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Fish farming | Wikipedia audio article


Fish farming or pisciculture involves raising
fish commercially in tanks or enclosures such as fish ponds, usually for food. It is the
principal form of aquaculture, while other methods may fall under mariculture. A facility
that releases juvenile fish into the wild for recreational fishing or to supplement
a species’ natural numbers is generally referred to as a fish hatchery. Worldwide, the most
important fish species produced in fish farming are carp, tilapia, salmon, and catfish.Demand
is increasing for fish and fish protein, which has resulted in widespread overfishing in
wild fisheries. China provides 62% of the world’s farmed fish. As of 2016, more than
50% of seafood was produced by aquaculture.Farming carnivorous fish, such as salmon, does not
always reduce pressure on wild fisheries. Carnivorous farmed fish are usually fed fishmeal
and fish oil extracted from wild forage fish. The 2008 global returns for fish farming recorded
by the FAO totaled 33.8 million tonnes worth about $US 60 billion.==Major species====
Categories==Aquaculture makes use of local photosynthetic
production (extensive) or fish that are fed with external food supply (intensive).===Extensive aquaculture===Growth is limited by available food, commonly
Zooplankton feeding on pelagic algae or benthic animals, such as crustaceans and mollusks.
Tilapia filter feed directly on phytoplankton, which makes higher production possible. Photosynthetic
production can be increased by fertilizing pond water with artificial fertilizer mixtures,
such as potash, phosphorus, nitrogen, and microelements.
Another issue is the risk of algal blooms. When temperatures, nutrient supply, and available
sunlight are optimal for algal growth, algae multiply at an exponential rate, eventually
exhausting nutrients and causing a subsequent die-off in fish. The decaying algal biomass
depletes the oxygen in the pond water because it blocks out the sun and pollutes it with
organic and inorganic solutes (such as ammonium ions), which can (and frequently do) lead
to massive loss of fish. An alternate option is to use a wetland system,
such as that used in the commercial fish farm Veta La Palma, Spain.
To tap all available food sources in the pond, the aquaculturist chooses fish species that
occupy different places in the pond ecosystem, e.g., a filter algae feeder such as tilapia,
a benthic feeder such as carp or [catfish, and a zooplankton feeder (various carps) or
submerged weeds feeder such as grass carp. Despite these limitations, significant fish
farming industries use these methods. In the Czech Republic, thousands of natural and semi-natural
ponds are harvested each year for trout and carp. The large Rožmberk Pond near Trebon,
built in 1590, is still in use.===Intensive aquaculture===
In these kinds of systems fish production per unit of surface can be increased at will,
as long as sufficient oxygen, fresh water and food are provided. Because of the requirement
of sufficient fresh water, a massive water purification system must be integrated in
the fish farm. One way to achieve this is to combine hydroponic horticulture and water
treatment, see below. The exception to this rule are cages which are placed in a river
or sea, which supplements the fish crop with sufficient oxygenated water. Some environmentalists
object to this practice. The cost of inputs per unit of fish weight
is higher than in extensive farming, especially because of the high cost of fish feed. It
must contain a much higher level of protein (up to 60%) than cattle feed and a balanced
amino acid composition, as well. These higher protein-level requirements are a consequence
of the higher feed efficiency of aquatic animals (higher feed conversion ratio [FCR], that
is, kg of feed per kg of animal produced). Fish such as salmon have an FCR around 1.1
kg of feed per kg of salmon whereas chickens are in the 2.5 kg of feed per kg of chicken
range. Fish do not use energy to keep warm, eliminating some carbohydrates and fats in
the diet, required to provide this energy. This may be offset, though, by the lower land
costs and the higher production which can be obtained due to the high level of input
control. Aeration of the water is essential, as fish
need a sufficient oxygen level for growth. This is achieved by bubbling, cascade flow,
or aqueous oxygen. Clarias spp. can breathe atmospheric air and can tolerate much higher
levels of pollutants than trout or salmon, which makes aeration and water purification
less necessary and makes Clarias species especially suited for intensive fish production. In some
Clarias farms, about 10% of the water volume can consist of fish biomass.
The risk of infections by parasites such as fish lice, fungi (Saprolegnia spp.), intestinal
worms (such as nematodes or trematodes), bacteria (e.g., Yersinia spp., Pseudomonas spp.), and
protozoa (such as dinoflagellates) is similar to that in animal husbandry, especially at
high population densities. However, animal husbandry is a larger and more technologically
mature area of human agriculture and has developed better solutions to pathogen problems. Intensive
aquaculture has to provide adequate water quality (oxygen, ammonia, nitrite, etc.) levels
to minimize stress on the fish. This requirement makes control of the pathogen problem more
difficult. Intensive aquaculture requires tight monitoring and a high level of expertise
of the fish farmer. Very-high-intensity recycle aquaculture systems
(RAS), where all the production parameters are controlled, are being used for high-value
species. By recycling water, little is used per unit of production. However, the process
has high capital and operating costs. The higher cost structures mean that RAS is economical
only for high-value products, such as broodstock for egg production, fingerlings for net pen
aquaculture operations, sturgeon production, research animals, and some special niche markets
such as live fish.Raising ornamental coldwater fish (goldfish or koi), although theoretically
much more profitable due to the higher income per weight of fish produced, has been successfully
carried out only in the 21st century. The increased incidences of dangerous viral diseases
of koi carp, together with the high value of the fish, has led to initiatives in closed-system
koi breeding and growing in a number of countries. Today, a few commercially successful intensive
koi-growing facilities are operating in the UK, Germany, and Israel.
Some producers have adapted their intensive systems in an effort to provide consumers
with fish that do not carry dormant forms of viruses and diseases.
In 2016, juvenile Nile tilapia were given a food containing dried Schizochytrium in
place of fish oil. When compared to a control group raised on regular food, they exhibited
higher weight gain and better food-to-growth conversion, plus their flesh was higher in
healthy omega-3 fatty acids.==Fish farms==
Within intensive and extensive aquaculture methods,numerous specific types of fish farms
are used; each has benefits and applications unique to its design.===Cage system===Fish cages are placed in lakes, bayous, ponds,
rivers, or oceans to contain and protect fish until they can be harvested. The method is
also called “off-shore cultivation” when the cages are placed in the sea. They can be constructed
of a wide variety of components. Fish are stocked in cages, artificially fed, and harvested
when they reach market size. A few advantages of fish farming with cages are that many types
of waters can be used (rivers, lakes, filled quarries, etc.), many types of fish can be
raised, and fish farming can co-exist with sport fishing and other water uses.Cage farming
of fishes in open seas is also gaining popularity. Given concerns of disease, poaching, poor
water quality, etc., generally pond systems are considered more simple to start and easier
to manage. Also, past occurrences of cage-failures leading to escapes, have raised concern regarding
the culture of non-native fish species in dam or open-water cages. On August 22, 2017,
there was a massive failure of such cages at a commercial fishery in Washington state
in Puget Sound, leading to the release of nearly 300,000 Atlantic salmon in non-native
waters. This is believed to risk endangering the native Pacific salmon species.Though the
cage-industry has made numerous technological advances in cage construction in recent years,
the risk of damage and escape due to storms is always a concern.Semi-submersible marine
technology is beginning to impact fish farming. In 2018, 1.5 million salmon are in the middle
of a year-long trial at Ocean Farm 1 off the coast of Norway. The semi-submersible US$300
million project is the worlds first deep-sea aquaculture project, and includes 61-meter
(200 ft)-high by 91-meter (300 ft)-diameter pen made from a series of mesh-wire frames
and nets, designed to disperse wastes better than more conventional farms in sheltered
coastal waters, and therefore, be able to support higher fish packing density.====Copper-alloy nets====Recently, copper alloys have become important
netting materials in aquaculture. Copper alloys are antimicrobial, that is, they destroy bacteria,
viruses, fungi, algae, and other microbes. In the marine environment, the antimicrobial/algaecidal
properties of copper alloys prevent biofouling, which can briefly be described as the undesirable
accumulation, adhesion, and growth of microorganisms, plants, algae, tube worms, barnacles, mollusks,
and other organisms.The resistance of organism growth on copper alloy nets also provides
a cleaner and healthier environment for farmed fish to grow and thrive. Traditional netting
involves regular and labor-intensive cleaning. In addition to its antifouling benefits, copper
netting has strong structural and corrosion-resistant properties in marine environments.
Copper-zinc brass alloys are deployed in commercial-scale aquaculture operations in Asia, South America,
and the USA (Hawaii). Extensive research, including demonstrations and trials, are being
implemented on two other copper alloys: copper-nickel and copper-silicon. Each of these alloy types
has an inherent ability to reduce biofouling, cage waste, disease, and the need for antibiotics,
while simultaneously maintaining water circulation and oxygen requirements. Other types of copper
alloys are also being considered for research and development in aquaculture operations.In
Southeast Asia, the traditional cage farming platform is called kelong.===Irrigation ditch or pond systems===These use irrigation ditches or farm ponds
to raise fish. The basic requirement is to have a ditch or pond that retains water, possibly
with an above-ground irrigation system (many irrigation systems use buried pipes with headers.)
Using this method, water allotments can be stored in ponds or ditches, usually lined
with bentonite clay. In small systems, the fish are often fed commercial fish food, and
their waste products can help fertilize the fields. In larger ponds, the pond grows water
plants and algae as fish food. Some of the most successful ponds grow introduced strains
of plants, as well as introduced strains of fish.
Control of water quality is crucial. Fertilizing, clarifying, and pH control of the water can
increase yields substantially, as long as eutrophication is prevented and oxygen levels
stay high. Yields can be low if the fish grow ill from electrolyte stress.====Composite fish culture====
The composite fish culture system is a technology developed in India by the Indian Council of
Agricultural Research in the 1970s. In this system, of both local and imported fish, a
combination of five or six fish species is used in a single fish pond. These species
are selected so that they do not compete for food among them by having different types
of food habitats. As a result, the food available in all the parts of the pond is used. Fish
used in this system include catla and silver carp which are surface feeders, rohu, a column
feeder, and mrigal and common carp, which are bottom feeders. Other fish also feed on
the excreta of the common carp, and this helps contribute to the efficiency of the system
which in optimal conditions produces 3000–6000 kg of fish per hectare per year.
One problem with such composite fish culture is that many of these fish breed only during
monsoon. Even if fish are collected from the wild, they can be mixed with other species,
as well. So, a major problem in fish farming is the lack of availability of good-quality
stock. To overcome this problem, ways have now been worked out to breed these fish in
ponds using hormonal stimulation. This has ensured the supply of pure fish stock in desired
quantities.===Integrated recycling systems===One of the largest problems with freshwater
pisciculture is that it can use a million gallons of water per acre (about 1 m3 of water
per m2) each year. Extended water purification systems allow for the reuse (recycling) of
local water. The largest-scale pure fish farms use a system
derived (admittedly much refined) from the New Alchemy Institute in the 1970s. Basically,
large plastic fish tanks are placed in a greenhouse. A hydroponic bed is placed near, above or
between them. When tilapia are raised in the tanks, they are able to eat algae, which naturally
grow in the tanks when the tanks are properly fertilized.The tank water is slowly circulated
to the hydroponic beds, where the tilapia waste feeds commercial plant crops. Carefully
cultured microorganisms in the hydroponic bed convert ammonia to nitrates, and the plants
are fertilized by the nitrates and phosphates. Other wastes are strained out by the hydroponic
media, which double as an aerated pebble-bed filter.This system, properly tuned, produces
more edible protein per unit area than any other. A wide variety of plants can grow well
in the hydroponic beds. Most growers concentrate on herbs (e.g. parsley and basil), which command
premium prices in small quantities all year long. The most common customers are restaurant
wholesalers.Since the system lives in a greenhouse, it adapts to almost all temperate climates,
and may also adapt to tropical climates. The main environmental impact is discharge
of water that must be salted to maintain the fishes’ electrolyte balance. Current growers
use a variety of proprietary tricks to keep fish healthy, reducing their expenses for
salt and wastewater discharge permits. Some veterinary authorities speculate that ultraviolet
ozone disinfectant systems (widely used for ornamental fish) may play a prominent part
in keeping the tilapia healthy with recirculated water.
A number of large, well-capitalized ventures in this area have failed. Managing both the
biology and markets is complicated. One future development is the combination of integrated
recycling systems with urban farming as tried in Sweden by the Greenfish Initiative.===Classic fry farming===
This is also called a “flow through system” Trout and other sport fish are often raised
from eggs to fry or fingerlings and then trucked to streams and released. Normally, the fry
are raised in long, shallow, concrete tanks, fed with fresh stream water. The fry receive
commercial fish food in pellets. While not as efficient as the New Alchemists’ method,
it is also far simpler and has been used for many years to stock streams with sport fish.
European eel (Anguilla anguilla) aquaculturalists procure a limited supply of glass eels, juvenile
stages of the European eel which swim north from the Sargasso Sea breeding grounds, for
their farms. The European eel is threatened with extinction because of the excessive catch
of glass eels by Spanish fishermen and overfishing of adult eels in, e.g., the Dutch IJsselmeer,
Netherlands. Although European eel larvae can survive for several weeks, the full life
cycle has not yet been achieved in captivity.==Issues==The issue of feeds in fish farming has been
a controversial one. Many cultured fishes (tilapia, carp, catfish, many others) require
no meat or fish products in their diets. Top-level carnivores (most salmon species) depend on
fish feed of which a portion is usually derived from wild-caught fish (anchovies, menhaden,
etc.). Vegetable-derived proteins have successfully replaced fish meal in feeds for carnivorous
fishes, but vegetable-derived oils have not successfully been incorporated into the diets
of carnivores. Research is underway to try to change this, such that even salmon and
other carnivores could be successfully fed with vegetable products. The F3 Challenge
(Fish-Free Feed Challenge), as explained by a report from Wired in February 2017, “is
a race to sell 100,000 metric tons of fish food, without the fish. Earlier this month,
start-ups from places like Pakistan, China, and Belgium joined their American competition
at the Google headquarters in Mountain View, California, showing off feed made from seaweed
extracts, yeast, and algae grown in bioreactors.” Not only do the feeds for carnivorous fish,
like certain salmon species, remain controversial due to the containment of wild caught fish
like anchovies, but they are not helping the health of the fish, as is the case in Norway.
Between 2003 and 2007, Aldrin et al., examined three infectious diseases in Norwegian salmon
fish farms–heart and skeletal muscle inflammation, pancreas disease, and infectious salmon anemia.
In 2014, Martinez-Rubio et al., conducted a study in which cardiomyopathy syndrome (CMS),
a severe cardiac disease in Atlantic salmon (Salmo salar), was investigated pertaining
the effects of functional feeds with reduced lipid content and increased eicosapentaenoic
acid levels in controlling CMS in salmon after infection with Piscine Myocarditis Virus (PMCV).
Functional feeds are defined as high-quality feeds that beyond purposes of nutrition, they
are formulated with health promoting features that could be beneficial in supporting disease
resistance, such as CMS. In choosing a clinical nutrition approach using functional feeds
could, potentially move away from chemotherapeutic and antibiotic treatments, which could lower
the costs of disease treatment and management in fish farms. In this investigation three
fishmeal-based diets were served–one made of 31% lipid and the other two made of 18%
lipid (one contained fishmeal and the other krill meal. Results demonstrated a significant
difference in the immune and inflammatory responses and pathology in heart tissue as
the fish were infected with PMCV. Fish fed with functional feeds with low lipid content
demonstrated milder and delayed inflammatory response and therefore, less severe heart
lesions at earlier and later stages after have PMCV infection.Secondly, farmed fish
are kept in concentrations never seen in the wild (e.g. 50,000 fish in a 2-acre (8,100
m2) area.). However, fish tend also to be animals that aggregate into large schools
at high density. Most successful aquaculture species are schooling species, which do not
have social problems at high density. Aquaculturists feel that operating a rearing system above
its design capacity or above the social density limit of the fish will result in decreased
growth rate and increased feed conversion ratio (kg dry feed/kg of fish produced), which
results in increased cost and risk of health problems along with a decrease in profits.
Stressing the animals is not desirable, but the concept of and measurement of stress must
be viewed from the perspective of the animal using the scientific method.Sea lice, particularly
Lepeophtheirus salmonis and various Caligus species, including C. clemensi and C. rogercresseyi,
can cause deadly infestations of both farm-grown and wild salmon. Sea lice are ectoparasites
which feed on mucus, blood, and skin, and migrate and latch onto the skin of wild salmon
during free-swimming, planktonic nauplii and copepodid larval stages, which can persist
for several days. Large numbers of highly populated, open-net salmon farms can create
exceptionally large concentrations of sea lice; when exposed in river estuaries containing
large numbers of open-net farms, many young wild salmon are infected, and do not survive
as a result. Adult salmon may survive otherwise critical numbers of sea lice, but small, thin-skinned
juvenile salmon migrating to sea are highly vulnerable. On the Pacific coast of Canada,
the louse-induced mortality of pink salmon in some regions is commonly over 80%. In Scotland,
official figures show that more than nine million fish were lost to disease, parasites,
botched treatment attempts and other problems on fish farms between 2016 and 2019.A 2008
meta-analysis of available data shows that salmon farming reduces the survival of associated
wild salmon populations. This relationship has been shown to hold for Atlantic, steelhead,
pink, chum, and coho salmon. The decrease in survival or abundance often exceeds 50%.Diseases
and parasites are the most commonly cited reasons for such decreases. Some species of
sea lice have been noted to target farmed coho and Atlantic salmon. Such parasites have
been shown to have an effect on nearby wild fish. One place that has garnered international
media attention is British Columbia’s Broughton Archipelago. There, juvenile wild salmon must
“run a gauntlet” of large fish farms located off-shore near river outlets before making
their way to sea. The farms allegedly cause such severe sea lice infestations that one
study predicted in 2007 a 99% collapse in the wild salmon population by 2011. This claim,
however, has been criticized by numerous scientists who question the correlation between increased
fish farming and increases in sea lice infestation among wild salmon.Because of parasite problems,
some aquaculture operators frequently use strong antibiotic drugs to keep the fish alive,
but many fish still die prematurely at rates up to 30%. Additionally, other common drugs
used in salmonid fish farms in North America and Europe include anesthetic, chemotherapeutic,
and anthelmintic agents. In some cases, these drugs have entered the environment. Additionally,
the residual presence of these drugs in human food products has become controversial. Use
of antibiotics in food production is thought to increase the prevalence of antibiotic resistance
in human diseases. At some facilities, the use of antibiotic drugs in aquaculture has
decreased considerably due to vaccinations and other techniques. However, most fish-farming
operations still use antibiotics, many of which escape into the surrounding environment.The
lice and pathogen problems of the 1990s facilitated the development of current treatment methods
for sea lice and pathogens, which reduced the stress from parasite/pathogen problems.
However, being in an ocean environment, the transfer of disease organisms from the wild
fish to the aquaculture fish is an ever-present risk.The large number of fish kept long-term
in a single location contributes to habitat destruction of the nearby areas. The high
concentrations of fish produce a significant amount of condensed faeces, often contaminated
with drugs, which again affects local waterways. Aquaculture not only impact the fish on the
farm, but it also involves environmental interactions with other species, which in return are attracted
or repelled by the farms. Mobile fauna, such as crustaceans, fish, birds, and marine mammals,
interact with the process of aquaculture, but the long-term or ecological effects as
a result of these interactions is still unknown. Some of these fauna may be attracted or demonstrate
repulsion. The attraction/ repulsion mechanism has various direct and indirect effects on
wild organisms at individual and population levels. The interactions that wild organisms
have with aquaculture may have implications on the management of fisheries species and
the ecosystem in relation to how the fish farms are structured and organized.However,
if the farm is correctly placed in an area with a strong current, the ‘pollutants’ are
flushed out of the area fairly quickly. Not only does this help with the pollution problem,
but water with a stronger current also aids in overall fish growth.
Concern remains that resultant bacterial growth strips the water of oxygen, reducing or killing
off the local marine life. Once an area has been so contaminated, the fish farms are moved
to new, uncontaminated areas. This practice has angered nearby fishermen.Other potential
problems faced by aquaculturists are the obtaining of various permits and water-use rights, profitability,
concerns about invasive species and genetic engineering depending on what species are
involved, and interaction with the United Nations Convention on the Law of the Sea.
In regards to genetically modified, farmed salmon, concern has been raised over their
proven reproductive advantage and how it could potentially decimate local fish populations,
if released into the wild. Biologist Rick Howard did a controlled laboratory study where
wild fish and GMO fish were allowed to breed. In 1989, the AquaBounty Technologies developed
the Aqua Advantage salmon. The concerns and critiques of cultivating this GMO fish in
aquaculture are that the fish will escape and interact with other fish ultimately leading
to the reproduction with other fishes. However, the FDA, has determined that while net pens
would not be the most appropriate to prevent escapes, that raising the salmon in Panama
waters would result effective in the prevention of escape because the water conditions there
would fail to support long-term survival of the salmon in the case that they escaped.
Another method of preventing Aqua Advantage fish from impacting the ecosystems in the
case they escape suggested by the FDA was to create sterile triploid females. This way
concerns on reproducing with other fishes would be out of the question. The GMO fish
crowded out the wild fish in spawning beds, but the offspring were less likely to survive.The
colorant used to make pen-raised salmon appear rosy like the wild fish has been linked with
retinal problems in humans.===Labeling===
In 2005, Alaska passed legislation requiring that any genetically altered fish sold in
the state be labeled. In 2006, a Consumer Reports investigation
revealed that farm-raised salmon is frequently sold as wild.In 2008, the US National Organic
Standards Board allowed farmed fish to be labeled as organic provided less than 25%
of their feed came from wild fish. This decision was criticized by the advocacy group Food
& Water Watch as “bending the rules” about organic labeling. In the European Union, fish
labeling as to species, method of production and origin, has been required since 2002.Concerns
continue over the labeling of salmon as farmed or wild-caught, as well as about the humane
treatment of farmed fish. The Marine Stewardship Council has established an Eco label to distinguish
between farmed and wild-caught salmon, while the RSPCA has established the Freedom Food
label to indicate humane treatment of farmed salmon, as well as other food products.==Indoor fish farming==
An alternative to outdoor open ocean cage aquaculture, is through the use of a recirculating
aquaculture system (RAS). A RAS is a series of culture tanks and filters where water is
continuously recycled and monitored to keep optimal conditions year round. To prevent
the deterioration of water quality, the water is treated mechanically through the removal
of particulate matter and biologically through the conversion of harmful accumulated chemicals
into nontoxic ones. Other treatments such as ultraviolet sterilization,
ozonation, and oxygen injection are also used to maintain optimal water quality. Through
this system, many of the environmental drawbacks of aquaculture are minimized including escaped
fish, water usage, and the introduction of pollutants. The practices also increased feed-use
efficiency growth by providing optimum water quality.One of the drawbacks to recirculating
aquaculture systems is the need for periodic water exchanges. However, the rate of water
exchange can be reduced through aquaponics, such as the incorporation of hydroponically
grown plants and denitrification. Both methods reduce the amount of nitrate in the water,
and can potentially eliminate the need for water exchanges, closing the aquaculture system
from the environment. The amount of interaction between the aquaculture system and the environment
can be measured through the cumulative feed burden (CFB kg/M3), which measures the amount
of feed that goes into the RAS relative to the amount of water and waste discharged.
The environmental impact of larger indoor fish farming system will be linked to the
local infrastructure, and water supply. Areas which are more drought-prone, indoor fish
farms might flow out wastewater for watering agricultural farms, reducing the water affliction.From
2011, a team from the University of Waterloo led by Tahbit Chowdhury and Gordon Graff examined
vertical RAS aquaculture designs aimed at producing protein-rich fish species. However,
because of its high capital and operating costs, RAS has generally been restricted to
practices such as broodstock maturation, larval rearing, fingerling production, research animal
production, specific pathogen-free animal production, and caviar and ornamental fish
production. As such, research and design work by Chowdhury and Graff remains difficult to
implement. Although the use of RAS for other species is considered by many aquaculturalists
to be currently impractical, some limited successful implementation of RAS has occurred
with high-value product such as barramundi, sturgeon, and live tilapia in the US, eels
and catfish in the Netherlands, trout in Denmark and salmon is planned in Scotland and Canada.==Slaughter methods==Tanks saturated with carbon dioxide have been
used to make fish unconscious. Their gills are then cut with a knife so that the fish
bleed out before they are further processed. This is no longer considered a humane method
of slaughter. Methods that induce much less physiological stress are electrical or percussive
stunning and this has led to the phasing out of the carbon dioxide slaughter method in
Europe.===Inhumane methods===
According to T. Håstein of the National Veterinary Institute, “Different methods for slaughter
of fish are in place and it is no doubt that many of them may be considered as appalling
from an animal welfare point of view.” A 2004 report by the
EFSA Scientific Panel on Animal Health and Welfare explained: “Many existing commercial
killing methods expose fish to substantial suffering over a prolonged period of time.
For some species, existing methods, whilst capable of killing fish humanely, are not
doing so because operators don’t have the knowledge to evaluate them.” Following are
some of the less humane ways of killing fish. Air asphyxiation amounts to suffocation in
the open air. The process can take upwards of 15 minutes to induce death, although unconsciousness
typically sets in sooner. Ice baths or chilling of farmed fish on ice
or submerged in near-freezing water is used to dampen muscle movements by the fish and
to delay the onset of post-death decay. However, it does not necessarily reduce sensibility
to pain; indeed, the chilling process has been shown to elevate cortisol. In addition,
reduced body temperature extends the time before fish lose consciousness.
CO₂ narcosis Exsanguination without stunning is a process
in which fish are taken up from water, held still, and cut so as to cause bleeding. According
to references in Yue, this can leave fish writhing for an average of four minutes, and
some catfish still responded to noxious stimuli after more than 15 minutes.
Immersion in salt followed by gutting or other processing such as smoking is applied to eel.===More humane methods===
Proper stunning renders the fish unconscious immediately and for a sufficient period of
time such that the fish is killed in the slaughter process (e.g. through exsanguination) without
regaining consciousness. Percussive stunning involves rendering the
fish unconscious with a blow on the head. Electric stunning can be humane when a proper
current is made to flow through the fish brain for a sufficient period of time. Electric
stunning can be applied after the fish has been taken out of the water (dry stunning)
or while the fish is still in the water. The latter generally requires a much higher current
and may lead to operator safety issues. An advantage could be that in-water stunning
allows fish to be rendered unconscious without stressful handling or displacement. However,
improper stunning may not induce insensibility long enough to prevent the fish from enduring
exsanguination while conscious. Whether the optimal stunning parameters that researchers
have determined in studies are used by the industry in practice is unknown.==Gallery====See also==Aquaculture of catfish
Aquaculture of salmonids Kelong

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