Maximum sustainable yield and fisheries: WACE Year 12 Earth and Environmental Science
“Explain maximum sustainable yield and apply it to managing fisheries and other renewable resources”
Maximum sustainable yield is the largest harvest that can be taken from a renewable resource repeatedly without reducing its ability to replenish. For a population, growth is fastest at an intermediate size, so harvesting at that point gives the highest sustainable catch. Taking more than the yield, or fishing a stock too low, slows recovery and can cause collapse. Managers use quotas, size limits and closures to keep harvest at or below the sustainable yield.
What this dot point is asking
SCSA wants you to explain maximum sustainable yield, why it occurs at an intermediate population size, and how it guides management of renewable biological resources such as fisheries. This is the quantitative core of sustainable management, so be precise about the population logic.
Population growth and yield
A harvested population grows at a rate that depends on its size.
- A very small population grows slowly because there are few breeding individuals.
- A population near its environmental carrying capacity grows slowly because resources limit it.
- Growth is fastest at an intermediate size, where there are enough breeders but resources are not yet limiting.
Because the surplus you can harvest each year equals the population's growth, the largest sustainable harvest, the maximum sustainable yield, comes from holding the population at the intermediate size where growth peaks.
Maximum sustainable yield occurs at an intermediate population size, often around half the carrying capacity, because that is where the population grows fastest and so produces the largest harvestable surplus each year. Harvesting either a near-empty or a near-full population yields less.
Overexploitation and collapse
Problems arise when harvest exceeds the sustainable yield or when the stock is driven too low.
- Taking more than the annual growth shrinks the population year on year.
- Once the population falls below the fast-growth size, it grows even more slowly, so recovery becomes harder.
- Continued overharvesting can cause a stock collapse, where the population crashes and may not recover, taking the fishery and dependent ecosystems with it.
Maximum sustainable yield (MSY): the largest harvest sustainable indefinitely. Carrying capacity: the maximum population an environment can support. Overexploitation: harvesting faster than a resource can replenish. Stock collapse: a sudden, often irreversible crash in a harvested population. Quota: a legal limit on how much can be harvested.
Managing a fishery
Fisheries management aims to keep harvest at or safely below the sustainable yield despite uncertainty about stock size.
- Catch quotas cap the total allowable catch.
- Size and bag limits protect juveniles so they can breed before being caught.
- Closed seasons and areas protect spawning and let stocks recover.
- Monitoring of catch, effort and stock size feeds back into setting the next quota.
Western Australia's western rock lobster fishery is a well-known example of a managed renewable resource, using catch controls and stock monitoring, and it has been recognised for sustainable management.
Limits of the concept
Maximum sustainable yield is a useful target but a simplification. Real populations fluctuate with the environment, stock size is hard to measure accurately, and fishing exactly at the yield leaves little margin for error. Managers therefore often aim below the theoretical maximum to reduce the risk of collapse.
A curve plots population growth (harvestable surplus) against population size, peaking at an intermediate size. The peak is the maximum sustainable yield, and the population size beneath it is the target stock. Explain that harvesting at the peak gives the most catch sustainably, while harvesting beyond it shrinks the stock and risks collapse.
Beyond single-stock thinking
Real fisheries do not exist in isolation, and a modern evaluation of maximum sustainable yield must acknowledge this. Targeting one species at its MSY can still damage the wider ecosystem: removing a predator or a key prey species shifts the food web, and fishing gear catches non-target species (bycatch) and can damage habitat such as the sea floor. Environmental variability also moves the goalposts, because a run of poor recruitment years lowers the surplus the stock can produce, so a quota set in good years becomes an overharvest. This is why management increasingly uses an ecosystem-based approach and harvest strategies with reference points set below MSY, treating the single-species yield curve as a starting point rather than the final answer.
- Saying MSY is at maximum population
- It is at an intermediate size, where growth is fastest.
- Confusing yield with population
- Yield is the harvestable surplus (growth), not the stock size.
- Ignoring collapse risk
- Overharvesting below the fast-growth size can crash a stock irreversibly.
- Treating MSY as exact
- Stock estimates are uncertain, so managers usually aim below the theoretical maximum.
Exam-style questions
Questions in the style of SCSA 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.
WACE 2022-style6 marksA fish stock has a carrying capacity of 80000 tonnes. Its annual growth (harvestable surplus) is 0 tonnes at very low and at carrying-capacity biomass, and peaks at about 9000 tonnes when biomass is near 40000 tonnes. A fleet currently lands 11000 tonnes per year from a stock of 40000 tonnes. Use the data to determine whether this harvest is sustainable and predict the consequence.Show worked answer →
A 6 mark data question rewards comparing harvest with surplus and predicting the outcome.
- Compare
- The maximum sustainable yield is the peak surplus, about 9000 tonnes per year at a biomass near 40000 tonnes (half of carrying capacity). The current catch of 11000 tonnes exceeds the maximum surplus of 9000 tonnes.
- Judgement
- The harvest is not sustainable: taking 11000 tonnes when the stock can replace at most 9000 tonnes removes about 2000 tonnes more than annual growth, so biomass falls.
- Consequence
- As biomass drops below 40000 tonnes, the surplus also falls (left of the peak), so each year the gap between catch and replacement widens. Continued overharvesting drives the stock toward collapse unless the catch is cut below the surplus.
Markers reward identifying MSY as the peak surplus, the catch-exceeds-surplus comparison, and the runaway decline toward collapse.
WACE 2020-style7 marksExplain why maximum sustainable yield occurs at an intermediate population size, and evaluate the use of MSY as the basis for managing a real fishery such as the WA western rock lobster fishery.Show worked answer →
A 7 mark answer needs the population logic plus an evaluation of MSY in practice.
- Why intermediate
- Growth rate (harvestable surplus) is near zero at very low biomass (few breeders) and near carrying capacity (resources limit growth), and is greatest at an intermediate size where breeders are plentiful but resources not yet limiting. The surplus you can take equals growth, so the largest sustainable catch comes from holding the stock at that intermediate, fast-growth size, often around half carrying capacity.
- Evaluation
- MSY gives a clear, quantitative target and underpins quotas, size limits and closures (as used to manage WA western rock lobster, a fishery recognised for sustainability). But it is a simplification: stock size is hard to measure, populations fluctuate with the environment, and fishing exactly at MSY leaves no safety margin. Managers therefore usually aim below MSY and adjust quotas using monitoring data.
- Judgement
- MSY is a useful guide but should be applied conservatively with ongoing monitoring, not treated as a precise, fixed catch.
Markers reward the growth-rate-versus-size reasoning and a balanced evaluation with a precautionary judgement.