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Harvesting Fish

Harvesting Fish
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. It is used to stock streams with sport fish. Fish are fed external food supply.
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  • 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. It is used to stock streams with sport fish. Fish are fed external food supply.

  • Limiting for 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 mixture, such as potash, phosphorus, nitrogen and micro-elements.

  • Because most fish are carnivorous, they occupy a higher place in the trophic chain and therefore only a tiny fraction of primary photosynthetic 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. The aqua culturist will choose fish which occupy different places in the pond ecosystem, in order to tap all available food sources in the pond. For instance a filter algae feeder such as tilapia, a benthic feeder (various carps) or submerged weeds feeder such as carp. Despite these limitation 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. 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 treatment. The exception to this rule is cages which are placed in a river or sea, which supplements the fish crop with sufficient oxygenated water. 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.

  • 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. Catfish can breathe atmospheric 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.

  • When fish densities are high the risk of infections by parasites like fish lice, fungi, intestinal worms (such as nematodes), bacteria, and protozoa 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.

  • Raising ornamental cold water fish (goldfish), although theoretically much more profitable due to the hgher income per weight of fish produced, has never been successfully carried out until very recently. 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. 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.

  • Fish cages are placed in open water resources to contain and protect fish until they can be harvested. 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. Concerns of disease, poaching, poor water quality, etc. lead some to believe that in general, pond systems are easier to manage and simpler it start. Also past occurrences of cage-failures leading to escapes, have raised concern regarding the culture of non-native fish species in open-water cages. Even though the cage-industry has made numerous technological advances in cage construction in recent years, the concern for escapes remains valid.

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