Open-water aquaculture includes mollusc culture in shallow salt- and fresh-water areas, seaweed farming in coastal seas and pen and cage culture in sea and fresh-water bodies. As is obvious, in selecting sites for such systems of culture the main considerations are the hydro-graphic and climatic conditions. In spite of some limited success in extending certain types of aquaculture to deeper and more exposed coasts, the most suitable and preferred areas continue to be sheltered bays, estuaries, lagoons, straits, lakes and reservoirs, protected from strong winds and rough seas. While moderate currents and water flows are necessary to maintain water quality and removal of waste products from farm sites, frequent storms and turbulent seas will make it difficult to practise most types of aquaculture. Winds will directly affect culture installations above water, whereas waves affect both the sub-merged structures and the animals under culture. In most cases low current velocities are preferred.
In systems like the ones for bottom culture of molluscs, the nature of the sea or river bed is important. Suitable stable substrates are needed for the attachment of the animals. Most modern open-water culture is of the off-bottom type, where the water conditions and quality are more important.
Since mollusc culture is based largely on natural food organisms that the molluscs filter from the environment, it is essential to select sites with high primary production. Though some experimental work has been done on artificial feeding of certain molluscs, in commercial farming production is dependent on the growth of plankton or algae. In order to make natural food available to the animals the current velocity should not exceed 5 cm/s.
Even though controlled reproduction and hatchery production of seed are possible in mollusc farming, in many places aquaculturists depend on wild spat for culture. In such cases, it is advisable to select sites where there is an abundance of spat. A breeding population of the species nearby is, of course, necessary, but it does not necessarily follow that the spat will settle in the immediate neighbourhood. The larvae may be carried away by currents, so sufficient shelters and suitable current speeds are necessary to keep the larvae in the area. Field observations, supplemented by experimental spat setting, may be a necessary basis for a decision on site suitability.
In the farming of seaweed such as laver fertilizers are used to increase growth, but naturally fertile areas are still selected as in open-water situations fertilization can only be a complement to natural productivity. Movement of water prevents the increase of pH which can be caused by the consumption of carbon dioxide in seaweed-growing areas. Therefore it is necessary to select sites with an adequate current. A current of about 10– 30 cm/s is considered suitable, depending on the content of nutrients in the water. Waters deficient in nutrients should have a current of 30 cm/s and those rich in nutrients about 10 cm/s. Since periodic exposure of leafy thalli is important for growth in some seaweeds, it is necessary to select a place with a tidal range of 1–1.5m or more.