Aquatic food production systems

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 the water for food for thousands of years. In this chapter, we will explore why some parts of the ocean are like "underwater deserts" while others are "supermarkets," how we catch wild fish, and how we are increasingly "farming" the water through aquaculture. Don't worry if some of the technical terms seem a bit much at first—we will break them down step-by-step!

1. Marine Productivity: Why the Ocean Isn't Equal

Just like plants on land need sunlight and fertilizer to grow, marine productivity (how much "life" is produced in the sea) depends on two main things: Light and Nutrients.

Light Levels: The Photic Zone

Most marine life starts with tiny plant-like organisms called phytoplankton. Because they photosynthesize, they must stay near the surface where there is light.
• Photic Zone: The top layer of the ocean where there is enough light for photosynthesis.
• Aphotic Zone: The deep, dark layers where photosynthesis is impossible because light cannot reach it.

The Role of Nutrients

Even if there is plenty of light, phytoplankton cannot grow without nutrients (like nitrates and phosphates). These nutrients act like "sea-fertilizer."

A Comparison of Productivity

• Open Oceans: These are often like "biological deserts." Even though there is plenty of light at the surface, the nutrients have all sunk to the bottom where it is too dark for plants to grow.
• Coastal Areas: These are much more productive because nutrients are washed into the sea from the land by rivers.
• Upwelling Areas: This is where deep, cold, nutrient-rich water is pushed up to the surface. These are the "supermarkets" of the ocean with massive amounts of fish!

Did you know? Some of the world’s most productive fishing grounds are off the coast of Peru, thanks to a massive upwelling of nutrients!

Key Takeaway: High productivity requires both light (at the surface) and nutrients. If you only have one, you won’t get many fish.

2. Fishing: Our Last Great Hunting Activity

Fishing is unique because it is the last time humans hunt wild animals on a massive, industrial scale. To do this, we use several different methods, each with its own pros and cons.

Common Fishing Methods

Pelagic Trawling: Dragging a net through the "mid-water" to catch fish like mackerel.
Demersal Trawling: Dragging a heavy net along the seabed to catch fish like cod or shrimp.
Purse Seining: Circling a school of fish with a large "curtain" of netting and then pulling the bottom shut like a drawstring bag.
Drift Netting: Large nets that hang vertically in the water and catch fish by their gills as they swim into them.
Long Lining: Using a main fishing line that can be miles long, with thousands of smaller baited hooks branching off it.
Shellfish Traps: Baited cages (like lobster pots) placed on the seabed.

Environmental Impacts of Fishing

Fishing isn't just about catching dinner; it changes the whole ocean environment:
• Overfishing: Catching fish faster than they can reproduce, leading to a population decline.
• By-catch: Accidentally catching species you didn't want (like dolphins in tuna nets or small, young fish).
• Ghost Fishing: When lost or discarded nets keep catching and killing animals for years.
• Seabed Damage: Heavy demersal trawls can "plough" the ocean floor, destroying coral reefs and seagrass beds.
• Changed Age Structure: Fishermen usually catch the biggest fish. This leaves only the small, young ones, which may not be old enough to breed yet.

Quick Review Box:
By-catch: Wrong species caught.
Ghost fishing: Abandoned nets still killing.
Demersal: Bottom of the sea.
Pelagic: Open water/surface.

3. Managing Fish Stocks: How Much Can We Take?

To keep fishing sustainable, we need to know how many fish are in the sea. This is harder than it sounds because we can't see them!

Maximum Sustainable Yield (MSY)

The Maximum Sustainable Yield (MSY) is the largest amount of fish we can catch without shrinking the population for the future.

To calculate this, scientists look at:
• Biomass: The total "weight" of fish in an area.
• Recruitment: How many new "baby" fish are joining the population.
• Growth: How fast individual fish are getting bigger.
• Mortality: How many fish are dying from natural causes.

Methods of Reducing Impact

We use several strategies to protect fish stocks:
• Catch Quotas: Limits on how many fish a boat can bring back to port.
• Net Design: Using a larger mesh size so small young fish can escape, or adding escape panels for turtles and dolphins (acoustic deterrents or "pingers" also help).
• No-take Zones: Areas where fishing is completely banned to let the ecosystem recover.
• Minimum Catch Size: Making it illegal to keep fish that are too small and haven't bred yet.

Key Takeaway: Management is all about balance. If we catch only the "interest" (the new growth) and keep the "capital" (the breeding adults), fishing can last forever.

4. Aquaculture: Farming the Water

Since wild fish stocks are struggling, we are turning to Aquaculture (fish farming). This can be Extensive (low energy, like a simple pond) or Intensive (high energy, high technology, like a salmon farm).

Principles of Aquaculture

To run a successful fish farm, you must control several factors:
• Species Selection: Choosing fish that grow fast and are hardy.
• Stock Selection: Using breeding and genetic control to create better fish.
• Disease Control: Fish in cages are close together, so diseases spread fast. Farmers use antibiotics or vaccines.
• Abiotic Factors: Controlling temperature, dissolved oxygen, and light levels to maximize growth.

Environmental Impacts of Aquaculture

Fish farming isn't perfect. It can cause:
• Organic Waste: Excess fish food and "poo" can pollute the water around the farm.
• Escapes: Farmed fish might escape and compete with wild fish or change their genetics.
• Pesticide Use: Chemical treatments used to kill parasites like sea lice can harm other marine life.

Can Aquaculture Replace Fishing?

One big problem is Trophic Level Efficiency. If we farm carnivorous fish (like salmon), we have to catch wild sardines and anchovies to feed them. It takes several kilograms of wild fish to produce just one kilogram of farmed salmon. This isn't very efficient!

Analogy: Intensive aquaculture is like a "feedlot" for cows, but underwater. It produces a lot of food but requires a lot of "inputs" and creates a lot of waste.

Key Takeaway: Aquaculture helps provide food, but we must manage the pollution and the "fish-to-feed-fish" problem to make it truly sustainable.

Summary and Memory Aids

• Productivity: Needs Light + Nutrients. Coastal/Upwelling = Good. Open Ocean = Bad.
• Fishing Impacts: Remember "B.O.G.S." — By-catch, Overfishing, Ghost fishing, Seabed damage.
• Management: MSY is the "sweet spot" of catching just enough.
• Aquaculture: Intensive (high control/high impact) vs Extensive (low control/low impact).

Don't worry if you find the different types of nets confusing at first. Just remember: Demersal = Destroying the bottom; Pelagic = Mid-water; Long-lines = Lots of hooks. You've got this!