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Fish farming

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File:Aquaculture-demonstration facility.jpeg
A demonstration aquaculture facility

Fish farming is the principal form of aquaculture, while other methods may fall under mariculture. It involves raising fish commercially in tanks or enclosures, usually for food. 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. Fish species raised by fish farms include salmon, catfish, tilapia, cod and others.

Basics of fish production

Basically, there are two kinds of aquaculture: extensive aquaculture based on local photosynthetical production and intensive aquaculture, in which the fishes are fed with external food supply. The management of this two kinds of aquacultural systems is completely different. See also: Principles of fish production

Extensive (pond) aquaculture

Limiting for fish growth here is the available food supply by natural sources, commonly zooplankton feeding on pelagic algae or benthic animals, such as certain crustaceans and mollusks. Tilapia species filter feed directly on phytoplankton, which makes higher production possible. The photosynthetical production can be increased by fertilizing the pond water with artificial fertilizer mixtures, such as potash, phosphorus, nitrogen and micro elements. Because most fishes are carnivorous, they occupy a higher place in the trophic chain and therefore only a tiny fraction of primary photosynthetical production (typically 1%) will be converted into harvestable fish.

As a result, without additional feeding the fish harvest will not exceed 200 kilograms of fish per hectare per year, equivalent to 1% of the gross photosynthetic production.

A second point of concern is the risk of algal blooms. When temperatures, nutrient supply and available sunlight are optimal for algal growth, algae multiply their biomass at an exponential rate, eventually leading to an exhaustion of available nutrients and a subsequent die-off. The decaying algal biomass will deplete the oxygen in the pond water and pollute it with organic and inorganic solvents (such as ammonium ions), which can (and frequently do) lead to massive loss of fish.

In order to tap all available food sources in the pond, the aquaculturist will choose fish species which 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.

Sources:

  • Introduction to Aquaculture, college notes, Department of Aquaculture, Wageningen University
  • Aquaculture: training manual, second edition, Donald R. Swift, ISBN 0-85238-194-8

Intensive (closed-circulation) aquaculture

In this kind 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. A clever way to achieve this is the combination of 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 fresh water. 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 food, which must contain a much higher level of protein (up to 60%) than, e.g., cattle food and a balanced amino acid composition as well. This frequently is offset by the lower land costs and the higher productions which can be obtained due to the high level of input control.

Essential here is aeration of the water, as fish need a sufficient oxygen level for growth. This is achieved by bubbling, cascade flow or liquid oxygen. Catfish, Clarias ssp. can breathe atmospherical air and can tolerate much higher levels of pollutants than, e.g., 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.

Especially when fish densities are high, the risk of infections by parasites like fish lice, fungi (Saprolegnia ssp., intestinal worms, such as nematodes or trematodes, bacteria (e.g., Yersinia ssp, Pseudomonas ssp. and protozoa (such as Dinoflagellates) is much higher than in animal husbandry because of the ease in which pathogens can invade the fish body (e.g. by the gills). The same holds for water pollution or depletion of oxygen in the water, which can ruin a fish crop within minutes. This means, intensive aquaculture requires tight monitoring and a high level of expertise of the fish farmer.

Systems of fish farming

The main systems of fish farming are treated below.

Integrated recycling systems

One of the largest problems with freshwater aquaculture 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 Alchemists 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 grows in the tanks when the tanks are properly fertilized.

The tank water is slowly circulated to the hydroponic beds where the tilapia waste feeds a commercial crop such as parsley. 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 doubles 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, 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 waste water 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.

Reference: Freshwater Aquaculture: A Handbook for Small Scale Fish Culture in North America, by William McLarney

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). This is a low-investment way to produce fish from an existing structure. Often the fish sell for premium prices since they are fresh, and produced inland. If the ponds raise sport species, they can be advertised as "fishing ponds," and access can be sold directly to fishermen.

Using this method, one can store one's water allotment 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. Salting the water is not recommended because it can salinize the fields. Yields can be low if the fish grow ill from electrolyte stress.

Cage system

These use synthetic fiber cages in existing water resources. The advantage is that many types of water can be used (rivers, lakes, filled quarries, etc.), many types of fish can be raised, and the fish farming can co-exist with sport fishing and other forms of use. However, fish are vulnerable to disease, poaching, and low levels of dissolved oxygen. In general, pond systems are easier to manage, and simpler to start.

Classic fry farming

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 pelletized fish food. 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. As per 2005, no one managed to breed the European eel in captivity.

Controversy

In most areas where large-scale fish farming has been established, controversy has followed. Critics say that fish farms generate high levels of disease and parasites that infect wild fish stocks inhabiting water near the farms. For example, wild salmon runs in Norway and Scotland have seen large increases in sea lice infections after the introduction of fish farms nearby. A study published March 2005 in the scientific journal, Proceedings of the Royal Society B, shows that the transfer of parasitic sea lice from British Columbia salmon farms to wild salmon populations is much larger and more extensive than previously believed. The British Columbian and Canadian governments are allowing the expansion of open net cage salmon farming, arguing that recent studies can be interpreted in such a way that the high sea lice counts in wild salmon are not the fault of fish farms.

Supporters of fish farms argue that sea lice are a naturally occurring species, and that they are found naturally on wild salmon, sticklebacks, and herring. Pacific salmon species have lived with sea lice for centuries, though not in the numbers now being seen, and have developed a natural tolerance to this parasite. Sea lice production is influenced, not only by the presence of fish farms, but by many other factors such as high water temperature and high salt content. Using these facts, the industry and government pro fishfarm side of the controversy argues that fishfarm expansion can continue while the relationships of sea lice infections among farmed and wild salmon are studied further and, because of concerns in British Columbia, the Canadian government through the Department of Fisheries and Oceans has undertaken a number of research projects in the area.

Opponents of open net fishfarms want a moratorium on expansion and in some cases fallowing of existing farms, in order to protect BC wild salmon, based on the current science. "There is no doubt that salmon farms producing sea-lice are the major contributing factor to the sea-trout collapse." - Dr. Paddy Gargan, Senior Research Officer for Ireland's Central Fisheries Board 2003. "In the area without salmon farms, no heavy salmon lice infections were recorded." "In two fiords with salmon farms 48.5% to 86% of wild Atlantic salmon postsmolts were killed as a direct consequence of sea lice infections. - P Bjorn, B. Finstad and R. Kristofferson 2001 and Institute of Marine Research, Bergen. "It should be accepted "beyond reasonable doubt" that sea lice from salmon farms are killing sea trout returning to Scotland. - Professor David McKay, regional director of the Scottish EPA, 1999.

The northcoast of BC is fishfarm free but there is a battle raging over the Skeena River, the second largest commercial salmon river in Canada, with millions of wild salmon and world renowned for its steelhead. Industry and government are proposing open net farms for the Skeena river estuary in the wild Skeena salmon outmigration routes. The local people, including First Nations, rely heavily on wild salmon and are overwhelmingly opposed to these Skeena estuary fishfarms. See Save Our Skeena Salmon.

Other risks of fish farms include the introduction of foreign fish species into habitats where they have not previously been known. As an example, Atlantic salmon are farmed in Pacific waters off the coast of British Columbia. Some of these Atlantic salmon have escaped and it is feared they will create populations that will compete with already threatened Pacific salmon stocks for resources.

The DFO’s Atlantic Salmon Watch Program shows that there have been cases of escaped farmed Atlantic salmon surviving and then breeding with other escaped salmon in BC streams. It is not yet clear if there are any established Atlantic salmon populations. Some studies have shown that farmed salmon have a low survival rate in the wild because they are used to being fed. Throughout the 20th century, millions of Atlantic salmon were released throughout the west coast of the United States and Canada in an attempt to establish recreational and commercial fisheries. These attempts were not successful. However in the 21st century many runs of wild salmon are weaker and so the Atlantics may be finding it easier to establish. See Department of Fisheries and Oceans.

Proponents of fish farms also suggest that farming salmon reduces fishing pressure on existent wild stocks, and so allows ecosystems to replenish themselves over time. Countering this argument, fishfarm opponents point out that the BC and Alaska wild salmon commercial fishery is controlled by wild stock assessment(regardless of fishfarm production) and other safe forms of aquaculture like Tilapia or trout farming can be used as well to supply fish to the market.

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