Where fish are found and types of fishing
Fish need food. The base of the ocean food chain is plankton, which needs sunlight and nutrients. So the richest fishing grounds are:
- Continental shelves: shallow sea (less than about 200 m) next to land.
- Upwelling zones: where cold, nutrient-rich deep water rises, for example off Peru.
- Where warm and cold currents meet, such as near Newfoundland and Japan.
Kinds of fisheries
- Inland: rivers, lakes, reservoirs and ponds (freshwater).
- Marine: coastal (near shore, small boats) and deep-sea (large vessels, far out).
- Artisanal (traditional): small boats, hand nets, local markets.
- Commercial (industrial): trawlers and factory ships, large catches for export.
Common gear
Trawl net: a bag net dragged through the water or along the bottom. Purse seine: a wall of net closed around a shoal like a drawstring bag. Gill net: a hanging net that traps fish by the gills. Long line: one long line with thousands of hooks.
Aquaculture: farming the water
Aquaculture is raising water animals and plants under control: feeding them, protecting them and harvesting when ready. Mariculture is aquaculture in the sea.
- Ponds: carp and tilapia in fresh water, shrimp in brackish coastal ponds.
- Sea cages: salmon in Norway and Chile, sea bream in the Mediterranean.
- Ropes and rafts: mussels, oysters and seaweed (huge in China, Korea and Japan).
Benefits: steady supply, jobs, less pressure on wild stocks. Problems: cutting mangroves for shrimp ponds, waste and chemicals polluting water, diseases spreading to wild fish, and wild fish caught to make feed.
Processing and distribution: the cold chain
Fish spoils within hours in warm weather, so the trip from net to plate must be fast and cold.
- On board / harbour: fish are sorted and packed in ice.
- Processing: cleaning, filleting, then preserving by freezing, drying, salting, smoking or canning. By-products become fish meal and fish oil.
- Cold storage: frozen at about −18 °C or colder.
- Distribution: refrigerated trucks, containers and air freight to local markets, supermarkets and export.
This unbroken low-temperature path is the cold chain. A break anywhere lets bacteria grow. Fish and seafood are among the most traded foods in the world.
Overfishing, coastal change and sustainable management
A fish stock grows back each year by breeding. If we catch more than this regrowth, the stock shrinks, and next year it can regrow even less. This is overfishing. The cod of the north-west Atlantic collapsed in the early 1990s and has still not fully recovered.
Other harms: bycatch (dolphins, turtles and young fish caught by accident), bottom trawls scraping the seabed, and fleets fishing far from home with subsidies.
Maximum sustainable yield
A stock grows fastest at about half its natural size. The biggest catch that can be taken every year forever is the maximum sustainable yield (MSY).
Management tools
- Total allowable catch and quotas
- Closed seasons (e.g. monsoon ban) and closed areas
- Minimum mesh and fish size
- Marine protected areas and limits on fleet size
- Shared rules for shared seas, such as the European Union's Common Fisheries Policy, which sets quotas for member countries
The changing coast
Harbours, fish-processing plants, shrimp farms, tourism and housing transform coastlines, remove mangroves and dunes, and can increase erosion and pollution. Planning must balance jobs with healthy coasts.
Try it
In free play, find the yearly catch that gives the biggest total over 20 years without the stock falling. Then visit a fish market and ask where each fish came from and whether it was farmed or wild.
Key formulas and definitions
- Stock next year = stock + growth − catch
- Sustainable if catch ≤ natural growth
- Growth is fastest near half the maximum stock (MSY point)
- Total production = capture fisheries + aquaculture
- Cold chain: keep fish at 0 °C (fresh) or ≤ −18 °C (frozen)
Worked examples
1. A lake has 10,000 fish. They add 2,000 new fish a year. Fishers catch 3,000 a year. What happens?
Each year the stock falls by 1,000 (2,000 − 3,000), and as it shrinks it grows back even less. Overfishing.
2. Why are continental shelves good fishing grounds?
They are shallow, so sunlight reaches the bottom and nutrients from rivers and the seabed feed plankton, the base of the food chain.
3. A shrimp farm removes mangroves. Give two effects.
Coast loses its natural storm and erosion protection; young wild fish lose their nursery habitat.
4. Why is a monsoon fishing ban useful?
Many fish breed then. Leaving them alone lets more young fish survive and refill the stock.
Common mistakes
- Thinking the ocean is so big that fish can never run out. Many stocks have collapsed.
- Calling all aquaculture harmless. Badly run farms pollute water and destroy mangroves.
- Thinking the best catch is the biggest catch this year. Long-term total is higher with moderate catches.
- Forgetting the cold chain: fresh fish left warm for a few hours is no longer safe.