This section of the Hyper book will consider how locations for oyster cultivation projects might or should be chosen. Good site selection is critical to the success of any aquaculture venture, and there are some obvious considerations.
The selection of a suitable site is crucial to the success or failure of an oyster farm. Growth and survival of oysters are influenced by a range of physical, biological and chemical factors including sea water temperature and salinity, water flow rate and phytoplankton content, exposure to air and wind, substrate type, predators, competitors and fouling organisms, dissolved nutrients, oxygen and pollutants. Many of these are subject to seasonal and annual variation and it is advisable to monitor the conditions at your prospective site for at least a year before any commercial culture begins and carry out a pilot study to see how well oysters grow and survive.
In the UK, oysters start to grow in the spring when sea water temperatures reach 8-9 oC. Growth rate reaches a maximum in July or August when temperatures peak (usually 16-18 oC) and then falls off again as the temperature drops to below 8-9 oC in November or December. Oysters can die in very cold winters if exposed to chill winds and air temperatures close to freezing. When exposed to the air, they close tightly to prevent desiccation of the internal tissues. They can respire an aerobically (i.e. without oxygen) when out of water but have to expel toxic metabolites when re-immersed as the tide comes in. To grow well, native oysters need sheltered areas with fully saline water (> 30 parts per thousand) and tidal flows of 1–2knots (50-100 cm sec –1). Plots low down the beach which allow access at spring tides for essential husbandry and maintenance are most suitable.
Oysters feed only when they are immersed, therefore for optimum growth they should be kept submerged as much as possible.
Most coastal sites have sufficient quantities of algae in the water to support cultivation. However, some species of algae can cause shellfish to accumulate bio toxins in their flesh. Routine testing is carried out to monitor bio toxin levels and once they exceed permitted values shellfish beds are closed (statutory or voluntary) and stock can no longer be harvested or offered for sale. The beds remain closed until two consecutive samples return values below the threshold levels. Such closures can adversely affect a business so this factor should be considered when selecting a site. Unfortunately, past track record (where available) can only offer limited guidance, it cannot guarantee that a problem will not occur in the future.
Also, sites near large urban and industrial developments are unsuitable for shellfish cultivation because of potential pollutants in the water.
Tri-butyl tin (TBT), a substance once used extensively in antifoulant paints, has been a particular problem for oyster growers. Although its use is now banned, TBT can remain in sediments for some time, therefore sites next to boatyards and marinas should generally be avoided.
Site access and ownership
Any cultivation site should be readily accessible for bringing gear on to the site and for transporting harvested clams away to market. Ownership of the area and its availability are important considerations in the initial site selection.
Many shellfish cultivation operations directly co-exist beside and even within designated environmentally sensitive areas including statutory sites such as Sites of Special Scientific Interest (SSSIs), Special Areas of Conservation (SACs),Special Protected Areas (SPAs) and Ramsar sites, as well as local voluntary sites. It is the co-existence with such sites that industry operations possibly face the greatest challenge. The Habitats Directive 92/43/EEC and the Birds Directive79/409/EEC make provision for the conservation of wildlife habitats and of birds through the designation of SACs and SPAs respectively. Designated areas can, and are encouraged, to include estuaries, shallow bays and coastal waters. Within such areas, cultivation practices are likely to be subject to local management plans.
As legislation on these and other aspects can be changed, it is wise to consult the appropriate regulating bodies for the most recent information.