Marine Science (9693) | Marine aquaculture

Marine Aquaculture (Fisheries for the Future)

Welcome to the final topic in the 'Fisheries for the Future' section! As wild fish stocks face increasing pressure from modern fishing technology (which you studied in 8.2), we need alternative ways to feed a growing human population.

This chapter, Marine Aquaculture (or simply 'fish farming'), provides that solution. It is the controlled cultivation of aquatic organisms, and understanding it requires balancing major economic potential against serious environmental risks. Don't worry if the impacts seem complicated—we will break down the pros and cons clearly!

1. Defining Aquaculture: Intensive vs. Extensive (8.3.1)

Aquaculture is the farming of aquatic organisms, including fish, shellfish, and algae, in controlled or semi-controlled environments.

Aquaculture systems are categorised based on how controlled the environment is and the level of inputs (food, energy, labour).

A. Intensive Aquaculture Techniques

This is the high-tech, high-input approach. Think of it like a crowded, efficient factory farm for fish.

• Definition: Organisms are kept in high densities and are totally reliant on external inputs (feed, oxygen, water exchange) for survival.
• Inputs: High labour, high technology (e.g., monitoring sensors, pumping systems), and high quality formulated feed.
• Yield: Very high biomass produced per unit area.
• Named Example: Farming salmon in large net pens or sea cages. These systems require formulated pellets for feed and are highly managed to control water quality and disease.

B. Extensive Aquaculture Techniques

This approach is lower density and relies more on natural ecosystem processes. Think of it like a pasture where animals graze naturally.

• Definition: Organisms are kept at low densities and rely primarily on natural productivity (e.g., phytoplankton or detritus) as their food source. Little artificial feed is used.
• Inputs: Low labour, low technology, and minimal or no artificial feed input.
• Yield: Lower biomass produced per unit area, but often with lower running costs.
• Named Example: Cultivating marine mussels attached to ropes or rafts in the ocean. The mussels are filter feeders and get all their necessary nutrients from the naturally occurring plankton in the sea water.

2. The Process of Aquaculture (8.3.2)

The basic process generally involves three stages: Hatchery (producing young stock), Nursery (growing small stock), and Grow-out (reaching market size).

A. Salmon Aquaculture

Salmon farming usually involves a combination of fresh and salt water stages due to the salmon's complex life cycle (anadromous).

1. Hatchery Stage (Freshwater): Eggs are hatched in controlled freshwater tanks. The young fish (alevins, then fry) are fed formulated pellets.
2. Smoltification Stage (Freshwater to Saltwater): When the young salmon become smolts (ready to transition to saltwater), they are moved to sea cages or net pens located in sheltered coastal areas.
3. Grow-out Stage (Saltwater): Salmon are grown to market size, typically over 1-2 years. Feeding is managed carefully, often using automated systems. Health and water quality are monitored intensively.
4. Harvesting: Fish are humanely harvested and processed.

B. Marine Mussel Aquaculture

Mussel farming is typically extensive and involves simpler, low-cost structures.

1. Spat Collection: Mussel larvae (called spat) are collected naturally. Farmers hang collector ropes in areas known for high larval settlement. The larvae attach themselves.
2. Raft/Longline Installation: The spat-covered ropes are transferred to growing structures (rafts or longlines) suspended in the water column.
3. Grow-out (Filter Feeding): Mussels grow by filter feeding on natural phytoplankton in the water. No external feed is usually required.
4. Harvesting: Mussels are harvested when they reach market size, usually by pulling the ropes out of the water using machinery.

C. Shrimp Aquaculture

Shrimp farming is often carried out in coastal ponds, which can be either extensive or semi-intensive/intensive.

1. Hatchery/Stocking: Larvae are sourced either from dedicated indoor hatcheries or collected from the wild. These are released into prepared grow-out ponds.
2. Pond Management: Ponds require careful management of water quality (salinity, pH, oxygen) and water exchange.
3. Feeding: In intensive systems, formulated feed pellets are added multiple times per day. In extensive systems, shrimp graze on algae and small invertebrates growing in the pond.
4. Harvesting: Ponds are often drained or netted, and the shrimp are collected.

3. Requirements for Long-Term Success (8.3.3)

A successful aquaculture project needs careful planning, considering numerous biological, environmental, social, and economic factors. Ignoring any of these can lead to failure, both ecological and financial.

Key Requirements Explained:

• Availability of Stock: A reliable supply of eggs, larvae, or juveniles (stock) is essential. This usually means running a dedicated hatchery or managing broodstock (parent fish). Reliance on wild stock can be unsustainable.
• Availability of Clean Water: The water must have appropriate salinity, temperature, and high dissolved oxygen. Pollution from outside sources (like agricultural run-off) can wipe out stock quickly.
• Availability of Feed and Efficiency of Use of Feed: For intensive systems (like salmon), feed is the largest operational cost. The feed must be nutritionally complete and cheap. Maximising the Feed Conversion Ratio (FCR) (the amount of feed needed to produce 1 kg of biomass) is crucial for profitability.
• Availability of Labour: Skilled technicians and biologists are needed for daily management, monitoring water quality, and treating diseases.
• Disease Management: High stocking densities create stress and allow pathogens (like sea lice or viruses) to spread rapidly. Effective prevention and treatment protocols are vital.
• Availability of Location: The site must be sheltered (to prevent storm damage), have adequate water depth and flow (to disperse waste), and meet legal/regulatory requirements.
• Market Demand and Access to Market: There must be a reliable consumer demand for the product (e.g., people must want to buy farmed salmon). Efficient infrastructure (transport, processing) is needed to get the product to the consumer quickly.
• Return on Investment (ROI): Ultimately, the project must be profitable. Running costs (feed, labour, energy) must be lower than the revenue generated by selling the harvest.

4. Principal Impacts of Aquaculture (8.3.4)

Aquaculture has profound effects—both beneficial (positive) and detrimental (negative)—on the environment, economy, and society. A Level students must be able to discuss these impacts in detail.

A. Negative Environmental Impacts

• Habitat Destruction: Coastal ecosystems, particularly mangrove forests, are often cleared to create space for shrimp ponds. This destroys valuable nursery habitat for wild species and removes crucial coastal protection.
• Overexploitation of Feedstocks: Many farmed species (especially carnivorous fish like salmon and tuna) require feed derived from wild-caught fish (e.g., anchovies or sardines) processed into fishmeal and fish oil. This puts unsustainable pressure on these wild forage fish populations.
• Pollution: Aquaculture farms produce significant waste:
  • Eutrophication: Excess fish faeces and uneaten feed settle below the cages, leading to high nutrient levels (nitrogen/phosphorus) that can cause algal blooms and reduce dissolved oxygen.
  • Chemicals: Antibiotics and pesticides (used for parasite control, like sea lice treatments) are released directly into the marine environment.
• Escape of Cultured Stock: Escaped farmed fish (which are often selectively bred) can breed with native wild stocks, leading to genetic dilution. This may reduce the fitness and survival ability of the wild population.
• Introduction of (Potentially) Invasive Species: If non-native species are introduced for farming, and they escape, they can become an invasive species, out-competing native organisms for resources.
• Spread of Disease and Parasites: High density in farms means diseases (like Infectious Salmon Anaemia, ISA) and parasites (like sea lice) spread rapidly. Water flow can carry these pathogens to nearby wild populations.
• Competition for Resources: Farmed organisms and structures can compete with native species for space, oxygen, and natural food sources.

B. Positive Social and Economic Impacts (and mitigation of negative impacts)

The primary benefit of aquaculture is its contribution to food security and the reduction in the exploitation of native stocks.

• Reduction in the Exploitation of Native Stocks: By providing an alternative, reliable source of seafood, aquaculture reduces the fishing pressure on wild populations, allowing them to recover (e.g., farmed shrimp reduces the need to trawl for wild shrimp).
• Economic Impacts: Aquaculture creates thousands of jobs (social impact) in rural coastal communities, ranging from labourers and technicians to processors and distributors. It provides a reliable income source that is not dependent on the unpredictable nature of wild catch.
• Social Impacts: It ensures a consistent supply of protein for the global population, contributing positively to food stability and affordability.
• Development of Sustainable Practices: Research is constantly aiming to reduce negative impacts, for example:
  • Developing vegetarian-based feeds to reduce reliance on fishmeal.
  • Implementing integrated multi-trophic aquaculture (IMTA), where waste from farmed fish feeds lower trophic level organisms (like mussels or seaweed), thereby cleaning the water.