Oyster Farming For Dummies

Oyster Farming For Dummies

How Is Oyster Farming Done?

LIFE CYCLE OF NATIVE OYSTERS

Native oysters are described as protandrous alternating hermaphrodites. This means that when they reach maturity, they function first as a male and then they go into alternate cycles of female and male stages for the rest of their life. The number of cycles each year depends on the length of the breeding season, so in the UK native oysters usually spawn twice during the summer, once as a male and once as a female.

A minimum water temperature of 16oC is required before they spawn. The males release sperm into the surrounding sea water which is taken in through the inhalent siphon of the female. The eggs produced by the female are fertilised inside the parent’s shell and the larvae are brooded within the mantle cavity until they have a fully-formed shell (at around 0.170 mm). This usually takes about 10 days. The parent oyster releases the larvae into the sea water where they drift in the plankton and feed on natural phytoplankton. The number of larvae released is related to the size of the parent oyster.

Also Read: How do I set up an oyster farm?

After 2 to 3 weeks, depending on local environmental conditions, the larvae are mature and they develop a foot. At this stage, they are called pediveligers. They sink to the seabed and explore the sediment surface with their foot until they find a suitable surface on which to settle permanently. The pediveligers cement firmly to a hard surface such as shell or stone. Next, they go through a series of morphological and physiological changes, a process known as metamorphosis(which takes 3 to 4 days), to become ‘immature’ adults called spat (or seed).

LIFE CYCLE OF PACIFIC OYSTERS

As a species not native to the UK, the Pacific oyster seldom reproduces naturally in the wild. Sea water temperatures around the UK are generally too low to support maturation or successful larval development and settlement. All juvenile oysters used for culture are produced by specialist hatchery facilities.

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Pacific oysters are alternating hermaphrodites. On maturity, they can function as either a male or female. The following season, they change sex and then have alternate cycles of female and male stages for the rest of their life. However, a few remain as true hermaphrodites, acting as both male and female at the same time.

In their native environments, Pacific oysters release eggs and sperm into the sea water where fertilisation occurs. The larvae are plank tonic, drifting in the surface layers for approximately 2 to 3 weeks before they sink to the bottom to take up a sessile lifestyle attached to shell, stones or rocks.

How Do I Set Up An Oyster Farm?

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.

Also Read: How long does it take to grow oysters?

SITE SELECTION

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.

Algal Situation

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.

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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.

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