Water motion is recognised as a prime factor in successful cottonii production. Consequently, the most viable cottonii farming sites can be viewed as a compromise between high water movement promoting rapid growth, and water movement that allows the plants and support structures to be retained without breakage. Water motion is caused by two factors: tidal current flow and the wind. Wind driven currents are weaker and generally considered less important than tidal currents, although wind-driven wave action can be very significant in promoting good cottonii growth. The most productive farming sites in other countries rely heavily on tidal currents, and in a few cases turbulent wave action also sustains a high productivity rate. Cottonii will grow at relative growth rates of 8% per day in the absence of any wind-driven wave action, relying solely on high tidal current flow.
Cottonii seaweed grows well in salinities ranging from 33 to 35 parts per thousand, and will tolerate even higher salinities. The species is intolerant of lower than normal salinities, which are generally rare on outer-island coral reef flats, occurring only for short periods when heavy rain coincides with a period of low water movement. Lower than normal salinities are more common in lagoon environments, and cottonii survival will be dependent on the proximity to flushing with oceanic water, and the duration of low salinity periods. Commercial growth rates are achieved in shallow water temperatures ranging from 24 degrees C to 30.5 degrees C. Plants can tolerate higher temperatures but the impact on growth rate has not been reported. It is suspected that temperatures above 30 degrees C, which in the eastern and central Pacific are likely to coincide with an EI Nino weather pattern, will have a negative impact on growth.
There is little published data on the nutrient requirements of cottonii seaweed. Nor is extensive seawater analysis of proposed farming sites seen as a necessary precursor to production trials. A more useful, and successfully proven approach, is to use cottonii plants themselves as a biological indicator of environmental conditions. From experience gained at other Pacific Island production sites, the necessary physical parameters can be assessed with a reasonable degree of certainty. Further confirmation of suitable physical and chemical environmental factors can then be established by measuring the cottonii growth rate at selected sites. A sustainable development is not dependent on a thorough prior knowledge of the environment. Expensive long-term physio-chemical monitoring of the environment may well provide valuable data, but it has not been essential to the establishment of sustainable cottonii farming at numerous sites in South East Asia, and in Kiribati.
The influence of biological factors, such as fish grazing, is the most difficult to predict, and can only be assessed with any certainty by extensive cottonii farming over an extended period of time. Certain Siganid fish species common in the Pacific can have a severe detrimental impact on cottonii farming. Where fish grazing is a problem, there may well be a production threshold that must be reached before the problem ceases to influence viability because of the large overall stock volume. In other cases, such as the first farming attempt in Vava'u, Tonga, in the 1980s, the grazing intensity is such that the production threshold is never attained with the off-bottom farming technique.
Any site selection must take into account the current location of village populations. Seaweed farms need permanent and regular care to achieve maximum returns to the farmer. Proposed sites for farm development that are within walking or cycling distance from existing population centres will provide higher net financial returns to the farmers than those sites that can only be accessed using canoes or and boats with outboard motors. There are also logistical and economic advantages in all the farmers from one village being located as close together as possible at a single site. If the initial farming activity is carefully controlled, the risks are minimised and the potential for a commercially viable production is increased. If a production base is established, later new entrant farmers can be free of controls, and can choose to farm at distant locations. Living for periods outside the village to farm seaweed then becomes a personal choice for those involved.
Cottonii has been successfully farmed using off-bottom constant-depth structures in a variety of reef environments from shallow flats of fringing reefs to sub-tidal flats of barrier reefs, to shallow flats associated with broken patch-reef formations. Over time floating rafts and floating long-line methods have developed, leading to successful farming in deeper water away from coral reef flats. The most consistent growth rate occurs when the plant remains submerged throughout the tidal cycle, but brief periods of tidal emergence during spring low tides are frequently tolerated in productive farming situations.
