Shellfish
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Shrimp/prawn farming
The names shrimp and prawn are sometimes used interchangeably, however there
are differences between the two. Shrimp are generally smaller with a narrow
pointed rostrum (tail end of the body) and a sculptured or spiny carapace
(shell). Farmed prawns represent a third of the global harvest, with developing
countries accounting for 99% of production. However, prawn farming has a high
environmental cost, with loss of wetland and mangroves, degradation of habitats,
pollution and depletion of wild fish stocks.
Prawns are raised in ponds or enclosures until they reach harvestable size.
There are three categories of farm: extensive/traditional; semi-intensive
and intensive. The Environmental Justice Foundation advises consumers to only
buy shrimp/prawns with recognised, credible, environmental, Fair Trade and
organic labels. For more information see: http://www.ejfoundation.org/
Oysters
 There
are two species of oyster farmed in the UK: the European Flat or native oyster,
Ostrea edulis and the Pacific cupped oyster, Crassostrea gigas.
Native oysters are virtually all dredged from wild stocks and some of these
may be re-laid to grow on more sheltered inshore beds until they reach marketable
size. The Pacific oyster was introduced in the 1970s. They are bred in hatcheries
and then grown to market size by: 1. Placing them in plastic mesh bags fastened
onto steel or timber trestles with rubber bands and submerged; 2. Placing
them in elongated cages suspended between a series of poles submerged in seawater
during high tide or 3. By re-laying larger oysters loose onto the seabed where
there is firm gravel ground. The best farming areas are sheltered sites where
some mixing of marine and fresh water occurs.
Mussels
 Mussels
are the most common species of shellfish farmed in the UK. In England and
Wales, the mussel spat are collected from naturally occurring ephemeral beds
and re-laid in sites in more sheltered inshore waters for them to grow to
marketable size. In Scotland, mussels are predominantly rope grown which entails
suspending ropes from floating buoys in areas that have naturally occurring
mussels in the water. The rope provides a substrate for spat attachment, where
they continue to grow until reaching harvestable size, when they are collected
by hand from the ropes.
Other farmed shellfish
There are other species of shellfish farmed within the UK and Ireland, though
not in the same numbers as mussels and oysters. These include scallops, abalone,
carpet shells, sea urchins and cockles. Scallops are farmed by collecting
spat from the wild and growing them to marketable size either in special bags
or by relaying them on the seabed. Harvesting is normally carried out by divers
using hand gathering, his method of collection ensures no damage to the seabed
or other marine organisms occurs.
Water quality
Water quality is very important for the production of shellfish and is regulated
by two directives. The EC Shellfish Waters Directive (79/923/EEC) seeks to
protect or improve shellfish waters in order to support shellfish life and
growth and thus contribute to the high quality of shellfish products directly
edible by man.
The EC Shellfish Hygiene Directive (91/492/EEC) monitors the suitability of
all shellfish for human consumption. Harvesting sites are monitored and classified
in terms of concentrations of coliform bacteria and salmonella. Sites are
graded from A to C depending on how much purification (depuration) the shellfish
require before consumption.
Environmental impacts
As shellfish require no feed inputs and no chemical treatments the environmental
impacts of farming mussels, oysters and other shellfish in the UK are minimal,
as long as the shellfish are harvested in such a way as to not damage the
seabed, i.e. avoiding the use of dredges especially in sensitive marine areas.
The only output from shellfish is pseudofaeces, a waste product generated
by filter feeders; this can accumulate on the seabed in areas of low water
flushing or high densities of wild or farmed stocks.
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