AP Environmental Science

## Learning Outcomes

- Explain how overfishing lowers fish populations by removing fish faster than they can be replaced
- Distinguish between **overfishing** and **overfished**, a common AP Environmental Science exam trap
- Understand how stock biomass, breeding adults, and recruitment are linked
- Describe how harvesting above **maximum sustainable yield (MSY)** causes long-term decline
- Interpret simple fishery trends and use data to support an environmental claim
- Identify ecological consequences beyond target-species decline, including bycatch and food-web disruption
- Evaluate management strategies that reduce overfishing and support recovery

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand how overfishing reduces fish populations, with a focus on the following syllabus points:

- Know that overfishing means harvesting fish faster than populations can replenish themselves
- Identify the difference between **overfishing** as an excessive harvest rate and **overfished** as a depleted stock condition
- Explain how loss of breeding adults lowers reproduction and recruitment
- Understand that fish populations do not increase without limit and that sustainable harvest depends on replacement rate
- Recognise **maximum sustainable yield (MSY)** as the largest long-term average catch that can be maintained
- Describe how bycatch and destructive fishing methods worsen population decline
- Use evidence that biologically sustainable marine stocks have declined over time
- Identify management strategies such as catch limits, marine sanctuaries, and reduced subsidies

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between **overfishing** and **overfished**?
2. How does removing too many breeding adults reduce future fish populations?
3. Why is **maximum sustainable yield** not the same as the biggest possible catch in one year?
4. How can bycatch reduce fish populations even when those species are not the fishing target?
5. Name two management strategies that can reduce overfishing.

## Introduction

Overfishing is one of the clearest examples of unsustainable resource use in AP Environmental Science. Fish are a major food source for humans, but fish populations cannot keep supplying food if they are harvested faster than they can reproduce. When this happens, stock size falls, recovery becomes harder, and the entire fishery is put at risk.

This topic matters because AP questions often move beyond a simple definition. You may be asked to explain a cause-and-effect chain: too much harvest removes breeding adults, breeding declines, fewer young fish enter the population, biomass falls, and long-term yield drops. Strong answers use this chain clearly.

> **Key Term: overfishing**
> Catching fish at a rate faster than the population can replace itself.

### How overfishing actually reduces fish populations

Fish populations are not fixed. They change through births, growth, immigration, emigration, natural deaths, and harvesting. Overfishing reduces fish populations when the rate of removal by humans is greater than the population’s ability to replace those lost individuals.

A simple way to think about it is this: if a fish population adds 1,000 fish through reproduction and survival in a year, but fishing removes 1,500, the population shrinks. If that happens repeatedly, the stock declines further each year.

> **Key Term: fish stock**
> A population of a fish species that is managed or assessed as a unit.
>
> **Key Term: stock biomass**
> The total biological mass of a fish stock, often used to estimate the size and health of the population.

This decline is especially serious because harvesting does not remove fish randomly from an unlimited pool. Fishing often removes larger and older individuals, which are commonly important breeders. That means the population loses not just numbers, but also reproductive capacity.

The APES-level cause chain is:

- too many fish are harvested
- the number of adult breeders drops
- fewer eggs are produced
- fewer juveniles survive to join the population
- stock biomass decreases
- future catches become smaller and less sustainable

This is why overfishing is not just “people catch a lot of fish.” It is specifically about crossing the line where harvest exceeds replacement.

A second reason populations fall is that many marine systems depend on limited productivity. Most ocean surface waters are low in nutrients, and primary production is often restricted. Since fish ultimately depend on energy entering the food web through phytoplankton and other producers, fisheries do not have unlimited growth potential. Human harvest can exceed that limit.

> **Test Tip:** When an FRQ asks how overfishing reduces fish populations, do not stop at “too many fish are caught.” Add at least one extra step about breeding adults, reproduction, or recruitment to earn fuller credit.

### The crucial distinction: overfishing vs overfished

Students often lose marks by mixing up two related but different terms.

> **Key Term: overfished**
> A stock whose population size or biomass is already too low to support sustainable harvest at the target level.

**Overfishing** describes what is happening now: the harvest rate is too high.

**Overfished** describes the condition of the stock: the population is already depleted.

A stock can be:

- experiencing **overfishing** now, but not yet **overfished**
- **overfished**, but no longer experiencing **overfishing** if managers reduced catches
- both **overfishing** and **overfished**
- neither, if managed sustainably

This distinction matters because it separates cause from condition.

Example:

- If a fishery has recently expanded and catches suddenly exceed replacement, that stock may be undergoing **overfishing** even if biomass is still moderate.
- If a stock collapsed years ago and managers have already cut fishing sharply, the stock may still be **overfished** because biomass remains too low, even though **overfishing** is no longer occurring.

That distinction is a favorite exam angle because it tests understanding, not memorisation.

> **Key Term: recruitment**
> The addition of young fish into a population when they survive to an age or size that matters for the stock.

Recruitment is central here. When too many adult fish are removed, especially spawning adults, recruitment usually falls. Fewer young fish survive to join the breeding or catchable population. This creates a delayed effect: even if fishing pressure drops later, recovery may still take years or decades because the stock has too few breeders left.

> **Test Tip:** If a multiple-choice option says “overfished means too much fishing is happening right now,” it is wrong. That defines overfishing, not overfished.

### Why MSY matters and why exceeding it causes decline

AP Environmental Science expects you to know the idea of **maximum sustainable yield**, usually shortened to MSY.

> **Key Term: maximum sustainable yield (MSY)**
> The largest long-term average catch that can be taken from a fish stock under existing environmental conditions.

MSY is about long-term sustainability, not the biggest catch humans can take in a single year. That difference is essential.

Imagine a fish population follows a broad logistic pattern:

- when the population is very small, there are too few breeders, so growth is slow
- when the population is near carrying capacity, resources limit further growth, so growth is again slow
- at an intermediate population size, growth is highest

That middle zone is where the stock can replace harvested fish most effectively. MSY is based on harvesting around that level, not above it.

If fishing goes above MSY for too long:

1. harvest exceeds replacement
2. stock biomass begins to fall
3. the breeding population shrinks
4. recruitment weakens
5. future sustainable yield gets smaller
6. the stock may become overfished

This is why catching “as much as possible” is not rational in the long run. Short-term gain can reduce long-term yield.

Some courses use a simplified logistic model:

- population growth is greatest at about half the carrying capacity
- in that simplified model, MSY occurs around that intermediate level

You do not usually need to calculate MSY on the APES exam for this topic, but you do need the logic behind it.

A common mistake is thinking MSY means “safe maximum.” In real fisheries, environmental conditions change, food webs shift, and scientists estimate stock size with uncertainty. If MSY is overestimated, managers may set catches too high and accelerate decline.

> **Key Term: biologically sustainable stock**
> A fish stock with abundance high enough to maintain long-term productivity and sustainable harvest.

Global evidence supports the concern. The share of biologically sustainable marine fish stocks has fallen markedly since the 1970s. That trend shows that overfishing is not a small local problem but a global pattern of depletion.

### Other pressures that make stock decline worse

Overfishing reduces target populations directly, but in many fisheries the impact is broader than the target species alone.

One major issue is **bycatch**.

> **Key Term: bycatch**
> Non-target organisms caught during fishing operations.

Bycatch can include juvenile fish, seabirds, turtles, sharks, marine mammals, and noncommercial fish. This matters for two reasons.

First, bycatch may directly reduce populations of species that were not meant to be harvested. If juveniles are removed before reproducing, recovery becomes harder.

Second, bycatch wastes biological productivity. If organisms are caught and discarded dead, the food web loses those individuals without any intended food benefit.

Fishing methods linked to high bycatch include:

- drift nets
- long lines
- trawling

Some fisheries use devices to reduce this damage, such as Turtle Excluder Devices in shrimp trawling, but bycatch remains a major problem.

Another issue is habitat damage. Bottom trawling can scrape seafloor habitats, damaging coral communities, seamount marine lifes, and nursery areas. That means overfishing is not only removing fish; it may also destroy the habitats fish need for breeding, feeding, or juvenile survival.

Food-web disruption is another consequence. If a major predator is overfished, its prey population may increase sharply. If a major prey species is overharvested, predators may decline. Either way, the result can be a trophic imbalance.

A classic APES way to explain this is:

- overharvest a key species
- change in food availability or predation pressure
- shift in population sizes at other trophic levels
- altered biodiversity and ecological stability

This is why healthy fisheries are also an environmental issue, not just a food supply issue.

### Management strategies and why they work

Because overfishing is caused by harvest rates that exceed replacement, management must reduce fishing pressure or improve stock recovery.

A major strategy is catch limits based on stock assessment.

> **Key Term: stock assessment**
> A scientific evaluation of a fish population’s size, status, and sustainable harvest level.

If managers know stock biomass and recruitment trends, they can set annual catch limits below harmful levels. Some systems also use an **overfishing limit (OFL)**, meaning a catch level above which overfishing is likely.

Another strategy is reducing effort by limiting:

- licenses
- number of boats
- fishing days
- gear types
- harvest seasons

Marine sanctuaries or marine protected areas can also help by creating spaces where populations can recover with less direct harvest. These areas matter most when they protect breeding grounds, nursery habitats, or key feeding areas.

Governments can also remove harmful subsidies. If subsidies keep too many fishing boats operating, they increase pressure on already stressed stocks. Removing those supports can reduce excess harvest capacity.

Import controls and labelling also matter. If consumers and governments support sustainably caught fish and restrict products from unsustainable fisheries, markets can reward better practices.

Freshwater systems need different but linked restoration actions. If overfished freshwater food webs are also stressed by erosion, invasive species, blocked migration routes, and wetland loss, recovery depends on reducing those pressures too.

Management does not guarantee quick recovery. Some stocks rebuild slowly because depleted breeding populations, changed age structure, altered food webs, and climate conditions all affect recovery rates.

### Case study logic: Atlantic cod and fishery collapse

A well-known fishery example is Atlantic cod in the Northwest Atlantic. For decades, cod were heavily exploited. Improved technology, industrial fishing, and insufficient limits kept harvest pressure too high.

The basic case-study chain is:

- chronic high fishing mortality
- declining biomass
- reduced spawning stock
- weaker recruitment
- severe depletion
- fishery collapse or moratorium

In Canadian waters off Newfoundland and Labrador, the northern cod moratorium in **1992** became one of the clearest examples of what happens when overfishing persists for too long.

This case matters because it shows three exam-relevant points:

- abundant stocks can still collapse
- reducing fishing late is less effective than preventing decline early
- recovery may take decades

The cod case also shows why “just stop fishing and it will bounce back quickly” is too simple. Environmental conditions, prey supply, predator relationships, and reduced breeding biomass can all slow recovery.

If an FRQ asks for one consequence of overfishing, “fishery collapse” is a valid answer if you explain it properly: the stock falls so low that catches are no longer sustainable or economically viable.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Overfishing means fish are harvested faster than the population can replace itself
- Fish populations decline when harvest exceeds reproduction and survival
- Stock biomass is the total biological mass of a fish stock
- Removing too many breeding adults lowers reproductive capacity
- Reduced spawning leads to weaker recruitment of young fish into the population
- Overfishing describes an excessive harvest rate, while overfished describes a depleted stock condition
- Maximum sustainable yield is the largest long-term average catch, not the biggest short-term catch
- Harvesting above MSY for too long causes biomass decline and lowers future yield
- Bycatch reduces non-target populations and can remove juveniles before reproduction
- Destructive methods such as bottom trawling can damage habitats and nursery areas
- Overfishing can disrupt food webs and reduce biodiversity
- Management strategies include catch limits, stock assessments, marine sanctuaries, gear controls, and reduced subsidies

## Key Terms and Concepts

- overfishing
- fish stock
- stock biomass
- overfished
- recruitment
- maximum sustainable yield (MSY)
- biologically sustainable stock
- bycatch
- stock assessment


Impacts of Overfishing - Overfishing reduces fish populations