Like other animals, fish suffer from diseases and parasites. To prevent disease they have a variety of defenses.
Non-specific defenses include the skin and scales, as well as the mucus layer secreted by the
epidermis that traps and inhibits the growth of
microorganisms. If
pathogens breach these defenses, fish can develop an
inflammatory response that increases blood flow to the infected region and delivers
white blood cells that attempt to destroy pathogens. Specific defenses respond to particular pathogens recognised by the fish's body, i.e., an
immune response.
In recent years,
vaccines have become widely used in aquaculture and also with ornamental fish, for example
furunculosis vaccines in farmed
salmon and
koi herpes virus in
koi.
Some species use
cleaner fish to remove external parasites. The best known of these are the
Bluestreak cleaner wrasses of the genus
Labroides found on
coral reefs in the
Indian and
Pacific Oceans. These small fish maintain so-called "cleaning stations" where other fish congregate and perform specific movements to attract the attention of the cleaners.
Cleaning behaviors have been observed in a number of fish groups, including an interesting case between two cichlids of the same genus,
Etroplus maculatus, the cleaner, and the much larger
Etroplus suratensis.
Evolution
Outdated evolutionary view of continual gradation (animation)
Fish do not represent a
monophyletic group, and therefore the "evolution of fish" is not studied as a single event.
Proliferation of fish was apparently due to the hinged
jaw, because
jawless fish left very few descendants.
Lampreys may approximate pre-jawed fish. The first jaws are found in
Placodermi fossils. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors.
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.
Importance to humans
Economic importance
Main article:
Fish as foodMain article:
AquacultureMain article:
Fish farmingRecreation
Main article:
FishkeepingConservation
The 2006
IUCN Red List names 1,173 fish species that are threatened with extinction.
Included are species such as
Atlantic cod,
Devil's Hole pupfish,
coelacanths,
and
great white sharks.
Because fish live underwater they are more difficult to study than terrestrial animals and plants, and information about fish populations is often lacking. However, freshwater fish seem particularly threatened because they often live in relatively small water bodies. For example, the
Devil's Hole pupfish occupies only a single 3 by 6 metres (10 by 20 ft) pool.
Overfishing
Main article:
OverfishingOverfishing is a major threat to edible fishes such as cod and
tuna.
Overfishing eventually causes
population (known as
stock) collapse because the survivors cannot produce enough young to replace those removed. Such
commercial extinction does not mean that the species is extinct, merely that it can no longer sustain a fishery.
One well-studied example of fishery collapse is the
Pacific sardine Sadinops sagax caerulues fishery off the California coast. From a 1937 peak of 790,000 long tons (800,000 t) the catch steadily declined to only 24,000 long tons (24,000 t) in 1968, after which the fishery was no longer economically viable.
The main tension between
fisheries science and the
fishing industry is that the two groups have different views on the resiliency of fisheries to intensive fishing. In places such as Scotland, Newfoundland, and Alaska the
fishing industry is a major employer, so governments are predisposed to support it.
On the other hand, scientists and conservationists push for stringent protection, warning that many stocks could be wiped out within fifty years.
Habitat destruction
A key stress on both freshwater and marine ecosystems is habitat degradation including
water pollution, the building of
dams, removal of water for use by humans, and the introduction of
exotic species.
An example of a fish that has become endangered because of habitat change is the
pallid sturgeon, a North American freshwater fish that lives in rivers damaged by human activity.
Exotic species
Introduction of
non-native species has occurred in many habitats. One of the best studied examples is the introduction of
Nile perch into
Lake Victoria in the 1960s. Nile perch gradually exterminated the lake's 500
endemic cichlid species. Some of them survive now in captive breeding programmes, but others are probably extinct.
Carp,
snakeheads,
tilapia,
European perch,
brown trout,
rainbow trout, and
sea lampreys are other examples of fish that have caused problems by being introduced into a lien environments.
Aquarium collecting
Culture
In the
Book of Jonah a "great fish" swallowed
Jonah the
Prophet. Legends of half-human, half-fish
mermaids have featured in stories like those of
Hans Christian Andersen and movies like
Splash (See
Merman,
Mermaid).
Among the deities said to take the form of a fish are
Ika-Roa of the
Polynesians,
Dagon of various ancient
Semitic peoples, the shark-gods of
Hawaiʻi and
Matsya of the
Dravidas of India. The
astrological symbol
Pisces is based on a constellation of the
same name, but there is also a second fish constellation in the night sky,
Piscis Austrinus.
Fish have been used figuratively in many different ways, for example the
ichthys used by early
Christians to identify themselves, through to the fish as a symbol of fertility among Bengalis.
Fish feature prominently in art and literature, in movies such as
Finding Nemo and books such as
The Old Man and the Sea. Large fish, particularly sharks, have frequently been the subject of
horror movies and
thrillers, most notably the novel
Jaws, which spawned a series of films of the
same name that in turn inspired similar films or parodies such as
Shark Tale,
Snakehead Terror, and
Piranha.
In the
semiotic of
Ashtamangala (
buddhist symbolism) the golden fish (Sanskrit: Matsya), represents the state of fearless suspension in
samsara, perceived as the harmless
ocean, referred to as 'buddha-eyes' or 'rigpa-sight'. The fish symbolizes the auspiciousness of all living beings in a state of fearlessness without danger of drowning in the Samsaric Ocean of Suffering, and migrating from teaching to teaching freely and spontaneously just as fish swim.
They have religious significance in Hindu, Jain and Buddhist traditions but also in
Christianity who is first signified by the
sign of the fish, and especially referring to
feeding the multitude in the desert. In the
dhamma of Buddha the fish symbolize happiness as they have complete freedom of movement in the water. They represent fertility and abundance. Often drawn in the form of
carp which are regarded in the Orient as sacred on account of their elegant beauty, size and life-span.[3]
The name of the Canadian city of
Coquitlam,
British Columbia is derived from
Kwikwetlem, which is said to be derived from a
Coast Salish term meaning "little red fish".
Terminology
Fish or fishes
Though often used interchangeably, these words have different meanings.
Fish is used either as singular noun or to describe a group of specimens from a single species.
Fishes describes a group of different species.
Shoal or school
A random assemblage of fishes merely using some localised resource such as food or nesting sites is known simply as an
aggregation. When fish come together in an interactive, social grouping, then they may be forming either a
shoal or a
school depending on the degree of organisation. A
shoal is a loosely organised group where each fish swims and forages independently but is attracted to other members of the group and adjusts its behaviour, such as swimming speed, so that it remains close to the other members of the group.
Schools of fish are much more tightly organised, synchronising their swimming so that all fish move at the same speed and in the same direction. Shoaling and schooling behaviour is believed to provide a variety of advantages.
Examples:
- Cichlids congregating at lekking sites form an aggregation.
- Many minnows and characins form shoals.
- Anchovies, herrings, and silversides are classic examples of schooling fishes.
While school and shoal have different meanings within biology, they are often treated as
synonyms by non-specialists, with speakers of
British English using "shoal" to describe any grouping of fish, while speakers of
American English often using "school" just as loosely.
http://en.wikipedia.org/wiki/Fish
![Drawing of animal with large mouth, long tail, very small dorsal fins, and pectoral fins that attach towards the bottom of the body, resembling lizard legs in scale and development.[43]](http://en.wikipedia.org/wiki/File:Dunkleosteus_BW.jpg)
