Fish

Fish are animals with backbones that live in water, breathe with gills and swim using fins instead of legs. There are more than 30,000 known kinds of fish, and people around the world raise and catch many of them for food, which makes fish one of the most important foods on Earth.

What Is a Fish?

Fish are a huge and varied group. It includes the jawless hagfish and lampreys, cartilaginous fish such as sharks and rays, and bony fish such as salmon, carp and goldfish. In fact, fish make up about half of all known vertebrates (animals with backbones).

Most fish are cold-blooded, or ectothermic, which means their body temperature changes along with the temperature of the water around them. A few fast swimmers, such as tuna and the great white shark, can keep parts of their bodies warmer than the water, which helps them swim powerfully.

Fish live in almost every kind of water, from cold mountain streams, home to fish such as char, to the deepest parts of the ocean. Snailfish have been filmed more than 8,000 meters (about 27,000 feet) below the surface.

Fish are also a valuable food for people. They are rich in protein and other nutrients, and oily fish such as salmon, mackerel and sardines contain omega-3 fatty acids, which are linked to a healthy heart and brain.

Fish Anatomy

Scales and Slime

Most fish are covered in scales that overlap like roof tiles and protect the fish from injury and infection. Some scales are rounded and smooth, while others have rough, toothed edges. A few fish, such as many catfish, have no scales at all, and pufferfish have scales that are changed into small spines.

A fish’s skin is also covered in a layer of slimy mucus. This slime traps germs before they can get into the body, and it helps the fish slip smoothly through the water.

Gills

Fish breathe with gills, feathery organs on each side of the head that are packed with tiny blood vessels. As water flows over the gills, oxygen passes into the blood and carbon dioxide passes out. In bony fish, the gills are protected by a hard flap called the operculum. The gills also help the fish get rid of waste, especially ammonia.

Swim Bladder

Many bony fish have a gas-filled bag inside their body called a swim bladder. By changing the amount of gas inside it, a fish can float at the depth it wants without having to keep swimming. Some fish even use their swim bladder to make sounds. Sharks and rays do not have a swim bladder. Instead, they rely on a large, oily liver and keep swimming to stay up.

Eggs and Babies

Fish have ovaries (females) or testes (males), and in some species the shape of a small tube near the vent, called the genital papilla, can show whether a fish is male or female. Most fish reproduce by spawning: the female releases eggs into the water and the male releases sperm to fertilize them outside the body. Sharks, skates and rays reproduce differently. Their eggs are fertilized inside the female’s body, and some lay egg cases while others give birth to live young.

Fish eggs, like the eggs of amphibians, are soft and jelly-like, with no hard shell. They would quickly dry out in the air, so they are usually laid in water. Some amphibians have found clever ways around this. The gray foam-nest tree frog (Chiromantis xerampelina) of Africa lays its eggs in a frothy foam nest on a branch hanging over water.

Fish People Raise and Eat

Carp

Carp are freshwater fish in the family Cyprinidae, one of the largest families of fish in the world. The common carp is native to Europe and Asia but has been spread by people to almost every part of the world. In places such as North America and Australia it is considered an invasive species because it muddies the water and crowds out native fish.

Common carp can live in many conditions, but they prefer large bodies of still or slow-moving water with soft, muddy bottoms and plenty of plants. Young carp often swim in small groups, while big adults tend to live alone.

Carp have been raised for food for thousands of years, especially in Asia and Europe. Two popular farmed varieties are the mirror carp, which has a few large, shiny scales, and the leather carp, which has almost none. The crucian carp is a separate, smaller species closely related to the goldfish.

In winter, carp slow down, rest near the bottom and stop eating. In spring, females lay many thousands of sticky eggs on plants in shallow water, and the eggs hatch in a few days. Carp grow quickly and can breed when they are about three years old. Most are around 1 to 2 feet (30 to 60 cm) long, but big carp can grow over 3 feet (1 m) and weigh more than 40 pounds (18 kg). They can live for decades.

Tilapia

Tilapia is the common name for about a hundred kinds of cichlid fish that come from Africa and the Middle East. They mostly live in fresh water, such as ponds, lakes, streams and rivers, though some can survive in slightly salty water. For thousands of years they have been caught by local fishers in Africa and the Levant, and today they are one of the most widely farmed fish in the world.

Tilapia need warm water and cannot survive long in cold water, so they rarely cause problems in places with cold winters. In warm places such as parts of Australia and the southern United States, though, escaped or released tilapia have become invasive.

A tilapia has a deep body that is flattened from side to side. Like other cichlids, it has a second set of jaws in its throat, called pharyngeal jaws. The front jaws grab food, and the throat jaws grind and process it, which makes tilapia very efficient eaters.

Because tilapia are so widely farmed, most tilapia sold today are hybrids of different species rather than pure wild types. Farmed tilapia sometimes have a muddy or musty taste. This comes from natural chemicals called geosmin and 2-methylisoborneol, which are made by blue-green algae (cyanobacteria) in the water. The taste does not mean the fish is unsafe or spoiled, but fish farmers check their fish to keep the flavor good.

Salmon

Salmon belong to the family Salmonidae, along with trout, char, whitefish and grayling. Atlantic salmon (genus Salmo) are native to rivers flowing into the North Atlantic, while Pacific salmon (genus Oncorhynchus) come from rivers flowing into the North Pacific. People have introduced salmon and trout to new areas such as Patagonia in South America and the Great Lakes of North America.

Salmon is a popular oily fish. It is rich in protein, vitamin D and omega-3 fatty acids. Huge numbers of salmon are now raised on fish farms. Some studies have found higher levels of certain pollutants in farmed salmon than in wild salmon, but these levels are usually well below safety limits, and major reviews have concluded that the health benefits of eating salmon outweigh the risks.

Catfish

Catfish (order Siluriformes) are a large group of ray-finned fish named for their barbels, the long feelers around the mouth that look like a cat’s whiskers. They have no scales. Some catfish have bare skin, while others are covered in bony armor plates.

Catfish come in an amazing range of sizes. Giants include the Mekong giant catfish of Southeast Asia and the wels catfish of Europe and Asia. At the other end are tiny species such as the candiru of South America. Many catfish feed on the bottom, eating whatever they can find, including dead plants and animals.

Many small catfish, especially Corydoras, are popular in home aquariums. Most catfish are active at night, though some feed during the day or at dawn and dusk.

Catfish have been farmed for food for many years in Africa, Asia, Europe and North America. In the United States, the channel catfish is the main farmed species. Catfish is a good source of protein, but it has less omega-3 than oily fish such as salmon.

Rohu

The rohu (Labeo rohita) is a large member of the carp family that lives in the rivers of South Asia. It eats both plants and small animals. Rohu can grow up to about 6 feet (2 m) long and weigh around 100 pounds (45 kg), though most are much smaller.

Rohu is a favorite food in Bangladesh, Pakistan, Nepal and many parts of India, including West Bengal, Odisha, Bihar, Assam and Uttar Pradesh. In some communities it is a traditional dish for weddings and other special occasions. It is also one of the most important farmed fish in South Asia.

Black Pomfret

The black pomfret (Parastromateus niger) is a flat, silvery-gray fish in the jack family (Carangidae). It is the only species in its genus. It lives in the warm coastal waters of the Indian Ocean and western Pacific, usually between about 50 and 350 feet (15 to 105 m) deep, and can grow to about 30 inches (75 cm) long. It is an important food fish in South and Southeast Asia.

Fish Farming

Fish farming, also called pisciculture, means raising fish in ponds, tanks or net pens, usually for food. It is the main form of aquaculture, the farming of water animals and plants. Farming fish and shellfish in the sea is called mariculture. Some farms, called hatcheries, raise young fish and release them into rivers and lakes to boost wild populations or for sport fishing. The most important farmed fish worldwide include carp, tilapia, salmon and catfish.

Fish farming is very old. People in China were raising carp in ponds more than 2,000 years ago. But it only became a huge modern industry from the 1970s onward. Today China produces more farmed fish than any other country, and aquaculture provides around half of all the fish people eat.

As the demand for fish grows, many wild fish populations have been overfished, and fish farms help meet the demand. However, farming meat-eating fish such as salmon does not always take pressure off the oceans, because these fish are often fed fishmeal and fish oil made from small wild fish.

Fish Around the World

• Most of the fish caught or farmed in the world is eaten by people. The rest is mainly turned into fishmeal and fish oil for animal feed and fish farms.
• Around 3 billion people get about a fifth of their animal protein from fish.
• People in developing countries, especially those living near coasts, often depend on fish much more than people in wealthier countries. In some countries, fish makes up a quarter or more of all the animal protein people eat.
• Asia eats the most fish of any continent, thanks to its large population and the high amount of fish each person eats.

How People Prepare Fish

People prepare fish in many delicious ways. It can be eaten raw, as in Japanese sashimi, marinated or cured, as in the Spanish and Latin American dish escabeche, or smoked, like smoked salmon. Herring is often pickled. Fish can also be grilled, steamed, baked, fried (as in British fish and chips) or gently poached in a flavored broth called court-bouillon.

Many of these methods, such as smoking, salting and pickling, began as ways to keep fish from spoiling before people had refrigerators. Today they are still popular because of the flavors and textures they create. Raw fish should only be eaten when it has been specially handled and prepared to be safe.

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