01 / 22

Aquaculture
Farming the Waters

Explore the science, sustainability, and environmental impacts of raising aquatic organisms for food, restoration, and commerce.

AP Environmental Science

What Is Aquaculture?

Aquaculture is the controlled cultivation of aquatic organisms: fish, shellfish, algae, and aquatic plants. It encompasses everything from freshwater tilapia ponds in Southeast Asia to offshore salmon cages in Norway.

Aquaculture now produces over 50% of all fish consumed globally, surpassing wild-capture fisheries for human food.

Why Aquaculture Matters

Food Security
Feeds a growing global population projected to reach \( 9.7 \) billion by 2050
Wild Stock Relief
Reduces pressure on overfished wild populations and marine ecosystems
Economic Growth
Employs millions worldwide and supports rural coastal economies

Types of Aquaculture Systems

Freshwater Ponds Tilapia, catfish, carp, trout Inland, low-cost Marine Net Pens Salmon, sea bass, tuna, cobia Coastal, open-water RAS (Recirculating) Any species, indoor controlled tanks High-tech, low waste Mariculture Oysters, mussels, seaweed, kelp Saltwater-only IMTA Multi-species, waste = food Most sustainable model

Open net pens, ponds and recirculation expose different pathways

Three cutaway scenes show a coastal net pen,earthen pond and indoor tank/filter system beside an authored pathway comparison.Open pens exchange water with the sea. Pond effluent depends on management, while recirculating systems capture solids and use biofiltration. Recirculation still requires makeup water, energy and waste handling.

Compare actual water,waste and escape pathways.

Compare actual water,waste and escape pathways.. Open pens exchange water with the sea. Pond effluent depends on management, while recirculating systems capture solids and use biofiltration. Recirculation still requires makeup water, energy and waste handling.Open pens exchange water with the sea. Pond effluent depends on management, while recirculating systems capture solids and use biofiltration. Recirculation still requires makeup water, energy and waste handling.

Core Principle: Carrying Capacity

Every aquaculture system has a carrying capacity: the maximum population an environment can sustain without degradation. Exceeding it leads to disease, waste accumulation, and ecosystem collapse.

Time Population K Sustainable stocking Overstocked (crash risk)

The Feed Conversion Ratio

A key efficiency metric in aquaculture is the Feed Conversion Ratio (FCR): how much feed is needed to produce one unit of body mass.

Salmon
FCR: \( \approx 1.2 \)
Chicken
FCR: \( \approx 1.8 \)
Beef Cattle
FCR: \( \approx 6{-}8 \)

Aquaculture vs. Wild-Capture

Aquaculture Wild-Capture
Source Farmed, controlled Harvested from wild
Sustainability Renewable if managed Risk of overfishing
Bycatch Minimal to none Significant problem
Habitat Impact Localized pollution risk Bottom trawling damage
Feed Source Requires fishmeal/soy Natural food web
Disease Risk Higher (dense populations) Lower (dispersed)

Environmental Impacts

1
Nutrient Pollution: Excess feed and waste release nitrogen and phosphorus, causing eutrophication and algal blooms in surrounding waters.
2
Habitat Destruction: Shrimp farming has destroyed over 35% of the world's mangrove forests, removing vital coastal buffers.
3
Invasive Escapes: Farmed fish that escape can outcompete or interbreed with wild populations, weakening genetic diversity.
4
Antibiotic Resistance: Routine use of antibiotics in crowded farms promotes resistant bacteria that spread to wild ecosystems.

Explain why escape and genetic introgression are separate steps.

Use this illustration to explore the concept described in its caption.

A salmon exits a damaged farm net pen. An authored sequence separates escape, survival, reproduction and possible ancestry change, with each later transition conditional.
An escaped farm fish must survive and reproduce before it can contribute ancestry to a wild population. Interbreeding and changes in local adaptation are possible outcomes rather than automatic consequences of escape.

The Eutrophication Cycle

Excess \(N\) & \(P\) from farms
 Algal bloom forms Sunlight blocked below Algae die, decompose Hypoxic "Dead Zone" cycle repeats

Sustainable Solutions

IMTA
Integrated Multi-Trophic Aquaculture: one species' waste feeds another (e.g., fish + seaweed + shellfish)
RAS
Recirculating Aquaculture Systems: filter and reuse 90-99% of water, preventing pollution discharge
Plant-Based Feed
Replacing fishmeal with soy, algae, or insect protein reduces pressure on wild forage fish
Certification
ASC (Aquaculture Stewardship Council) standards ensure responsible farming practices

Integrated multi-trophic nutrient recovery

A marine fish pen,mussel longline and seaweed longline accompany exact nutrient-form pathways and partial-recovery conditions.Fed fish release particulate organic matter and dissolved nutrients. Shellfish and seaweed recover different forms of material, so their placement and growth determine how much is retained in harvested biomass.

Different waste, different consumers

Different waste, different consumers. Fed fish release particulate organic matter and dissolved nutrients. Shellfish and seaweed recover different forms of material, so their placement and growth determine how much is retained in harvested biomass.Fed fish release particulate organic matter and dissolved nutrients. Shellfish and seaweed recover different forms of material, so their placement and growth determine how much is retained in harvested biomass.

How IMTA Works

Fish (Salmon) Fed species
dissolved \(N, P\)
 particulate waste Seaweed / Kelp Absorbs dissolved nutrients Mussels / Oysters Filter particulate matter Cleaner Water + Multiple Revenue Streams

How to Evaluate an Operation

1
Identify the species: Is it native or non-native? Carnivorous or herbivorous? Higher trophic level species require more feed resources.
2
Assess the system type: Open net pen (high escape/pollution risk) vs. RAS (low risk, high energy cost) vs. IMTA (balanced).
3
Check the FCR: Lower is better. Compare to terrestrial livestock alternatives.
4
Examine waste management: Where do nutrients and chemicals go? Is there a treatment system or are they discharged?
5
Consider the ecosystem context: Mangrove destruction? Wetland loss? Genetic impact on wild populations?

Example: Shrimp Farm in Thailand

A coastal shrimp farm converts 50 hectares of mangrove forest into aquaculture ponds. It uses antibiotics, imports fishmeal feed, and discharges untreated water.

Habitat Loss
Mangroves protect coastlines, store \( \approx 3{-}5 \times \) more carbon than terrestrial forests per hectare
Pollution
Untreated discharge causes eutrophication in nearby estuaries
Trophic Cost
Carnivorous shrimp need fishmeal: \( \approx 1{-}1.5 \) kg of wild fish per kg of shrimp

Example: Salmon RAS in Norway

An indoor RAS facility raises Atlantic salmon using recirculated, filtered water. It uses plant-based feed supplements and treats all waste before discharge.

No Escapes
Closed system prevents genetic contamination of wild Atlantic salmon
Water Reuse
Filters and recirculates \( \approx 95\% \) of water, minimal discharge
Energy Cost
High electricity demand for pumps, filters, and temperature control

Example: Oyster Restoration Aquaculture

Chesapeake Bay farmers raise native Eastern oysters. Each adult oyster filters \( \approx 190 \) liters of water per day, improving water clarity and quality.

Before Oysters
Turbid, low \(\text{O}_2\)
 algal overgrowth With Oyster Reefs
Clear water, higher \(\text{O}_2\)
 biodiversity returns

Common Mistakes

✕ All Aquaculture Is Bad
Students often assume all fish farming harms the environment. IMTA, RAS, and shellfish farming can be net-positive.
✕ Confusing Mariculture
Mariculture is saltwater-only aquaculture. All mariculture is aquaculture, but not all aquaculture is mariculture.
✕ Ignoring Trophic Level
Farming herbivorous fish (tilapia) is far more efficient than carnivorous species (salmon, tuna) that require fishmeal.

Aquaculture Systems Compared

Open Ponds Net Pens RAS IMTA
Cost Low Medium High Medium
Escape Risk Low High None Moderate
Pollution Moderate High Very Low Low
Water Use High Open ocean Minimal Open ocean
Energy Use Low Low High Low

Question 1 of 5

A shrimp farm that clears mangrove forest to create ponds would most directly cause which environmental impact?

Question 2 of 5

Which metric measures the efficiency of converting feed into animal biomass?

Question 3 of 5

In an IMTA system, seaweed is grown alongside salmon pens primarily to:

Question 4 of 5

Which is a significant disadvantage of Recirculating Aquaculture Systems (RAS) compared to open net pens?

Question 5 of 5

A scientist argues that farming tilapia is more environmentally sustainable than farming salmon. Which reasoning best supports this claim?