Exert topic: Carpet shell
by Rebecca Sherratt, Production editor, Fish Farming Technology & International Aquafeed
The farming of carpet shells
The farming of carpet shells proves to be a relatively popular business. Harvesting occurs primarily in France, Portugal and Spain, records of mollusc fishing and consumption in Spain dating all the way back to the sixteenth century. Intensive fishing first began around 1926, however, at this time regulations weren"t so rigidly enforced, and fishermen often used prohibited tools and fished for clams of all sizes.
It was not until 1935 that clam fishing was regulated, and rules began to formally be established, the first rule ever being that fishermen could only harvest up to 14kg of clams during each low tide, and this season would officially last from May to October.
Between 1997-2001 the total aquaculture production of carpet shells ranged from between 3,700-4,900 tonnes per year. Over the years, France has declined slightly in production, producing only 475 tonnes in 2004, and this has only continued to decline. Over the years, the UK, more specifically Ireland, have also begun producing and farming carpet shells.
Despite these large numbers of harvests, that have only continued to expand in the past decade, carpet shells, alongside many clams in general, repopulate at an exceedingly fast rate, and so they are thankfully not considered endangered in any way.
Carpet shells are available in a variety of subspecies, one notable one being the Japanese carpet shell, which are established in groups in California and British Columbia, alongside Japan. They have also been transferred from Japanese waters into the UK, Hawaii and Spain.
The farming process
Modern farmers have three main methods by which they can obtain carpet shells. The first way is via seed. Farmers obtain seed from their own parks, or via natural clam populations in the early Spring. Using a small shovel, they dig through sand to the clam seed, pass it through a sieve to separate seed from sand, and then relocate the seed into their own parks.
The seed is spread in densities of approximately 800 clams/m2. Adult carpet shells can also be dug up, usually from seaport areas, and spread into farmers parks. This is a relatively easy and low-maintenance process, although farmers must take care to periodically clean their parks of predators and mud.
Hatcheries are an alternative source for seed. Breeders, not exceeding 40mm, are maintained at 20°C for 30-40 days. They are fed with unicellular algae until the induction of the liberation of gametes takes place. A cycle of temperature changes is made to induce this shift. Raising temperatures from 10-to-26°C, and maintaining this temperature for approximately 10 minutes, before reducing it to 15°C for several minutes in a cycle, is the most effective method to cause this change in carpet shells.
Following this, the selected molluscs are isolated from other carpet shells in their own tanks, and eggs are filtered through a 40 µm mesh. They are then transferred to a 10-litre tank, where veliger larvae appear after 48 hours. These larvae are reared at densities of 3,000 larvae per litre and fed until metamorphosis occurs.
The third method is obtaining carpet shells from a nursery, where the clams are reared in greenhouses. Similar to hatcheries, they are fed on unicellular algae, or alternatively reared in meshed containers over culture tables. Another method is to pump environmentally-controlled water to inland tanks, where the clams are placed into tanks of roughly 50cm in diameter and 20cm long, with a bottom consisting of rigid mesh.
The actual harvesting process doesn"t require the same advanced tools and machinery that harvesting most aquaculture species involves, to most farmers" relief. In the Galician region, most fishermen harvest molluscs and clams by walking along the intertidal areas and gathering carpet shells with specialised hand shovels.
Boats are often also used to harvest carpet shells. Collections and harvests may range from between one tonne in size to even 12 tonnes. No specialised form of boat is needed for this type of harvesting, as fishermen use both oar-propelled boats and outboard engine boats.
Once harvested, the fishermen place the clams in specialised depurtation stations, where they are kept in tanks for a minimum of 42 hours. After this, the clams are packed in net bags of 0.5-1 and two kg, to be sent off to be packaged and eaten. Refrigerated trucks transport the goods, which remain at between 3-10°C, and clams typically have a shelf life of five days.
Japanese carpet shells are left longer than other forms of carpet shell, so they grow bigger and, therefore, can be sold at a higher price. In China, these clams are harvested after 10-16 months, once they reach 30mm in size. Mechanical harvesting can also be used, wherein suction or elevator dredges dig up clams electronically from the sand, with a lateral conveyor belt equipped to a tractor.
Risks and difficulties
Whilst carpet shells might be easy to harvest, they can be particularly susceptible to a variety of diseases, and so, therefore, correct maintenance and checking of your harvests is essential. Some diseases prevalent in carpet shells include:
Perkinsosis – Visible white cysts appear on clam gills, foot, guts and digestive gland,
Brown ring disease – Bacteria builds up around the clam"s periostracal lamina, causing a secretion of brown material, named conchiolin. Growth of clams will also be stunted, and their normal calcification process is disrupted,
Haplosporidiosis – Can cause lesions in clam digestive glands and gills,
Larval mycosis – This affects early veliger to post-metamorphic juveniles, typically up to 40 µm in diameter. This disease causes the disintegration of soft-tissue and clams release motile zoospores. Up to 90 percent of larvae infected can die within two days of infection.
Other difficulties with harvesting carpet shells is ensuring that they are sustainable. Whilst they do repopulate quickly, and their numbers remain healthy, areas in the past have been completely wiped clean of clams and molluscs due to over fishing. Pollution is another factor which can negatively impact carpet shells, alongside urbanisation and the lack of available environments for them to flourish in.
A sumptuous meal
Carpet shells are usually sold in markets, hotels and restaurants, especially in Madrid and Barcelona. Their prices vary greatly, depending on their availability in the market. Due to their status as non-threatened environmentally, it is estimated that they will remain relatively stable in price and shouldn"t increase in price due to any sustainability reasons.
Carpet shells are popular with vinegar, alongside a variety of sauces. In the Galician region, "ameixas a marineira" (or "mariner clams"), is a popular dish, what involves placing the clams in salted water and cooking them with special sauces, such as onion, garlic, parsley, bread grind and white wine. They also go great in spaghetti alle vongole and bilbao fish stew, so there is plenty to enjoy about the carpet shell!