines the stock's ability to recover, so a stock can collapse quite suddenly once it is pushed past a threshold and may take many years to recover, if it recovers at all. This is a clear example of a renewable resource made non-renewable by over-use.

Over-fishing also damages the wider ecosystem, not just the target species. Removing a species alters the food web, releasing its prey or starving its predators, and can shift the whole community to a different, often less valuable, state. These ecosystem effects, together with the loss of the fishery itself and the livelihoods that depend on it, are why fisheries must be managed to stay at or below the maximum sustainable yield, the subject of the management section.

Worked example

Interpreting a yield curve

A graph of sustainable catch against fishing effort peaks at an effort of 10 units, giving a catch of 50 units, and falls to zero at an effort of 20. Explain what the peak represents and what happens beyond it.

The peak
The peak (a catch of 50 units at an effort of 10) is the maximum sustainable yield, the largest catch that can be taken year after year without reducing the stock.
Beyond the peak
As effort rises above 10, fish are removed faster than the stock can renew, so the sustainable catch falls even though more effort is applied.
At high effort
At an effort of 20 the stock is so depleted that the sustainable catch approaches zero: the fishery is collapsing.

Result: The peak is the maximum sustainable yield; fishing harder than this depletes the stock and reduces the catch.

Typical mistakes

Assuming more effort always means more catch; beyond the maximum sustainable yield the catch falls as the stock is depleted.
Treating a fishery as inexhaustible because fish reproduce; over-fishing can make a renewable stock collapse.

Active revision

A fishery's catches fall each year even though more boats are fishing, and the fish landed are getting smaller. Explain what is happening and why.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Fishing methods and their impacts#

StandardAQA 7447 3.5.2

Fishing methods and their impacts

Fig. 2Fishing methods differ in selectivity, bycatch and habitat damage, which determine their sustainability.

Different fishing methods catch fish in different ways and with different environmental impacts, and a central issue is selectivity, how well a method catches the target species and size while avoiding everything else. Trawling drags a large net through the water or along the seabed; bottom trawling in particular can catch a great deal besides the target and damages the seabed habitat. Drift nets and gill nets hang in the water and catch fish by the gills; they can be indiscriminate and were notorious for catching dolphins and turtles. Long-lining sets baited hooks over great distances and can catch seabirds and sharks. Purse seining encircles a shoal with a net and can be relatively selective if used carefully.

The unwanted catch, the bycatch, is a major problem. Non-target fish, undersized fish, and other animals such as dolphins, turtles and seabirds may be caught and killed, often then discarded dead. Bycatch wastes life, harms populations of vulnerable species, and damages the wider ecosystem, so reducing it is a key aim of sustainable fishing, achieved by more selective gear, escape panels and devices, and avoiding sensitive areas and times.

The physical impact on habitats matters as much as the catch. Bottom trawling drags heavy gear across the seabed, destroying the structures and communities, such as corals and sponge beds, that provide habitat for many species and take a long time to recover. A fishing method must therefore be judged not only by how many target fish it lands but by its bycatch and its damage to the habitat, so that a method giving a high catch may still be unsustainable if it wrecks the ecosystem that supports the fishery.

Evaluating fishing methods means weighing efficiency and catch against selectivity, bycatch and habitat damage. The most sustainable methods catch the target species and size with little bycatch and little habitat damage, even if they are less efficient in the short term, because they protect the stock and the ecosystem on which future fishing depends. This connects directly to the management measures that regulate how, where and when fishing takes place.

Worked example

Evaluating a fishing method

A fleet uses bottom trawling to catch flatfish and lands large quantities. Evaluate the sustainability of this method.

The catch
Bottom trawling lands a large catch of the target flatfish efficiently, which is its advantage.
Bycatch and habitat
It also catches and discards much non-target life and drags heavy gear across the seabed, destroying habitats that take years to recover.
Judgement
Despite the large catch, the high bycatch and seabed damage undermine the ecosystem that supports the fishery, so the method is not sustainable as used.

Result: The large catch does not make bottom trawling sustainable, because its bycatch and seabed damage harm the supporting ecosystem.

Typical mistakes

Judging a method only by the size of the target catch and ignoring bycatch and habitat damage.
Assuming all trawling is the same; bottom trawling is especially damaging to the seabed.

Active revision

Explain why bottom trawling can be regarded as unsustainable even when it lands a large catch of the target fish.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Managing wild fisheries sustainably#

AdvancedAQA 7447 3.5.2

Managing a wild fishery

Fig. 3Sustainable fisheries management combines catch limits, gear rules, protected areas and enforcement.

Wild fisheries are managed to keep the catch at or below the maximum sustainable yield so that the stock persists. The main tool is the quota, a limit on the total weight that may be landed (the total allowable catch), set from scientific estimates of the stock and shared among fishing fleets. Quotas directly limit how much is taken, but they require reliable stock assessment, are hard to enforce, and can encourage the discarding of fish caught over quota, so they work best combined with other measures.

Other measures protect the fish that most need protecting. Regulating the mesh size of nets lets young, undersized fish escape to breed before they are caught, protecting the future of the stock; closed seasons protect fish during breeding, and closed areas and marine protected areas set aside places where fish can breed and grow undisturbed, from which young fish spill over into the surrounding fishery. Limiting the number, size or power of boats, and restricting the most damaging gear, control the effort applied. Together these measures manage not just how much is caught but which fish, and when and where.

Effective management depends on monitoring and enforcement. Stocks must be assessed regularly so that quotas can be adjusted as the stock changes, and rules are only useful if they are obeyed, which requires inspection and penalties. International cooperation is essential because fish move across boundaries and many fisheries are shared, so agreement between countries is needed and is often difficult to reach and to enforce, which is a legitimate weakness to evaluate.

Evaluating fisheries management means weighing effectiveness against practicality and cost, and recognising that measures work best in combination. A quota alone may fail if it is set too high, poorly enforced or evaded by discarding; mesh-size limits protect young fish but not the overall catch; marine protected areas conserve breeding stock but reduce the area open to fishing. The strongest management combines a scientifically set catch limit with gear rules, protected areas and enforcement, adjusted as the stock is monitored, so that the fishery stays productive indefinitely.

Worked example

Designing a management package

A cod stock is severely depleted. Recommend a combination of measures to allow it to recover and explain how each would help.

Limit the catch
Set a low quota based on scientific stock assessment so that fewer fish are removed than are added by breeding and growth, letting the stock rebuild.
Protect the breeders
Increase the mesh size so young cod escape to breed, and create a marine protected area and closed season to shelter spawning fish.
Enforce and monitor
Inspect catches and penalise breaches, cooperate with neighbouring countries that share the stock, and adjust the quota as monitoring shows the stock recovering.

Result: A low, enforced quota combined with mesh-size limits and a protected area, adjusted by monitoring, gives the stock the best chance to recover.

Typical mistakes

Assuming a quota alone solves over-fishing; it needs good stock data, enforcement and other measures to work.
Forgetting that mesh-size limits protect young fish so they can breed, rather than simply reducing the catch.

Active revision

Explain how increasing the minimum mesh size of nets and creating a marine protected area could together help a depleted fish stock recover.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

Aquaculture: promise and impacts#

StandardAQA 7447 3.5.2

Environmental impacts of a fish farm

Fig. 4Aquaculture relieves pressure on wild stocks but has its own impacts: pollution, disease, escapes and habitat loss.

Aquaculture, the farming of fish, shellfish and aquatic plants, has grown rapidly and now supplies a large and rising share of the aquatic food eaten by people. Its great advantage is that it produces food without taking wild fish from the sea, and it can be highly productive in a small area and give a controlled, reliable supply. In principle, therefore, aquaculture can relieve the pressure of over-fishing on wild stocks, which is a major argument in its favour.

However, aquaculture has significant environmental impacts that qualify this promise. Uneaten feed and the waste from crowded fish enrich the surrounding water and can cause the eutrophication met in earlier chapters; the crowding encourages disease and parasites (such as sea lice), which are treated with chemicals that enter the environment and can spread to wild fish; and farmed fish that escape can interbreed with or out-compete wild populations, reducing their genetic fitness. Building fish and shrimp farms can also destroy important habitats, notably the clearance of coastal mangroves for shrimp ponds.

A further concern is that many farmed fish are carnivorous and are fed on fishmeal and fish oil made from wild-caught fish, so that farming them can still draw on wild stocks and may use more fish as feed than it produces, undermining the claim that aquaculture spares wild fisheries. Farming lower-trophic species, such as shellfish and herbivorous fish, avoids much of this problem and is more efficient, because less energy is lost feeding a carnivore.

Evaluating aquaculture therefore means weighing its contribution to food supply and to relieving wild fisheries against its pollution, disease, escape and habitat impacts, and asking what species and methods are used. Well-managed aquaculture of lower-trophic species, sited and stocked to limit pollution and escapes, can be a sustainable source of food; poorly managed farming of carnivorous fish in sensitive habitats is not. As with fisheries, the sustainable path combines the benefit of production with the careful management of its impacts.

Worked example

Evaluating aquaculture

A coastal region proposes large salmon farms to increase food supply and reduce sea fishing. Evaluate this proposal.

Benefits
Farming produces fish reliably in a small area and could reduce the catch taken from over-fished wild stocks.
Impacts
Salmon are carnivores fed partly on wild-caught fish; the farms produce waste that can cause eutrophication, foster sea lice and disease, and risk escapes that harm wild salmon genetics.
Judgement
The proposal is only sustainable if the farms are well sited and stocked, waste and disease are controlled, escapes prevented, and feed sourced responsibly; otherwise it shifts rather than solves the problem.

Result: Salmon farming can help supply and spare wild stocks, but is sustainable only if its pollution, disease, escape and feed impacts are carefully managed.

Typical mistakes

Presenting aquaculture as impact-free; it causes eutrophication, disease, escapes and habitat loss.
Forgetting that farming carnivorous fish uses wild fish as feed, so it can still draw on wild stocks.

Active revision

Evaluate whether expanding salmon farming is a sustainable way to increase the supply of fish, considering its benefits and impacts.

Active recall

Recall the key points — then reveal.

Sources: AQA AS and A-level Environmental Science (7447) specification (AQA) · GCE AS and A level subject content (Department for Education)

References & sources

Sources

AQA

AQA AS and A-level Environmental Science (7447) specification

Department for Education

GCE AS and A level subject content

Aquatic food production systems

4 sections~13 min reading time3 competenciesLevel Standard 3 · Advanced 1

Aquatic food production systems

Key points

Interpreting a yield curve

Key points

Evaluating a fishing method

Key points

Designing a management package

Key points

Evaluating aquaculture

Sources

Aquatic food production systems

Key points

Interpreting a yield curve

Key points

Evaluating a fishing method

Key points

Designing a management package

Key points

Evaluating aquaculture

Sources