AP Environmental Science

## Learning Outcomes

- Explain why aquaculture can increase food supply but still damage the environments
- Identify the main environmental drawbacks of intensive aquaculture
- Understand how nutrient pollution from aquaculture can cause eutrophication and low dissolved oxygen
- Describe how disease, parasites, and escaped fish can affect wild populations
- Recall why shrimp aquaculture is linked to mangrove loss
- Distinguish between open-net pen aquaculture and more contained systems for exam questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand aquaculture environmental drawbacks, with a focus on the following syllabus points:

- Know that **aquaculture** is the farming of aquatic organisms such as fish, shellfish, and aquatic plants
- Identify **wastewater and nutrient pollution** from uneaten feed, feces, and metabolic waste
- Explain how excess **nitrogen** and **phosphorus** from aquaculture can lead to **eutrophication** and oxygen depletion
- Understand how crowded farming conditions can increase **disease and parasite transmission**
- Know why **escaped farmed fish** can compete with, infect, or interbreed with wild populations
- Recognise **mangrove clearing for shrimp aquaculture** as a major habitat destruction example
- Distinguish **open-net pen aquaculture** from more contained systems in terms of environmental risk
- Understand that impacts vary with species, system type, siting, and management rather than being identical in all aquaculture

## 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. How can fish farming contribute to eutrophication in nearby waters?
2. Why are open-net pen systems often seen as riskier than more enclosed systems?
3. Name two ways escaped farmed fish can harm wild fish populations.
4. Why does mangrove clearing for shrimp aquaculture matter beyond the farm site itself?
5. How can disease problems in aquaculture spread into natural wild habitatss?

## Introduction

## Introduction

Aquaculture is often presented as a partial answer to overfishing. That is true to a point: farming fish, shellfish, and aquatic plants can raise food production and reduce pressure on some wild stocks. But AP Environmental Science expects you to see the trade-off clearly. A system that produces food can also create environmental externalities if wastes, chemicals, disease, or habitat destruction are not controlled.

The key exam idea is cause and effect. Do not just memorise a list of drawbacks. You need to explain how intensive aquaculture changes water chemistry, species interactions, habitats, and food webs. Most questions reward students who can link the farming practice to the ecological consequence.

> **Key Term: aquaculture**
> The farming of aquatic organisms such as fish, shellfish, and aquatic plants for harvest.

### Nutrient pollution and water-quality decline

The most common aquaculture drawback tested on APES is **nutrient pollution**. In many fish farms, especially intensive finfish systems, farmers add large amounts of feed to grow fish quickly. Not all of that feed is eaten. In addition, fish release feces and dissolved metabolic wastes into the surrounding water. These wastes add **nitrogen** and **phosphorus** to the system.

> **Key Term: nutrient pollution**
> The addition of excess nutrients, especially nitrogen and phosphorus, to water.

Why does that matter? In many aquatic systems, nitrogen and phosphorus are limiting nutrients. When extra nutrients enter the water, algae and phytoplankton can grow much faster than normal. This may sound helpful at first because producers are increasing, but the later stages are the problem. Large algal growth reduces water clarity, blocks light from reaching submerged plants, and eventually creates a large mass of dead organic matter when algae die.

Decomposers then break down this dead material. As they respire, they consume dissolved oxygen from the water. That means the original nutrient input indirectly lowers oxygen levels.

The exam chain should be stated like this:

- uneaten feed and feces enter water
- nitrogen and phosphorus increase
- algal growth increases
- algae die and are decomposed
- biological oxygen demand rises
- dissolved oxygen falls
- fish and other organisms experience stress or die
  > **Key Term: eutrophication**
  > Nutrient enrichment of water that causes excessive algal growth and often lowers dissolved oxygen.

This is closely linked to Unit 8 pollution content, so AP questions may connect aquaculture to dead zones, fish kills, or declining biodiversity. If the farm’s waste output exceeds the surrounding environment’s ability to dilute and process it, local water quality declines. This is especially likely in poorly flushed waters or enclosed areas with weak circulation.

Open-water exchange can sometimes make the problem look smaller because wastes are spread out, but that does not remove them from the environment. It simply moves them into nearby water or sediments.

A related effect is **benthic** degradation. Solids such as feces and uneaten feed sink beneath cages or pens. This organic accumulation changes sediment chemistry and can lower oxygen levels at the bottom.

> **Key Term: benthic habitat**
> The bottom sediments and bottom-dwelling community in a water body.

Benthic impacts matter because many bottom organisms recycle nutrients, support food webs, and provide habitat for other species. When seafloor conditions shift toward oxygen-poor, organic-rich sediment, species richness can drop and tolerant organisms may replace more sensitive ones.

> **Test Tip:** In FRQs, do not stop at “waste pollutes water.” Extend the chain to “waste adds nitrogen and phosphorus, causing eutrophication, which lowers dissolved oxygen and harms aquatic life.”

### Disease, parasites, and chemical inputs

Aquaculture often places large numbers of similar organisms close together. That density is economically efficient, but ecologically risky. Crowded conditions make it easier for pathogens and parasites to spread from one host to another. If one fish becomes infected in a dense farm, the infection may spread much faster than it would in a wild population where individuals are more dispersed.

> **Key Term: monoculture**
> The cultivation or farming of a single species in a uniform, low-diversity system.

This is one reason aquaculture can reduce biodiversity even before any wastes leave the farm. A dense monoculture is less resilient than a more diverse natural system. Once disease appears, the whole stock may be vulnerable.

The risks are especially high in **open-net pen aquaculture**, where fish are raised in cages that float in natural waters and freely exchange water with the outside environment.

> **Key Term: open-net pen aquaculture**
> A fish-farming system using floating cages that allow direct water exchange with surrounding waters.

Because the water moves in and out of the pens, so can pathogens and parasites. That means disease is not just a farm-management problem. It may become an environmental problem if nearby wild fish are exposed.

The APES distinction to remember is this:

- **Nutrient pollution** harms water quality
- **Disease and parasite transmission** harms organism health and wild populations

Students often blur these together. Keep them separate in your explanation.

Farmers may respond to disease risk with antibiotics or other chemical treatments. These can reduce immediate losses, but they introduce another environmental issue: chemical residues in surrounding waters or sediments. Depending on the chemical used, non-target organisms may also be affected.

Chemical use is a secondary drawback compared with nutrient waste, but it is still important. AP scoring guidance has accepted examples such as excess organic waste, antibiotics, disease spread, and escaped fish. If the question asks for multiple drawbacks, chemical contamination is a good extra point after waste pollution and disease.

A useful comparison is with CAFOs on land. Just as concentrated livestock operations create waste, pathogen, and antibiotic problems, concentrated aquatic production can do something similar in water.

One more point: not all aquaculture systems use heavy chemical inputs. Vaccination, fallowing, screening, and lower densities can reduce drug use. But the exam focus here is the environmental drawback when management is poor or the system is intensive.

> **Test Tip:** If a question asks why intensive aquaculture can threaten wild fish, mention density. High density increases disease transmission within the farm and raises the chance of spread to nearby wild populations.

### Escaped fish and impacts on wild populations

Another major drawback is the escape of farmed fish into natural waterss. This can happen when nets tear, storms damage cages, equipment fails, or fish are released accidentally. Students sometimes think escape only matters if the fish become invasive. The real issue is broader than that.

Escaped fish can cause at least three different problems.

First, they may **compete** with wild fish for food, habitat, spawning sites, or shelter. Even if they do not establish a permanent invasive population, they can still reduce resources available to native species.

Second, escaped fish may **spread disease or parasites** to wild populations. If the farm population has been exposed to pathogens amplified by dense conditions, escape creates a direct pathway into natural food webs.

Third, escaped fish may **interbreed** with wild populations, changing the gene pool. Farmed strains are often selected for fast growth, captivity tolerance, or market traits, not for survival in natural habitatss. If those traits mix into wild populations, the result may reduce long-term fitness or alter local adaptations.

> **Key Term: genetic integrity**
> The maintenance of the natural genetic makeup and local adaptations of a wild population.

This is why APES materials mention that escaped farmed fish can compete or breed with wild fish. The concern is ecological and genetic, not just numerical.

A classic exam-ready example is carp released from fish farming causing problems in U.S. freshwater systems such as the Mississippi River and the Great Lakes. Even without memorising every case, you should understand the pattern: farmed species introduced outside controlled conditions may alter food webs and ecological balance balance.

Another linked drawback is feed dependence. Some farmed fish, especially carnivorous species, are fed fishmeal and fish oil made from wild-caught forage fish. That means aquaculture does not always reduce pressure on wild fisheries.

The logic is important:

- farmed carnivorous fish need feed
- feed may come from wild fish
- harvest pressure shifts from one fishery to another
- wild forage fish are removed from marine food webs
- predators that depend on those forage fish may be affected

This does not cancel all the benefits of aquaculture, but it does weaken the claim that aquaculture automatically solves overfishing.

### Habitat destruction and why mangrove loss matters

The most important habitat-loss example for this topic is shrimp aquaculture and mangrove clearing. In many regions, shrimp ponds have been built by converting coastal mangrove habitatss into aquaculture sites.

> **Key Term: mangrove**
> A salt-tolerant coastal tree or shrub formation found in tropical and subtropical estuaries and shorelines.

This is a strong APES example because mangroves provide several major environmental services:

- nursery habitat for fish and shrimp
- shoreline stabilisation and erosion control
- storm protection from hurricanes and surges
- filtration of sediments and pollutants
- carbon storage

So when mangroves are cleared, the impact extends beyond the footprint of the farm. You do not just lose trees. You lose breeding habitat, coastal protection, water-quality functions, and carbon storage.

That is why habitat destruction must be kept distinct from water pollution. They are different mechanisms.

- **Habitat destruction:** the habitat is physically removed or converted
- **Water pollution:** wastes degrade water chemistry and ecological conditions

Both are major drawbacks, but they occur in different ways and affect different parts of the environment.

FAO summaries report that pond shrimp aquaculture has been a major driver of mangrove loss, accounting for **31%** of all mangrove loss in **2000–2010** and **21%** in **2010–2020**. You do not need to use these figures in every answer, but they are useful evidence if a question asks for specific support.

Habitat conversion also matters because coastal habitatss often contain high biodiversity and support fisheries outside the farm itself. Removing mangroves can reduce the success of nearby wild populations by removing nursery areas juveniles need to survive.

A final distinction students should know is that not all aquaculture has the same environmental footprint. Shellfish and seaweed systems generally do not create the same feed-related nutrient problems as fed finfish systems. The strongest pollution concerns are tied to intensive finfish and shrimp systems, especially open-net pens and shrimp ponds.

So a high-quality exam answer will stay balanced:

- aquaculture can increase protein supply
- but intensive, poorly sited systems may create serious environmental costs

That balance is more accurate than saying all aquaculture is harmful.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Aquaculture is the farming of aquatic organisms such as fish, shellfish, and aquatic plants
- Intensive aquaculture can produce nutrient pollution from uneaten feed, feces, and metabolic waste
- Excess nitrogen and phosphorus can trigger eutrophication in nearby waters
- Eutrophication can lower dissolved oxygen and lead to aquatic stress or fish kills
- Organic waste from farms can accumulate on the bottom and damage benthic habitats
- Dense monocultures increase the risk of disease and parasite outbreaks
- Open-net pen systems allow direct movement of wastes, pathogens, and parasites into surrounding waters
- Escaped farmed fish can compete with wild fish, spread disease, and interbreed with wild populations
- Some carnivorous farmed fish depend on wild-caught fishmeal and fish oil, so aquaculture may still pressure wild fisheries
- Shrimp aquaculture can destroy mangroves, causing habitat loss and reducing coastal marine services
- Mangrove loss matters because mangroves provide nursery habitat, shoreline protection, filtration, and carbon storage
- Environmental impacts vary by species, farm design, site selection, and management

## Key Terms and Concepts

- aquaculture
- nutrient pollution
- eutrophication
- benthic habitat
- monoculture
- open-net pen aquaculture
- genetic integrity
- mangrove


Aquaculture - Aquaculture environmental drawbacks