A fish is any member of a paraphyletic group of organisms
that consist of all gill-bearing aquatic craniate animals that
lack limbs with digits. Included in this definition are the
living hagfish, lampreys, and cartilaginous and bony fish, as
well as various extinct related groups. Most fish
are ectothermic ("cold-blooded"), allowing their body
temperatures to vary as ambient temperatures change,
though some of the large active swimmers like white
shark and tuna can hold a higher core temperature. Fish are
abundant in most bodies of water. They can be found in
nearly all aquatic environments, from high mountain streams
(e.g., char andgudgeon) to the abyssal and
even hadal depths of the deepest oceans
(e.g., gulpers and anglerfish). At 32,000 species, fish exhibit
greater species diversity than any other group of vertebrates
Many animals that live in water are called
fish. Perch, crayfish, cuttlefish, jellyfish,
starfish, and even whales and dolphins all
live in water. Yet, of these animals, only the
perch is a true fish. Whales and dolphins are
warm-blooded mammals. The others
belong to the great group of animals
without backbones, called invertebrates.
The earliest organisms that can be classified as fish were soft-
bodied chordates that first appeared during the Cambrian period.
Although they lacked a true spine, they possessed notochords which
allowed them to be more agile than their invertebrate counterparts.
Fish would continue to evolve through the Paleozoic era, diversifying
into a wide variety of forms. Many fish of the Paleozoic
developed external armor that protected them from predators. The
first fish with jaws appeared in the Silurian period, after which many
(such as sharks) became formidable marine predators rather than just
the prey of arthropods.
Most fish exchange gases using gills on either side of the
pharynx. Gills consist of threadlike structures called filaments.
Each filament contains a capillary network that provides a
large surface area for exchanging oxygen and carbon
dioxide. Fish exchange gases by pulling oxygen-rich water
through their mouths and pumping it over their gills. In some
fish, capillary blood flows in the opposite direction to the
water, causing countercurrent exchange. The gills push the
oxygen-poor water out through openings in the sides of the
pharynx. Some fish, like sharks and lampreys, possess multiple
gill openings. However, bony fish have a single gill opening
on each side. This opening is hidden beneath a protective
bony cover called an operculum.
Early fish from the fossil record are represented by a group of
small, jawless, armored fish known as Ostracoderms. Jawless
fish lineages are mostly extinct. An extant clade,
the Lampreys may approximate ancient pre-jawed fish. The first
jaws are found inPlacodermi fossils. The diversity of jawed
vertebrates may indicate the evolutionary advantage of a
jawed mouth. It is unclear if the advantage of a hinged jaw is
greater biting force, improved respiration, or a combination of
Fish may have evolved from a creature similar to a coral-
like Sea squirt, whose larvae resemble primitive fish in important
ways. The first ancestors of fish may have kept the larval form
into adulthood (as some sea squirts do today), although
perhaps the reverse is the case.
Fishing is the activity of trying to catch fish. Fish are normally
caught in the wild. Techniques for catching fish includehand
gathering, spearing, netting, angling and trapping.
The term fishing may be applied to catching other aquatic
animals such as molluscs, cephalopods, crustaceans,
. The term is not normally applied to catching farmed fish, or
to aquatic mammals, such as whales, where the term whaling is
According to FAO statistics, the total number of commercial
fishermen and fish farmers is estimated to be 38 million.
fisheries and aquaculture provide direct and indirect employment
to over 500 million people. In 2005, the worldwide per capita
consumption of fish captured from wild fisheries was 14.4 kilograms,
with an additional 7.4 kilograms harvested fromfish farms. In
addition to providing food, modern fishing is also a recreational
There are many fishing techniques and tactics for catching fish. The
term can also be applied to methods for catching other aquatic
animals such as molluscs (shellfish, squid, octopus) and edible
Fishing techniques include hand
gathering, spearfishing, netting, angling and trapping. Recreational,
commercial and artisanal fishers use different techniques, and also,
sometimes, the same techniques. Recreational fishers fish for
pleasure or sport, while commercial fishers fish for profit. Artisanal
fishers use traditional, low-tech methods, for survival in third-world
countries, and as a cultural heritage in other countries. Mostly,
recreational fishers use angling methods and commercial fishers use
There is an intricate link between various fishing techniques and
knowledge about the fish and their behaviour
including migration,foraging and habitat. The effective use of fishing
techniques often depends on this additional knowledge. Some
fishermen follow fishing folklores which claim that fish feeding
patterns are influenced by the position of the sun and the moon.
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