IB ESS SLTopic 4 — Aquatic Food Production SystemsPaper 1 & 2Core idea~11 min read
Overfishing and Damaging Harvest Methods
A fish stock is a renewable resource, but only if you take less than it grows back. Push past that line and the resource stops renewing. The Grand Banks cod fishery survived five centuries of fishing and then collapsed in about thirty years — and it has never fully recovered.
📚 What you need to know
Rising demand has driven unsustainable harvesting practices that damage ecosystems as well as stocks.
Bottom trawling drags heavy nets over the seabed: habitat destruction, bycatch and sediment disturbance.
Ghost fishing is lost or abandoned gear that keeps catching and entangling marine life.
Poisons such as cyanide and explosives such as dynamite kill indiscriminately and wreck coral reefs. Both are illegal in many places.
Overexploitation is harvesting faster than a population can reproduce, which can end in fishery collapse.
Annual yield is the yearly gain in biomass; maximum sustainable yield (MSY) is the largest catch that can be taken every year without reducing the resource’s long-term productivity.
Fishing above MSY is often still profitable in the short term, which is why it keeps happening.
Four damaging practices
Bottom trawling
Heavy nets are dragged along the seabed, catching everything in their path.
Destroys habitats such as coral reefs and other seabed structures.
Produces significant bycatch — non-target species caught and usually discarded dead.
Disturbs sediment, causing sediment pollution and releasing pollutants that were trapped in it.
Bycatch is the part students forget to mention. Everything the net meets is caught, and non-target species are usually thrown back dead — so the ecological cost is far larger than the landed catch suggests.
Ghost fishing
Happens when fishing gear is lost or abandoned but keeps working — ghost nets are the classic example.
Continues catching fish and other marine animals, causing deaths that serve no purpose at all.
Entangles marine organisms, including endangered species such as turtles and seals.
Adds to marine debris and plastic pollution, since most modern gear is synthetic and does not break down.
Poisons
Substances such as cyanide are used to stun or kill fish so they are easier to collect, often for the live fish trade.
The poison harms a wide range of marine life, not only the target.
Cyanide kills the coral polyps that build the reef itself, causing reef degradation and long-term biodiversity loss.
Highly unsustainable and illegal in many countries.
Explosives
Dynamite and similar explosives stun or kill fish over a wide area so they can be scooped up.
They kill indiscriminately, including non-target species and juveniles.
They shatter coral reefs and other structural habitats, which take decades to regrow if they recover at all.
Also illegal in many places, and heavily damaging to the communities that rely on those reefs.
Notice that poisons and explosives are both short-term wins with long-term losses: they destroy the habitat that produces the next generation of fish. It is the tipping-point idea from Topic 2, applied underwater.
Overexploitation and maximum sustainable yield
Better boats, sonar, bigger nets and freezer holds have all raised how much can be taken. Overexploitation happens when fish are harvested faster than they can reproduce, and if it continues the population can fall so low that the fishery collapses and cannot recover.
Two definitions to keep separateannual yield = the yearly gain in biomass or energy through growth maximum sustainable yield = the largest amount that can be harvested each year without reducing long-term productivity
MSY is calculated from population size, growth rate and reproduction rate, and it is what quotas are supposed to be based on. Harvest at MSY and the catch can, in principle, continue indefinitely. Harvest above it and the stock shrinks year on year.
The gap between the blue peak and the red crossing point is the whole problem. Between those two points the catch is falling and the stock is being destroyed, but each individual boat is still making money — so nobody stops voluntarily.
Case study: cod on the Grand Banks
The Grand Banks, southeast of Newfoundland in Canada, were among the richest fishing grounds on Earth. Cod had been fished there since the fifteenth century.
🧩 How a five-century fishery collapsed in thirty years
1970s to 1980s. Catches stayed at peak levels even as the underlying cod population was falling.
Early 1980s. Scientists warned that the stock was being overfished. The warnings were not acted on.
Late 1980s. Cod stocks were severely depleted.
1992. The Canadian government imposed a moratorium, closing the fishery to allow recovery.
Why it happened: overfishing driven by high demand and advanced technology, combined with inadequate management — quotas were set too high and were not based on scientific advice, and enforcement was poor.
What it cost: around 40 000 jobs were lost in the fishing industry, and communities across Newfoundland and Labrador went into severe economic decline. Ecologically, the cod population crashed and recovery has been slow.
Where it stands now: cod numbers have improved somewhat, but decades later they remain far below historic levels. The ecosystem itself has changed — other species moved into the space cod used to occupy — so full recovery may take many more years, or may never happen. Management today is adaptive, trying to balance ecological recovery against the economic needs of fishing communities.
The transferable lesson. Collapse did not come from a lack of knowledge — the warnings existed. It came from setting quotas politically rather than scientifically, and from a system where stopping fishing was costly today while the collapse was a problem for later.
Worked examples
EXAM Q1
Define maximum sustainable yield and explain why exceeding it is a problem. [3]
Step 1: definition
The largest catch that can be taken each year without reducing the long-term productivity of the stock.
Step 2: what exceeding it does
Fish are removed faster than they reproduce, so the breeding population shrinks.
Step 3: the consequence
Yields fall in later years and the fishery may collapse beyond recovery.
Above MSY the resource stops being renewable“without compromising long-term productivity” is the phrase worth memorising
EXAM Q2
Explain two ecological impacts of bottom trawling. [4]
Impact 1: habitat destruction
Heavy gear flattens seabed habitats such as reefs, removing shelter and nursery grounds, so biodiversity falls.
Impact 2: bycatch
Non-target species are caught and usually discarded dead, including juveniles and protected species, which disrupts food webs.
The method removes the habitat as well as the catchsediment disturbance and pollutant release is a valid third impact
EXAM Q3
Using the Grand Banks, explain why fisheries collapse even when the risk is known. [4]
Point 1: it stays profitable
Catch falls past MSY but fishing remains commercially viable, so effort does not drop.
Point 2: management failure
Quotas were set too high and not based on scientific advice, and enforcement was weak.
Point 3: short-term pressure
Jobs and income depend on fishing now, so restrictions are politically difficult.
Point 4: the outcome
By 1992 the stock had collapsed; about 40 000 jobs went anyway, and recovery is still incomplete.
Delaying the cost did not avoid it — it made it largerthe irony is the strongest evaluation point available here
💡 Exam tip
Learn the four damaging practices by name with one impact each. That structure answers most questions here.
Always mention bycatch when discussing trawling or gillnets.
Define MSY with the phrase long-term productivity, and mention that quotas should be based on it.
For the cod case study, remember three things: the 1992 moratorium, about 40 000 jobs, and incomplete recovery.
Say collapse happens because fishing stays profitable past the sustainable point — it explains the behaviour.
Link back to resilience: a collapsed fishery can settle into a new stable state with different species.
⚠ Common mix-up
Confusing annual yield with maximum sustainable yield. One is what the stock grows; the other is what you can safely take.
Thinking a collapsed fishery bounces back once fishing stops. The Grand Banks shows it may not.
Mixing up trawling and trolling. Trawling drags nets; trolling uses hook and line and is far less damaging.
Ignoring the economics. Overfishing is a rational short-term decision for each boat, which is why regulation is needed.
Treating bycatch as a minor issue. For some fisheries it exceeds the target catch.
Assuming MSY is easy to calculate. It depends on estimates of population size and growth that are often uncertain.
Up next: Protecting Fish Stocks — the tools that work, from mesh size to marine protected areas, and who has to use them.
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