Stimulating investment in pearl farming in ... - World Fish Center

5.2. Stocking densityPage 16Mature pearl oysters filter water at the rate of approximately 20 litres per hour, or 480 litresper day. If the oysters are not sufficiently separated they will collectively strip the water ofoxygen and nutrients and ingest excessive quantities of each other’s waste products. Aregulatory goal should be to ensure that stocking densities do not exceed one oyster per cubicmetre within the water volume occupied by the farm.On a typical pearl farm, shell are hung on ‘chaplets’, which are vertical ropes of 2 – 5 metresin length, and carrying 10 – 20 shell, either drilled through the hinge and tied directly ontothe chaplet, or in panel nets attached to it. Chaplets are strung along a horizontal mainlinewhich may be up to 220 metres in length (this being the standard length of a coil of rope). Inoperational terms, achieving a stocking density of one shell per cubic metre of water wouldrequire the following:• a minimum spacing of 10 cm between shells on a chaplet;• chaplets spaced along the mainline at minimum intervals of one metre;• mainlines spaced from each other at a distance of 10 metres, and preferably 20metres or more.Based on these minimum requirements a typical farm might be stocked as follows:• 100 mainlines each of 220 metres length, spaced 10 metres apart;• 220 chaplets hanging from each mainline, spaced at 1m intervals – total 22,000chaplets;• each chaplet 2 metres long, and carrying 20 shell spaced at 10 cm intervals – total440,000 shell.The volume occupied by the farm would be 440,000 m 3 (1000 m x 220 m x 2 m) giving therequired volume:shell ratio of one shell per cubic metre of water. Greater spacing betweenshells, chaplets or mainlines would further improve (i.e., lower) the stocking density.To facilitate monitoring, stocking density regulations should be kept relatively simple. Inparticular no distinction should be made between spat and adult oysters – the samerequirement of no more than one shell per cubic metre should apply irrespective of the oysterlife stage. In order to minimise the impact of any disease or health problems should theyarise, farm sites should be restricted to a maximum of one million shell held under culture atany one location. Shell in excess of this number should be held at a separate location andsubject to the provisions of a separate licence.5.3. Farm size and locationThe farm site described in the preceding section would occupy a surface area of 1,000m x220 m = 220,000 m 2 , or 22 hectares. However a more usual farming arrangement wouldinvolve the separation of shell into several areas (for examples, zones containing newly-hungspat, pre-operative shell, and seeded oysters) within the main farm site. Greater mainlinespacing, which should be encouraged, would add further to the need for space, as also wouldinstallations such as over-water seeding sheds. As a result a typical farm site would require alease area of at least 100 – 150 ha. To reduce the risk of disease outbreaks spreading fromone holding to another, regulations should stipulate that farm sites must be separated by atleast five kilometres of water.Most well-run farming sites are situated at least 100 m from adjacent reefs and in water atleast 25 m deep, with sediment rather than live corals beneath. These practices distance theshell from benthic predators and parasites and allow free water exchange below the hanging