Marine animals
Fish such as yellowtail, sea bream, sea bass and tuna are raised in floating net cages in sheltered bays. The farmer feeds them, checks nets and cleans them. Shellfish such as oysters, mussels and scallops hang on ropes or rafts; they filter their own food from the water, so no feed is needed. Abalone, sea cucumber and shrimp are raised in tanks, shore ponds or on the seabed.
Stock enhancement (sea ranching) means raising young animals in a hatchery and releasing them into the sea so wild numbers grow. Other tools are artificial reefs, protected nursery areas and fishing limits. Released fish may be marked so people can see how many survive.
Marine plants
Marine plants are mainly seaweeds (algae): nori (laver), wakame, kombu (kelp), agar and carrageenan weeds. They are used as food, in agar and gels, in medicine and as fertiliser.
- Seed: tiny spores or young plants are put on ropes or nets.
- Grow: ropes hang on floating long-lines or nets are stretched in shallow water. Seaweed takes sunlight, nutrients and carbon dioxide from the water. No feed is added.
- Harvest: cut or pull when the plants have grown, then dry or process.
Seaweed farms need clean water, good currents and the right season. Seaweed beside fish farms also takes up extra nutrients.
Aquaculture techniques abroad
Different regions use different systems, depending on water, land and money:
- Earthen ponds: freshwater carp ponds in India and China, catfish ponds in the USA, tilapia ponds in Africa and Asia, shrimp ponds on coasts in Asia and Latin America. Paddle wheel aerators add oxygen.
- Net pens in cold seas: Atlantic salmon in Norway, Chile and Scotland.
- Raceways: long channels with flowing water for trout.
- Recirculating aquaculture systems (RAS): tanks inside buildings where water is filtered and reused, so very little water is used and waste is easy to collect. They cost more to run.
- Polyculture: different species in one pond using different foods, like rohu, catla and mrigal carp.
Keeping ornamental fish
Ornamental (pet) fish like goldfish, guppy, koi and tetra are kept for beauty. A good aquarium has:
- A tank with a cover and enough water volume (more water means steadier conditions).
- A filter to clean and move water, and an air pump for oxygen.
- A heater for tropical fish, and a thermometer.
- Gravel and plants for shelter and to take up waste.
Care: start the filter several days before adding fish so good bacteria grow (cycling). Add fish slowly. Feed small amounts once or twice a day. Change about a quarter of the water every week. Do not overcrowd. Check temperature, pH and ammonia.
Try it: pick the system
In free play, tap each system and say what it grows, what it needs and what can go wrong. Match: shellfish need no feed; seaweed needs light; shrimp ponds need oxygen; aquariums need a filter.
Key formulas and definitions
- Volume of a rectangular tank (litres) = length × width × height (cm) / 1000
- Volume of a round cage = π × r² × depth
- Stocking density = number of animals / area or volume
- Survival (%) = number harvested / number stocked × 100
- Key terms: net cage, long-line, stock enhancement, sea ranching, RAS, polyculture, aeration
Worked examples
1. An aquarium is 60 cm long, 30 cm wide and 40 cm high. How many litres of water does it hold when full?
Volume = 60 × 30 × 40 = 72 000 cm³. 1000 cm³ = 1 litre, so 72 litres.
2. A round sea cage has radius 5 m and net depth 8 m. Find the water volume inside (take π = 3.14).
Volume = 3.14 × 5 × 5 × 8 = 628 m³.
3. A farmer stocks 200 000 young shrimp in a 2 hectare pond. What is the stocking density per hectare?
200 000 / 2 = 100 000 shrimp per hectare.
4. A hatchery releases 50 000 young fish and a later survey estimates that 6 000 survive. What is the survival percent?
6000 / 50 000 × 100 = 12 percent.
5. Why do oysters and mussels need no feed in farms?
They are filter feeders. They pump seawater through their bodies and strain out tiny plankton, so the farmer does not feed them.
6. A seaweed rope grows from 20 cm to 160 cm in 8 weeks. By how many times did it grow and what is the average growth per week?
160 / 20 = 8 times. Gain = 140 cm in 8 weeks = 17.5 cm per week.
Common mistakes
- Thinking all aquaculture needs feed. Shellfish and seaweed need none.
- Adding all fish to a new aquarium on day one. The filter bacteria are not ready yet.
- Calling stock enhancement the same as farming to harvest. Released fish are meant to join wild stock.
- Using only small tanks. Larger water volume stays steadier and is easier to keep healthy.