Concrete or fiberglass tanks

1. Shapes of the tank are often circular, rectangular or oval where enters through nozzles in a manner that creates a rotary circulation within the tank and discharge occur through tank center by standpipe or bottom drain surrounded by screen. Draining out is designed through a central outlet comprising a standpipe surrounded by screen. Typically circular tanks is 4 m in diameter with a water depth of 0.75 m. the inflow of water is about 4 l/sec. that enables average stocking densities of 200 kg. Tank hygiene seems to be superior due to self cleaning of the central limit in practice that realizes the water vortex created by flow rates which may be strong for the fish. Usually maximum stocking rates in tanks where the water is changed every 1-2 hour would around 25-50 kg/m3. With aeration this could be as high as 150 kg/m3.

The fish are fed complete feed, usually 30-35% crude protein for fish over 50 g average weight. Security is much easier with a tank system because production is concentrated on a small site, which can completely fence in.

4. Floating cages, pens and enclosure:

Floating cages, modern cages are floating structures with a net suspended below. They may be square, rectangular or round. Floating cages may be small and of limited strength or they may be many thousands of cubic metres in volume and designed for use in the open water. Cages are used for fish culture in their grow-out phase, that is the months or years up to their market size.

Pens and Net enclosures (hapas) are used in shallow water, typically in ponds, to create a restricted environment for culture of fish and some crustaceans. They are not usually large, being in the order of tens of square metres or less. The walls of the enclosures may be closely spaced stakes, such as bamboo stems or mangrove branches, or wire and other mesh. This system of culture is practised mainly in developing countries. One interesting exception to the shallow water pen is the use of mesh fences or walls to enclose bottom dwelling scallops. These pens are of sufficient height to prevent the scallops from swimming over the wall. They may use floats to allow the mesh to rise and fall with the tide.

 5. Net enclosure (hapas) usually made from fine nylon, plastic mosquito netting or cotton mesh. Haps are very easy to manage; because fry cannot escape harvesting is much easier. Hapas can be any manageable size from 1-40 m3 with a depth of 1-2 m and suspended on poles. They can be simple squares or rectangles. There are also more complex designs, some with a series of nets separating brood-stock from fry.

Usual broodstock densities are 2-7 fish/m3, male:female sex ratio of 1:2 to 1:7.

Exercise 51. Test yourself by answering these questions and summarize given information.

Text C

 Water exchange

 

Read the text and translate. Be ready to discuss.

According to water exchange there are two main systems:

Static systems

Much global aquaculture production uses traditional pond culture methods.

These ponds are static, with no exchange of water during the culture period. There may be some topping up to offset evaporation. Static pond culture is usually extensive because of major problems in maintaining water quality under conditions of a large biomass of cultured fish per unit volume of static water. Increasing biomass requires increasing inputs of fertilizers and supplementary feeds to maintain productivity. This, in turn, requires management for such water quality problems as unacceptable levels of nitrogen compounds and low DO levels at night. With supplementary aeration it may be possible to maintain DO with a higher biomass and achieve greater productivity. Aerators are, however, often not available or feasible in rural regions where static pond culture is employed.

Open systems

Open system is the use of the environment as fish farm (e.g. cages), i.e. the

culture organisms are confined or protected within the farm in a vast amount of

water (e.g. a lake or an ocean) so that water quality is maintained by natural flows and processing. There is no artificial circulation of water through or within the system. Cage system classified as open systems when they are placed within a large body of water such as an ocean or an estuary. In these cage systems the fish are generally at high density and artificial feed is supplied. Water quality is, however, maintained by natural currents and tides. Therefore, these are intensive open systems. Open systems tend to have low operating costs, as there is no requirement for pumping. Capital costs vary greatly depending on the type of culture. Seasonal variation in environment result in large variation in growth rate and this is the most disadvantages of ope system.