Collecting water animals and plants
Collecting means taking small samples of living things from a pond, river or sea so we can study them. We use a plankton net (very fine), a dip net or a bucket.
The mesh is the set of holes in the net. Animals smaller than the holes slip out. Animals bigger than the holes are kept. So a fine mesh keeps more.
- Write the place, time, water temperature and depth.
- Put the sample in a bucket of the same water. Keep it cool and shaded.
- Take only what you need. Put the rest back.
Rearing fish and shellfish
Rearing means keeping and growing water animals in a tank or pond. They need clean water. The most important thing to check is dissolved oxygen, the oxygen mixed into the water.
- 5 mg per litre or more: good for most farmed fish.
- 3 to 5 mg per litre: low. Add air.
- Below 3 mg per litre: danger. Fish gasp at the surface.
Warm water holds less oxygen, and many fish use it up. Also check temperature and how clear the water is. Feed small amounts. Uneaten food rots and uses oxygen. Remove dead fish. Write every reading in a daily log.
Processing fishery products
Processing means turning the catch into food that is safe and lasts longer. Fish spoil fast because germs and enzymes work quickly when it is warm. So the first rule is: chill at once.
- Put fish on ice right after catching.
- Wash with clean water.
- Clean and cut with a clean knife and board.
- Pack, and keep cold until it is sold or cooked.
Other ways to make fish last: drying, salting, smoking and freezing. Keeping fish cold all the way from the boat to the buyer is called the cold chain. Wash your hands and tools every time.
Marine practice and boat safety
Marine practice is work done on a boat or ship. Safety comes first.
- Check the weather and waves before you leave.
- Everyone wears a life jacket, even good swimmers.
- Check the boat: fuel, engine, bailer, rope, first-aid box.
- Tell someone on shore where you are going and when you will be back.
- Sit low, hold on, and never stand on the side of the boat.
- If someone falls in, shout, throw a float and keep your eyes on them.
As a rule of thumb, small practice boats stay in harbour when waves are about 2.5 m or higher. Your teacher and the local sea rules decide.
Try it
At home: put fish or a vegetable in two bowls, one in the shade and one in cold water with ice. Smell and look after 2 hours. Then in the 3D, slide the temperature and the net hole and say what changes.
Key formulas and definitions
- Net keeps: animals bigger than the hole
- Dissolved oxygen: 5 mg/L or more = good; below 3 mg/L = danger
- Daily feed (g) = body weight of all fish (g) × feed % ÷ 100
- Keep fish at about 0 to 2 °C (on ice)
- Cold chain = boat → ice → store → buyer with no warm gap
Worked examples
1. A tank has 50 fish of 100 g each. You feed 2% of their weight per day. How many grams of feed?
Total weight = 50 × 100 = 5000 g. Feed = 5000 × 2 ÷ 100 = 100 g per day. Split it into 2 small meals of 50 g.
2. A sample has 20 plankton in five sizes, 4 of each (size 1 smallest to size 5). A net with hole size 3 lets sizes 1, 2 and 3 pass. How many are kept?
Only sizes 4 and 5 are bigger than the holes. That is 4 + 4 = 8 plankton kept. A finer net (hole size 1) would keep 16.
3. A tank reads 2.5 mg/L of oxygen at dawn. What do you do?
Below 3 mg/L is danger. Switch on the air pump or aerator at once, stop feeding, and check again in 30 minutes. Find out why: too many fish, warm water or rotting food.
Common mistakes
- Thinking a bigger net always catches more. Big holes let the small animals slip out.
- Feeding a lot "to help the fish grow". Extra food rots, uses oxygen and can kill them.
- Leaving the catch in the sun while you finish other work. Chill first, always.
- Skipping the life jacket because "I can swim". Cold water and waves tire even strong swimmers.