AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/5/5-8)
Unit 5 · Topic 5.8
5.8 Impacts of Overfishing
Overfishing means catching fish faster than their populations can replace themselves. It has collapsed some fisheries, reduced ocean biodiversity and hurt people who depend on fish, while methods like bottom trawling and drift nets also kill unwanted species and damage habitat.
Key terms
- overfishing
- fishery collapse
- bycatch
- bottom trawling
What overfishing is
A fishery is a population of fish (or other seafood) that's harvested, along with the people and boats that harvest it. A fishery is sustainable when the catch is no larger than the number of fish the population adds each year through reproduction and growth. Overfishing happens when the catch is larger, so the population shrinks year after year.
Modern fleets can find and catch fish with sonar, GPS and huge nets, so a population can be overfished quickly. The UN Food and Agriculture Organization estimates that more than a third of the world's assessed fish stocks are overfished.
Fishery collapse
When a population is overfished to a small fraction of its former size, it may collapse, meaning it's too small to support commercial fishing. Some collapsed fisheries recover slowly or not at all, especially for long-lived fish that reproduce late.
The most famous example is the Atlantic cod off Newfoundland, Canada. After decades of heavy fishing, the cod population crashed, and Canada closed the fishery in 1992. Tens of thousands of people lost their jobs, and the cod population has still not fully recovered decades later.
As large predatory fish are fished out, fleets turn to smaller species lower in the food web, a pattern called ‘fishing down the food web’. This shifts the whole ecosystem.
Harmful fishing methods
Bycatch is the unintended catch of non-target species, which are usually thrown back dead or dying. Bycatch can include endangered species and juvenile fish that never get the chance to reproduce.
| Method | How it works | Main problem |
|---|---|---|
| Bottom trawling | Heavy nets are dragged along the seafloor | Destroys seafloor habitats like coral and sponge beds; high bycatch |
| Drift nets and gillnets | Long walls of net hang in the water | Entangle dolphins, sea turtles, seabirds and sharks |
| Longlines | Lines miles long with thousands of baited hooks | Hook sea turtles, sharks and albatrosses |
| Purse seines | A net encircles a whole school of fish | Can catch dolphins and young fish along with the target |
Effects and solutions
Overfishing reduces biodiversity, disrupts food webs (removing predators or prey) and harms people who depend on fish for protein and income, especially in coastal communities in lower-income countries.
Solutions include science-based catch limits (quotas), assigning catch shares to boats, seasonal closures during spawning, minimum size limits so young fish can reproduce, gear changes like turtle excluder devices (TEDs) in shrimp nets, banning destructive methods in sensitive areas, and marine protected areas where fishing is limited or banned so populations can recover. Consumers can also choose seafood certified as sustainably caught.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
Calculating a decline
A hypothetical fishery's annual catch was 800,000 metric tons in 1980, 450,000 in 1990 and 20,000 in 1995. Calculate the percent decline from 1980 to 1995 and suggest what the data indicate.
Show the solutionHide the solution
- Step 1: Percent change = (new − old) ÷ old × 100 = (20,000 − 800,000) ÷ 800,000 × 100.
- Step 2: −780,000 ÷ 800,000 = −0.975, so −97.5%.
- Step 3: Unless fishing effort also fell sharply, a drop of almost 98% in the catch suggests the population collapsed from overfishing.
Answer: A 97.5% decline, indicating a fishery collapse.
- Example 2
Reducing bycatch
A shrimp trawl fishery catches many endangered sea turtles as bycatch. Propose a solution and explain one tradeoff.
Show the solutionHide the solution
- Step 1: Solution: require turtle excluder devices (TEDs), grates in trawl nets with an escape hatch that lets turtles out while shrimp pass through to the back of the net.
- Step 2: How it helps: turtles escape before drowning, so turtle deaths drop sharply.
- Step 3: Tradeoff: some shrimp may also escape, slightly reducing the catch, and fishers must pay to install the devices.
Answer: Require turtle excluder devices; they let turtles escape, though they cost money and may slightly reduce the shrimp catch.
Common mistakes
- Thinking a collapsed fishery recovers as soon as fishing stops. Many recover slowly or not at all.
- Defining bycatch as the target catch. Bycatch is the non-target species caught by accident.
- Proposing only ‘stop fishing’. Give a specific management tool, like quotas, size limits or marine protected areas.
On the exam
- Expect catch data to analyze. Calculate percent change and connect a sharp decline to overfishing.
- Link overfishing to the tragedy of the commons and sustainable yield when explaining causes and solutions.
Connected topics
Videos
Check yourself
3 questions on 5.8 Impacts of Overfishing. Pick an answer to see if you got it, and why.
For centuries, the Grand Banks off Newfoundland supported a huge Atlantic cod fishery. After decades of increasingly intense industrial trawling, cod numbers collapsed, and Canada closed the fishery in 1992, putting tens of thousands of people out of work. This case best illustrates that overfishing
Some tuna fishing nets also catch dolphins, sea turtles and sharks, which are thrown back dead or injured. These unintended catches are called
Which is the most direct environmental effect of bottom trawling, in which heavy nets are dragged along the seafloor?
0 of 3 answered