People have been catching fish from the sea, rivers and lakes for thousands of years. Fishing is still a source of food, and an important industry for many people living near the sea. Most fish are now caught by fleets using modern equipment. This is called commercial fishing. There are about 30,000 kinds of fish, but only a few hundred are caught commercially. The world’s major fishing grounds are the areas of relatively shallow waters near the coast.

Trawlers

Most sea fish are caught by fishermen working on trawlers. Trawlers are equipped with a large net that is trawled (dragged) along or just above the seabed. Many trawlers have freezers so the fish can be gutted and frozen on board. Trawlers are used to catch cod and tuna in deep waters. They also catch fish that live in the waters close to shore, such as haddock, plaice, sole and turbot.

Sonar

Modern ships are equpped with sonar technology. Pulses of ultrasound (very high-pitched sounds humans cannot hear) are transmitted through the water and bounce back to the ship. Shoals of fish show up on a computer screen.

Drift or gill nets hang down in the water and fish are trapped as they try to swim through. Unfortunately, dolphins and seabirds are often also snared in them.

Overfishing

Today, in many traditional fishing grounds, the numbers of fish are falling fast. This is because of overfishing: taking fish out of the sea faster than new fish can be born to replace them. Huge factory ships that use giant nets are the chief culprits. Ocean food webs may be upset or even destroyed. Catching too many sharks has led to many kinds being threatened with extinction.

Fish farming

The decline in fish stocks worldwide has led to a new way of supplying fish for food: fish farming. This is the raising of fish in cages or pens under water. Popular farmed species include salmon, carp, tilapia and catfish. Fish cages or pens are placed in the sea or estuaries. They contain and protect young fish until they can be harvested.