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Pacific oyster

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Pacific oyster

Description

Oysters are delicious and good for you with a distinctive taste of the sea. Pacific oysters are a non-native species that has been introduced to the UK for aquaculture.  It is much more commonly found offered for sale than the native or flat oyster. Pacific oysters are more irregularly shaped than their flat native cousins, and the edges of the shell have distinctive wavy, large frills.In some areas this species has escaped captivity and has become an invasive species, they are sometimes sold as 'rock oysters'. Oysters have to be depurated (sterilized) before being eaten so self collection is not advised.

Sustainability Overview

There are now two main sources of Pacific oysters available to consumers. Wild caught and farmed. Pacific oysters have been commerially farmed on the banks of the Camel estuary since the 1960's. And historically Pacific oyster farming has also taken place in the Helford, Fal, Fowey and Tamar Estuaries. They are usually cultured in mesh sacks which are turned by hand each tide. The culture of oysters is low impact and doesn’t add pollution to the water. The disadvantage is the risk of oyster larvae (spat) escaping and colonising nearby natural habitats. In the Camel estuary this hasn't happened, possibly due to the sandy nature of the environment in this area preventing the settlement of spat. However, in Cornwall's south coast estuaries and bays Pacific oysters have now formed wild populations, and in some areas they significantly alter the nature of the shore creating sharp intertidal oyster reefs, and smothering intertidal rock. 

Some fishermen already collect feral oysters but report that there is currenly not a large market for them. The oysters themselves are irregular in shape and sometimes very large (up to 30cm long), and a product sometimes sold as a 'Rock oysters'. We hope that better markets can be found for these feral (wonky) oysters to enable the oyster populations in our estuaries to be controlled by hand collection. Oysters found growing on rocky reefs on the intertidal are flatter and harder to remove without damaging them therefore they cant be sterilised. An alternative use for these needs to be found in order to control populations.  

Sustainability ratings for this species

Farmed

Farmed on the shore in semi rigid plastic mesh cages.

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Hand Collection

Cornwall

Hand collection of feral oysters is one method to keep wild populations of this non native species under control. Wild oysters are much more variable in shape and can be very large. They must be depurated before consumption.

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How we rate fish

Cornwall Good Seafood Guide rates fish on sustainability using a scale of 1 to 5.

1, 2 and 3 are recommended, Fish to avoid are rated 5.

We use the system devised by the Marine Conservation Society (MCS) so our scores are comparable with the scores produced by MCS for the UK and fisheries from all around the world. For more information on scoring click here.

Biology

Oysters belong to the commercially important group of bivalve molluscs which also includes mussels, clams and cockles. The Pacific oyster, now widely distributed, originated in northeastern Asia. Pacific oysters begin life as males, but may change sex more than once during their lifetimes. Sexes are separate, but hermaphrodites sometimes do exist. In habitats with a high food supply the sex ratio in the adult population tends to favour females, whereas in areas with low food abundances it tends to have a larger proportion of males. 

Warm water is required for Pacific oysters to spawn (usually taken to be about 20 degrees C), and they are very fecund, with females releasing anything up to 200 million eggs in a single spawning. The males release sperm, and the eggs are fertilised in the water column, developing into various stages of larvae before finally settling onto the bottom. Once the larva finds a suitable habitat it attaches to it permanently using cement secreted from a gland in its foot. It then metamorphoses into a juvenile ‘spat’. Growth can then be very rapid. Cultivated Pacific oysters are kept in special containers and need to be cleansed and purified prior to human consumption. But in the wild, the Pacific oyster, which can occur intertidally, often attaches itself to rocks and cannot be removed without breaking the lower shell; it cannot therefore be cleansed and purified. They also tend to settle on each other and rapidly form reefs, even in areas that were once just mud, and can thus completely transform the local habitat. 
 
Oysters have to be depurated before they are fit for human consumption. As filter feeding animals they collect plankton from the water around them as their main food source. Particularly during summer months there is a risk that toxic algae is concentrated in the gills of oysters. In the depuration process oysters are kept in recirculating seawater tanks, sterilized by Ultraviolet light for several days so that they are able to metabolise and excrete any potential toxins and bacteria. Always make sure that the oysters you eat have been properly depurated. 

Stock Info

Farmed oysters are bred in hatcheries and then grown on in the sea - usually in semi-rigid plastic mesh bags, supported by steel trestles secured in intertidal waters. Pacific oyster producers rely on spat from oyster hatcheries which have been manipulated (making them triploid - having three sets of dna) which ensures that they should not spawn in the wild although some escapees have been found in recent years in Cornish estuaries, where they appear to now have self sustaining populations. Research currently being carried out by Natural England, Cornwall Wildlife Trust and South Devon ANOB Estuaries Partnership, and funded by European and Marine Fisheries Fund is showing that feral populations are far greater than expected in Devon and Cornwall with numbers have exploded in recent years. Currently fishing levels on this species are minimal. Commercial collection is a potential method of controlling this species. 

Management

Marine aquaculture is regulated by DEFRA and environmental health. Although they spawn in summer farmed oysters are available all year round as after harvesting they are carefully depurated to remove impurities and toxic algae. 

Capture Info

Farmed on the shore often on steel tressels in semi rigid plastic mesh cages. 

Hand collected. 

Where to buy

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