Introduction to Aquatic Food Production
Welcome to your study notes on Aquatic Food Production Systems! This chapter is part of the Biological Resources section of your AQA A Level Environmental Science course.
We often think of farming as something that happens on land, but humans have relied on aquatic systems for wild hunting and cultivation for thousands of years. In this chapter, we will explore why some parts of the ocean are biological deserts while others are teeming with life, how commercial fishing operates and its environmental impacts, methods used to manage wild fisheries sustainably, and how aquaculture is used to farm aquatic organisms.
1. Marine Productivity: Why the Ocean Isn't Equal
Just like plants on land need sunlight and fertilizer to grow, marine productivity (the rate at which biomass is produced in aquatic systems) depends on two primary factors: Light and Nutrients.
Light Levels: The Photic Zone
Most marine life starts with tiny photosynthetic organisms called phytoplankton. Because they require sunlight for photosynthesis, they must stay in sunlit waters:
• Photic Zone: The upper sunlit layer of water where light intensity is high enough for photosynthesis to exceed respiration.
• Aphotic Zone: The deeper layers where sunlight cannot penetrate, meaning photosynthesis cannot take place.
The Role of Nutrients
Even with ample sunlight, phytoplankton cannot grow without essential inorganic nutrients, particularly nitrates ( \text{NO}_3^- ) and phosphates ( \text{PO}_4^{3-} ). These nutrients control primary biological productivity.
A Comparison of Productivity
• Open Oceans: These act as "biological deserts." Although sunlight is abundant in the photic zone, dead organic matter sinks, trapping nutrients in the deep, dark aphotic zone where light is absent.
• Coastal Areas: These have high productivity because runoff and rivers wash mineral nutrients from the land into shallow waters, where sunlight reaches near or down to the seabed.
• Upwelling Areas: Ocean currents push cold, nutrient-rich deep water upward into the photic zone. These support some of the most productive fisheries on Earth (for example, the Peruvian upwelling system).
Key Principle: Maximum marine productivity occurs only when high light levels and abundant nutrients coincide in the photic zone.
2. Fishing: Industrial Hunting
Fishing is unique as the last large-scale, industrial human hunting activity of wild animals. Commercial techniques target different depth zones and species.
Commercial Fishing Methods
Pelagic Methods (mid-water and surface species, e.g. tuna, mackerel, herring):
• Pelagic Trawling: Towing a net through mid-water depths without contacting the seabed.
• Purse Seining: Surrounding a shoaling school of pelagic fish with a vertical curtain of netting and cinching the bottom closed.
• Drift Netting: Suspending vertical nets near the surface that catch fish by entanglement behind their gills.
• Pelagic Long Lining: Floating surface or mid-water main lines spanning kilometres, carrying thousands of baited branch lines.
Demersal Methods (seabed-dwelling species, e.g. cod, haddock, flatfish, shellfish):
• Demersal Trawling: Dragging heavy weighted nets and otter boards along the ocean floor.
• Demersal Long Lining: Anchoring baited long lines along the seabed.
• Shellfish Traps: Baited pots or cages placed on the seabed for crustaceans (crabs, lobsters).
Environmental Impacts of Fishing
• Population Decline / Overfishing: Mortality from harvest exceeds recruitment and growth, depleting breeding populations.
• By-catch: Non-target species accidentally caught and discarded (e.g. juvenile fish, turtles, cetaceans, albatrosses).
• Ghost Fishing: Discarded or lost fishing gear continuing to trap and kill organisms indefinitely.
• Seabed Damage: Demersal gear scours the ocean bed, destroying benthic communities, cold-water corals, and nursery grounds.
• Changed Age Structure and Size Selection: Selectively removing the largest, fastest-growing individuals leaves smaller, younger fish with reduced reproductive capacity (megaspawners produce disproportionately more and higher-quality eggs).
3. Managing Fish Stocks & Sustainable Fisheries
To keep wild fisheries sustainable, harvests must not exceed the regenerative capacity of the stock.
Maximum Sustainable Yield (MSY)
The Maximum Sustainable Yield (MSY) is the greatest harvest of fish that can be taken continuously without depleting the long-term stock. Calculating MSY relies on assessing:
• Biomass: Total standing mass of the fish population.
• Recruitment: Addition of new individuals entering the catchable population through reproduction and maturation.
• Growth Rates: Increase in individual size and mass over time.
• Mortality: Natural deaths (predation, disease, senescence) versus fishing mortality.
Methods to Reduce Fishing Impacts
• Catch Quotas: Setting Total Allowable Catches (TACs) based on scientific estimates of MSY.
• Effort Limits: Regulating fishing capacity by restricting vessel size, engine power, gear size, and number of allowable days at sea.
• Net Design Modifications: Using larger mesh sizes, square mesh panels (which remain open under tension), and escape panels / By-catch Reduction Devices (BRDs).
• Acoustic Deterrents: Using acoustic "pingers" on nets to deter marine mammals.
• Ghost Gear Control: Deploying biodegradable materials in trap escape hatches and radio-tracked buoys to recover lost gear.
• Size Regulations: Setting minimum catch sizes to ensure individuals breed at least once, and maximum catch sizes to protect highly fertile "megaspawners."
• Spatial and Temporal Restrictions: Enforcing no-take zones (Marine Protected Areas) and closed seasons during peak spawning periods.
• Captive Rearing and Release: Stock enhancement programmes where larvae or juveniles are reared in hatcheries and released into wild waters.
4. Aquaculture: Farming Aquatic Systems
Aquaculture involves farming aquatic organisms (finfish, molluscs, crustaceans, algae) under controlled conditions. Operations range from low-input extensive systems (e.g. coastal oyster beds, carp ponds) to high-input intensive systems (e.g. salmon cages).
Principles and Productivity Controls in Aquaculture
• Species Selection: Choosing species suited to local abiotic conditions with high growth rates and high market value.
• Stock Selection & Genetic Control: Selective breeding for disease resistance, rapid growth, feed conversion efficiency, and docility (or triploidy to prevent reproductive diversion of energy).
• Control of Abiotic Factors: Regulating water temperature, dissolved oxygen levels, light levels (photoperiod control to manipulate maturity), and maintaining adequate water flow to supply oxygen and flush metabolites.
• Control of Biotic Interactions & Competition: Managing intraspecific competition (by size-grading fish and maintaining appropriate stocking densities) and interspecific competition/predation (using exclusion netting).
• Disease Control: Utilizing vaccinations, biological controls (e.g. cleaner wrasse for sea lice), lower stocking densities, and responsible, minimal use of veterinary medicines (antibiotics/antifungals).
Environmental Impacts of Aquaculture
• Organic Waste & Eutrophication: Uneaten feed and faecal excretion concentrate nutrients (nitrogen and phosphorus), risking local anoxia.
• Escapes & Genetic Pollution: Escaped domesticated fish compete with wild populations for food/spawning redds and dilute wild gene pools through cross-breeding.
• Pesticide and Chemical Release: Synthetic parasiticides (used for ectoparasites like sea lice) can be toxic to non-target wild crustaceans.
• Trophic Level Efficiency (Fish-in / Fish-out ratio): Cultivating carnivorous species (e.g. salmon, sea bass) requires wild-caught pelagic fish (anchoveta, sandeels) processed into fishmeal and fish oil, representing low ecological conversion efficiency.
Sustainable Aquaculture Approaches
• Lower Stocking Density: Reduces stress, parasite transmission, and the need for medication.
• Polyculture and Integrated Multi-Trophic Aquaculture (IMTA): Culturing species from different trophic levels together (e.g. finfish alongside filter-feeding bivalves to consume organic particulate matter and macroalgae/seaweed to absorb dissolved inorganic nutrients), closing nutrient loops.
Summary and Memory Aids
• Productivity: Requires Light + Nutrients. Highest in coastal and upwelling zones; lowest in open oceans.
• Fishing Gear: Pelagic = open water column (trawls, drift nets, purse seines, pelagic long lines); Demersal = seabed (otter trawls, demersal long lines, traps).
• Management Tools: Quotas, effort limits, mesh/escape panels, acoustic pingers, size limits (minimum and maximum), no-take zones, and biodegradable/tracked gear.
• Aquaculture Sustainability: IMTA, low stocking densities, herbivorous/lower-trophic species, and escape prevention.