A fish stock is renewable right up until it isn’t. Take fish faster than they can breed and the population does not decline gently — it can drop off a cliff and stay there for decades. The Grand Banks cod fishery is the case that taught everyone this the hard way.
📘 What you need to know
Damaging methods include bottom trawling, ghost fishing, and the use of poisons and explosives.
Bycatch is the capture of non-target species, and it is a major cost of industrial fishing.
Overexploitation means harvesting faster than a population can reproduce.
Continued overexploitation can cause a fishery to collapse beyond recovery.
Maximum sustainable yield (MSY) is the largest catch that can be taken year after year without depleting the stock.
Fishing above MSY often continues because it stays profitable in the short term.
Four ways of fishing that do real damage
Bycatch and habitat destruction are separate problems. Trawling does both at once, which is why it comes up most often in exam questions.
Why bycatch matters
Non-target species — young fish, turtles, dolphins, seabirds — are caught, killed and usually discarded.
Removing juveniles is especially damaging: those fish never get to breed, so future stocks shrink.
Removing predators or prey species alters the whole food web, not just one population.
Overexploitation and collapse
Overexploitation happens when fish are harvested faster than they can reproduce. Better boats, sonar, bigger nets and freezer capacity all made this far easier from the 1960s onwards.
What makes collapse so sudden is a positive feedback loop. A smaller population has fewer mature adults, so it produces fewer young, so it becomes even more vulnerable to the same level of fishing. The decline accelerates instead of levelling out.
The definition to learn
overexploitation = harvest rate > reproduction rate
Maximum sustainable yield
The annual yield of a natural resource is the gain in biomass or energy through growth over a year. The maximum sustainable yield is the largest share of that gain we can take every year and still keep the stock intact indefinitely.
The gap between the two lines is profit. Notice it stays positive well past the peak — which explains why fleets keep fishing a stock that is already declining, unless regulation stops them.
MSY is a moving target, not a fixed number. It depends on population size, growth rate and conditions in the sea, all of which change. That uncertainty is the heart of the HL stock assessment page later on.
Case study: the Grand Banks cod
Off Newfoundland, Canada, the Grand Banks supported cod fishing for roughly five centuries. It took about thirty years of industrial fishing to end it.
1970s to early 1980s. Catches stayed high even as the cod population fell. High catch was masking a shrinking stock.
Early 1980s. Scientists warned about overfishing. Quotas were set too high and not based on the scientific advice, and enforcement was weak.
Late 1980s. Stocks were severely depleted, and the decline accelerated.
1992. The Canadian government imposed a moratorium, closing the fishery to let cod recover.
Consequences. Around 40 000 jobs lost and severe economic decline in Newfoundland and Labrador communities.
Today. Only partial recovery. Decades on, cod remain far below historical levels, and the ecosystem has changed enough that full recovery may never happen.
The lesson examiners want. High catches are not evidence of a healthy stock. The Grand Banks produced peak catches while the population was already collapsing, because better technology found the last remaining fish.
Worked examples
WORKED EXAMPLE 1
Is this catch sustainable?
A fish stock has a biomass of 500 000 tonnes and grows by 8% per year. The fleet lands 55 000 tonnes a year. Assess whether the harvest is sustainable and find the change in stock.
Step 1: annual growth, which is the sustainable yield500 000 × 0.08 = 40 000 tonnesStep 2: compare with the catch55 000 − 40 000 = 15 000 tonnesStep 3: state the effect
Catch exceeds growth, so biomass falls by 15 000 tonnes in the first yearUnsustainable — the stock is being mined, not harvestedand it gets worse each year: a smaller stock grows by less, so the same catch takes an ever larger share
WORKED EXAMPLE 2
Reading the yield-effort graph [3]
Explain what happens to catch as fishing effort increases beyond the MSY point.
Point 1
Up to the peak, more effort gives more catchPoint 2
Beyond the peak, yield falls, because fish are removed faster than the stock can replenish
Point 3
Fishing continues anyway because earnings still exceed costsEffort keeps rising while the resource shrinksthis is a tragedy of the commons in miniature — good phrase to use if the question allows it
WORKED EXAMPLE 3
Explain why the cod stock collapsed rather than declining slowly [4]
Use feedback in your answer.
Link 1
Catch rate exceeded the reproduction rate for years, so biomass fell
Link 2
Fewer mature adults means lower reproductive potential, so recruitment of young fish falls
Link 3
The same fishing pressure now removes a much larger share of a smaller stock
Link 4
This is positive feedback: each decline speeds up the next
Decline accelerates into collapseadd that improved technology hid the problem, because catch stayed high while the stock fell
💡 Exam tip
Define MSY precisely: the largest catch that can be maintained indefinitely without depleting the stock.
Use the word bycatch and give examples of what gets caught.
Name the positive feedback in any collapse question. It is the difference between a good and a top answer.
Have the Grand Banks facts ready: 1992 moratorium, about 40 000 jobs, only partial recovery.
