Fishery collapse at Higher Level: The same fishery as SL, the Grand Banks cod, because it is the case the guide names, but told through the science of why the decline was missed, and a different second case: Peru's anchoveta. At HL, link the collapse to stock assessment (4.3.12), the risks of MSY (4.3.13) and recovery (4.3.14).
Practise this as you read
- Explain how overexploitation led to a collapse, step by step.
- Explain why a collapsed stock may not recover.
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When a fishery collapses: A fishery collapse happens when fish are caught faster than they can breed, year after year, until too few are left to rebuild the stock.
The points to remember
- Fish are renewable, but only if the catch is no more than the natural increase.
- Catch more, and the stock shrinks: fewer adults, so fewer young the next year.
- Warning signs: smaller fish, lower catch for the same effort, boats fishing further and deeper.
- Past a tipping point, the stock cannot recover even when fishing stops: collapse.
- World catches rose until about 1990, then levelled off at about 90 million tonnes.
Why world catches rose
- More people and more demand for fish.
- Better technology: sonar, bigger boats and nets.
- Fleets moved to new grounds and deeper water.
Why they levelled off
- Many stocks overfished and depleted.
- No new grounds left to fish.
- Quotas and rules; aquaculture took some demand.
- Climate change and pollution.
Renewable is not the same as unlimited: Overfishing can push a renewable stock so low that it cannot recover, so it behaves like a non-renewable resource. That is what a collapse is.
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The Grand Banks were one of the richest fishing grounds on Earth. People fished their cod for almost 500 years before the stock collapsed.
Up to the 1950s
Catches of about 200 000-300 000 tonnes a year, mostly by small boats near the coast.
1960s: the boom
Factory trawlers from Europe and the USSR fish all year with sonar; the catch peaks at 810 000 tonnes in 1968.
1977: the 200-mile limit
Canada pushes foreign fleets out, then builds up its own fleet; catches recover to about 250 000 tonnes.
2 July 1992: the moratorium
The stock has crashed; Canada bans cod fishing. About 35 000 people lose their jobs.
Reading the graph (HL). The 1968 peak of 810 000 tonnes was more than three times the catch of the 1950s. By 1992 the adult cod were about 1% of their early-1960s numbers, according to Myers and colleagues, and thirty years later the catch was still under 2% of the 1968 peak.
Scientists who studied the collapse, led by Ransom Myers, concluded that it was caused mainly by overfishing, not by changes in the sea.
The points to remember
- Too many boats, too efficient: factory trawlers, sonar and huge nets caught the shoals.
- Quotas set too high: scientists overestimated how many cod there were.
- Trawlers kept making big catches because cod gathered in dense shoals, so the stock looked healthy.
- Small young cod were caught and thrown away, so fewer grew up to breed.
- Politics: thousands of jobs depended on the catch, so cuts came too late.
- Cold water in the early 1990s may have added stress, but was not the main cause.
Remember it as: Too many boats, too high quotas, too late.
Example (HL): why the science missed it. Stock estimates relied partly on the catch per hour of the offshore trawlers. Because the last cod crowded into dense winter shoals that sonar found easily, catch rates stayed high while the stock shrank: the data hid the decline. Quotas set from those estimates let fishing take two to three times the intended share of the stock.
Name the cause: overexploitation: 'The cod died out' or 'the water got colder' does not explain a collapse. Say that catches were greater than the stock's natural increase for many years, and give the reasons why.
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A collapse is lasting: thirty years after the moratorium, the northern cod were still far below their old numbers. The costs reached far beyond the fish.
The points to remember
- Cod breed only at 6-7 years old, and the big old females, which lay the most eggs, were gone.
- With so few adults, too few young are produced to rebuild the stock.
- The food web changed: their food (capelin) crashed; shrimp and snow crab boomed.
- Jobs: about 35 000 people lost work; whole villages emptied as young people left.
- Food: a cheap source of protein disappeared from local tables.
- In 2024 Canada reopened a small fishery: 18 000 tonnes, about 2% of the 1968 catch.
Example (HL): a new food web. With the cod gone, the shrimp and snow crab they used to eat multiplied, and by the 2000s the shellfish catch was worth more than the cod catch had been. Some scientists think the new web, with many more harp seals too, now keeps the cod from returning.
Removing a top predator: When a top predator such as cod is fished out, its prey increase and the whole food web changes: a trophic cascade. The ecosystem may settle into a new state that does not go back.
Not every crash is a lasting collapse. Some stocks recover when fishing stops; others crash because of nature and fishing together. Reading a catch graph means linking each rise and fall to a cause.
Reasons a catch rises
- More boats at sea.
- Better technology.
- New fishing grounds.
- The stock recovers after a closure.
- Higher quotas or more demand.
Reasons a catch falls
- Overfishing: fewer fish left.
- Stricter quotas, fewer boats, bigger mesh.
- More predators or less prey.
- Less demand in the markets.
- Climate change or El Niño moves or kills fish.
Example (HL): Peru's anchoveta, 1972. The catch reached about 12 million tonnes in 1970, the largest of any fishery. In 1972 an El Niño cut the food supply while fishing went on, and the catch fell below 2 million tonnes by 1973. The stock recovered only in the 1990s.
Link every reason to a rise or a fall: 'Overfishing' alone is not enough: say that overfishing reduced the stock, so the catch fell. Each reason earns a mark only when it is tied to the direction of the change.
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How this comes up: Paper 2 Section B can ask you to explain how overexploitation led to the collapse of a named fishery [7]; Paper 1 gives a catch graph and asks for factors behind the changes [2].
Many fish stocks have been harvested faster than they can replace themselves.
Explain how overexploitation led to the collapse of a named fishery.
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