What is a marine information system?
Data means facts and numbers, such as sea temperature or wave height. An information system collects data, stores it, joins it and shows it in a way people can use.
In the sea, data comes from many places: buoys (floating instruments), ships, satellites and coastal stations. A marine information system collects all this and turns it into maps and warnings. Examples are a sea-surface temperature map, a wave forecast, a tide table and a storm alert.
Why do we need it? The sea is large, always moving and dangerous. Data lets us plan work and stay safe.
Information systems for ship operation
A ship needs to answer three questions: Where am I? What is around me? Where should I go?
- GPS (Global Positioning System): satellites send time signals. The receiver uses them to work out latitude and longitude.
- Radar: sends radio waves and shows other ships and land as dots on a screen, even at night or in fog.
- AIS (Automatic Identification System): each ship sends its name, position, speed and direction by radio. Other ships and ports can see it. This avoids collisions.
- Electronic chart (ECDIS): a digital sea map that shows the ship's position, depth and the planned route.
- Echo sounder: measures the depth under the ship so it does not touch the bottom.
- Weather and sea-state data: warns of storms and big waves along the route.
Together these are often called the ship's navigation and communication systems.
How sonar finds fish and depth
Sonar means sound navigation and ranging. A transducer under the ship sends a short sound pulse down. Sound travels in sea water at about 1500 m/s. When it hits a fish school or the sea floor it bounces back. The ship measures the time between sending and hearing the echo.
Depth = speed x time / 2. We divide by 2 because the sound goes down and comes back up.
Fish have a swim bladder full of gas. It reflects sound strongly, so fish schools show as bright marks on the fish-finder screen.
Fisheries information systems
Information technology helps at every step from sea to plate.
- Finding fish: satellite maps of sea temperature and plankton colour show where fish gather. Fish finders show the school under the boat.
- Catch records (logbooks): how much of which fish was caught, where and when. Scientists use this to check that fish stocks are not overfished.
- Vessel monitoring: a tracker on the boat shows that it stays inside allowed areas and seasons.
- Aquaculture sensors: in fish farms, sensors measure oxygen, temperature and pH, and alarms warn the farmer.
- Cold chain and traceability: temperature loggers watch fish during storage and transport. A barcode or QR code on the pack tells where and when the fish was caught.
- Market information: apps show today's prices so the fisher can choose where to sell.
Sharing data over networks, and staying safe
Data moves by radio, satellite link, mobile network and the internet. Near the coast a phone network works. Far at sea ships use satellite communication. A shore data centre stores the data and runs programs that make forecasts.
Data must be correct, up to date and protected. A wrong position or a hacked AIS signal can cause accidents. So systems use backups, passwords and checks.
Try it
In the 3D: go to the last step. Move the depth slider to 150 m. Predict the echo time first (hint: 2 x 150 / 1500 seconds). Then read the number below the scene.
At home: clap near a far wall in a quiet place, or shout in an empty corridor. Count how long the echo takes to return. A phone stopwatch helps. Distance = 340 x time / 2 (sound in air is 340 m/s).
Key formulas and definitions
- Depth = (speed of sound x echo time) / 2
- Speed of sound in sea water is about 1500 m/s
- Echo time = 2 x depth / speed
- GPS gives latitude and longitude
Worked examples
1. A ship's echo sounder gets an echo after 0.2 s. Sea water, speed 1500 m/s. How deep is the sea?
Depth = 1500 x 0.2 / 2 = 150 m.
2. A fish school is 75 m below the boat. How long does the echo take?
Time = 2 x 75 / 1500 = 0.1 s.
3. Which system should a captain use to avoid hitting another ship in fog? Give two.
Radar (shows other ships as dots) and AIS (other ships send their position and course by radio).
4. The echo from the sea floor comes back after 0.4 s. A fish echo comes after 0.1 s. How far is the fish above the floor?
Floor: 1500 x 0.4 / 2 = 300 m. Fish: 1500 x 0.1 / 2 = 75 m. Fish is 300 - 75 = 225 m above the floor.
Common mistakes
- Forgetting to divide by 2. The echo time covers the trip down and back.
- Mixing GPS with sonar. GPS uses satellites to give position; sonar uses sound to measure depth and find fish.
- Thinking radar uses sound. Radar uses radio waves; sonar uses sound waves.
- Using 340 m/s for sea water. Sound is faster in water, about 1500 m/s.