Sustainable fishing and bycatch (QCE Marine Science Unit 4)
“Explain overfishing and bycatch, describe the methods used to manage fisheries sustainably (quotas, size and gear limits, seasons), and evaluate their effectiveness using Australian examples”
Overfishing happens when fish are caught faster than they can reproduce, so the stock declines. Sustainable fishing aims to keep catches at or below the maximum sustainable yield, the largest catch a stock can replace each year. Bycatch is the unwanted capture of non-target species such as turtles and dolphins. Managers use quotas, size and bag limits, gear restrictions, closed seasons and areas, and bycatch reduction devices. Each tool has strengths and limits, so effective management usually combines several and relies on monitoring and enforcement.
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What this dot point is asking
QCAA wants you to explain why fishing can become unsustainable, what bycatch is, and to describe and evaluate the tools used to manage fisheries. Evaluation means judging how well each tool works and its limits, with Australian examples such as the Queensland prawn trawl fishery. This complements the broader fisheries and resource-management dot point.
Overfishing and sustainable yield
Fish populations can replace themselves through reproduction. Overfishing occurs when fishing removes individuals faster than the population can replace them, so the stock shrinks and, if it continues, can collapse. Sustainable fishing aims to stay at or below the maximum sustainable yield (MSY), the largest catch that can be taken year after year without reducing the stock. Catching above MSY erodes the breeding population and the future catch.
A particular danger is removing too many large, mature fish, because big females produce hugely more eggs than small ones. Targeting the biggest fish can therefore cut reproduction sharply, which is why size limits matter.
Bycatch
Bycatch is the capture of species the fishery is not targeting, including undersized fish, non-commercial species, and protected animals such as turtles, dolphins, sharks and seabirds. Much bycatch is killed and discarded. It is a major problem in non-selective methods like trawling. In the Queensland prawn trawl fishery, for example, large amounts of fish and other animals were historically caught alongside the prawns, including marine turtles.
Australian prawn trawlers are required to fit turtle excluder devices (TEDs) and bycatch reduction devices (BRDs) in their nets. A TED is a grid that lets large animals such as turtles escape through an opening while prawns pass through to the back of the net; a BRD lets smaller fish escape. These devices dramatically cut the death of turtles and unwanted fish while keeping most of the prawn catch, and they are a clear, examinable example of a technological management solution.
Management tools
Sustainable fisheries management uses a toolkit, usually in combination.
- Catch quotas cap the total or individual catch, directly limiting how much is removed. They work only if the science behind the limit is sound and the catch is monitored.
- Size and bag limits protect young fish so they can breed at least once, and protect very large breeders, by setting minimum (and sometimes maximum) sizes.
- Gear restrictions limit net mesh size, hook type or banned methods to make fishing more selective and reduce bycatch and habitat damage.
- Closed seasons and areas protect fish during spawning or in nursery habitats, linking to the marine-protected-area dot point.
- Bycatch reduction devices (TEDs and BRDs) cut the unwanted catch directly.
- Monitoring and licensing track stocks and limit the number of fishers (effort).
Evaluating effectiveness
Each tool has strengths and weaknesses, which is what evaluation questions want.
- Quotas can rebuild stocks but depend on accurate stock assessment and are undermined by illegal or unreported catch.
- Size limits protect breeders but allow continued pressure on the legal-size population.
- Gear rules and BRDs reduce bycatch and habitat harm but can lower catch efficiency, so fishers may resist them.
- Closed areas protect spawning and let stocks recover but shift effort elsewhere.
The strongest fisheries management combines several tools, backs them with good science, and depends on compliance and enforcement. Australia is often cited as having relatively well-managed fisheries, but global overfishing of many shared stocks shows how hard sustainable management is where rules are weak.
Australian context
The Queensland east coast trawl and prawn fisheries, requiring TEDs and BRDs, are the standout examples for bycatch management. Australian fisheries also use quotas, size limits and seasonal and area closures, and parts of fisheries operate inside the Great Barrier Reef Marine Park zoning system, tying this dot point to MPAs.
- Confusing overfishing with pollution
- Overfishing is removing fish faster than they breed; it is a separate pressure from pollution, though both stress stocks.
- Forgetting big breeders
- Removing the largest fish cuts reproduction sharply because large females produce far more eggs.
- Treating one tool as enough
- Effective management combines quotas, size and gear limits, closures and monitoring.
- Describing tools without evaluating
- QCAA wants the strengths, limits and the role of enforcement, not just a list.
Exam-style questions
Questions in the style of QCAA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice2 marksDescribe an aspect of fisheries management that applies the precautionary principle.Show worked answer →
For 2 marks, name a management measure and show how it acts cautiously under uncertainty.
The precautionary principle says that where there is uncertainty about a stock, managers should err on the side of caution to avoid serious or irreversible harm, rather than wait for full proof of decline.
Example aspect. Setting a conservative total allowable catch (TAC) or quota below the estimated maximum sustainable yield. When data on stock size are uncertain, the quota is deliberately set low so that fishing does not accidentally overexploit the population. (Other valid answers: closing a fishery or area when stock data are incomplete, or using maximum economic yield, which leaves a larger safety margin.) The key idea is acting to protect the stock despite incomplete information.
Practice3 marksA table shows southern bluefin tuna survey data. 2005: harvest 789.6 tonnes (53 060 fish), 11 070 released, release rate 21%, total allowable catch 5120 tonnes. 2015: harvest 742.1 tonnes (41 623 fish), 15 016 released, release rate 36%, total allowable catch 5665 tonnes. Contrast the data from 2005 and 2015 to draw a conclusion about changes in the sustainability of the fishery.Show worked answer →
For 3 marks, contrast the figures (1-2 marks) and reach a supported conclusion (1 mark).
Contrast the data. From 2005 to 2015 the harvest fell from 789.6 to 742.1 tonnes and the number of fish caught fell from 53 060 to 41 623. Over the same period the release rate rose from 21 to 36 per cent, and the total allowable catch rose slightly (5120 to 5665 tonnes), so the actual harvest stayed well below the TAC.
Conclusion. The fishery appears to be managed more sustainably in 2015: fishers are taking fewer fish than allowed and releasing a much larger proportion, reducing fishing pressure on the stock and allowing more fish to survive and breed. (A cautious answer notes the lower number of fish caught could also reflect a smaller stock, so the rising release rate is the clearest sign of improved sustainability.)