The production cycle begins with the reproductive stage and seed production. Seed can be obtained either by gathering wild spat or by hatchery production. Most commercial Japanese carpet shell culture is based on intertidal on-bottom culture using wild seed or hatchery products (2-3 mm size).
Natural spat supply
Together with the colourful clam Ruditapes variegata
, R. philippinarum
is one of the two predominant and traditional species in China, where it is mostly cultured using natural seed collected from muddy beach sand areas. In this country, seed collection involves the selection and preparation of seed collection beds, eradication of predators, and routine maintenance. Techniques have been developed to increase seed supply: these consist of shallow ponds (up to several ha in area) made in the lower intertidal area, from which all competitors, seaweed and predators are removed and the bottom smoothed. These ponds can be inoculated with Chaetoceros
, a suitable food for larvae and postlarvae. Each pond can be used for rearing 2-3 batches/yr. About 75 to 150 million seed clams (0.5 cm) can be grown per ha. These clams grow up to 1.5 cm by May the following year and are then replanted at 1.8 million/ha for another year to reach a 3.5 cm marketable size (19-45 tonnes/ha).
In European waters, spat supply is based both upon hatchery products as well as harvested wild seed. The availability of wild seed is a result of the invasiveness pattern of this species in areas where it was formerly cultured using hatchery-produced spat. In contrast, most of the seed supply in North America is based upon hatchery production.
The main operations consist of conditioning the breeding stock to facilitate gametogenesis; spawning and larval rearing to metamorphosis; and growing the resulting juveniles to a suitable size, either in nurseries or directly in the sea.
Broodstock conditioning takes 30-40 days at 20 °C. Spawning is induced either by thermal shock stimulus or by adding drops of sperm, or by stripping. Fertilized eggs are filtered through a 40 µm sieve and maintained in 10 litre containers until the veliger stage. Larvae are then collected by sieving and distributed into containers at a 3 000/l. Larvae are fed everyday for the first week at 30 cells/µl, then every second day until larval metamorphosis is reached (2 weeks). Salinity for breeding and rearing must be between 24 and 35‰ (tolerance range 13.5-35‰). A 15-28 °C temperature range is optimal for growth, although the species can survive at 0 ° and 35 °C for short periods of time. Pediveliger clam larvae can be prepared for shipping at this stage following a screening and counting process, and then deployment into shipping material (nytex, paper coffee filter), wrapped in several layers of moist towelling. Then, this is placed in an insulated shipping box along with an ice pack (without contact with the larvae). Alternatively, pediveliger larvae are settled in setting tanks or micronursery trays using nylon screens and a flow-through recirculating system with frequent water exchange (e.g., water tables, downwellers). Anytime the clams are removed from the water, care must be taken to ensure they do not dry out or become too warm.
Hatchery rearing requires the production of suitable microalgal species as food. Usually, the phytoplankton species used are flagellates such as Isochrysis galbana
, Pavlova lutherii
and Tetraselmis suecica
sp. Combinations of flagellates with diatoms such as Skeletonema costatum
, Chaetoceros calcitrans
, C. gracilis
and Thalassiosira pseudonana
provide a well balanced diet, facilitating gametogenesis and larval development. Food quantity depends on larval density.
Although clams have a protective shell, the shells will break if not handled with care during the screening and sorting processes.
A nursery system using an up-welling approach may be used for pre-growing seed up to 10-15 mm shell length. Slightly larger clams (6-7 mm) at 3 000/m² are deployed on the seabed enclosed in 4 mm mesh bags (1.5 m x 2 m). Other culture practices for pre-growing include placing 4-5 mm seed at a density of 10 000/m² in wooden frames covered by plastic netting, stacked underwater (in Italy) or in mesh covered wooden frames, or in mesh bags on trestles around the low spring tide area (in Ireland). Stocking densities are progressively thinned with increased growth. During the nursery stage, grow-out facilities must be cleaned, clams graded and predators such as crabs, removed. Although tray culture can be used for the early growing stage, clams grow better in the soil. Floating upwellers such as FLUPSYS (a rack structure which supports a series of containers along a central channel from where the water is forced either by using a propeller or a paddlewheel) are in use mostly in North America. Clams should be size graded to ensure that all animals are of similar size; otherwise, larger clams can out-compete the smaller ones, leading to retarded growth.
At about 10-15 mm shell length, clams are ready for seeding in the substrate. The ongrowing operation is best established on intertidal sites sheltered from extreme wind, wave and tidal action. Alternatively, 400 m² oyster ponds can be used to rear clams up to the market size. Suitable substrates for on-growing sites usually consist of gravel, sand, mud and shell. Although Japanese carpet shells clam will survive in a variety of soils, a too soft bottom limits access and the type of equipment used for planting, husbandry and harvesting. Prior to seeding, the area must be properly prepared and predators removed. The plot system of cultivation uses strips of mesh, deployed over the seeded clams and ploughed in along the edges of the plot to limit crab and bird predation.
Growth and survival are directly related to stocking density. Clams (10-15 mm) are seeded at 200-300/m² and simultaneously covered with 4 mm aperture netting (1.5 wide, 300 m long) to protect them from excessive predation. In Europe, a planting machine has been developed that simultaneously ploughs in the netting and sows the seed. Deflector plates backfill the trenches, burying the edges of the net. Nets should be cleaned to avoid fouling organisms and siltation, and monitored for predators. Depending on local carrying capacity, clams will grow up to 40 mm in about 2 to 3 years.
In China, Japanese carpet shell seeds (5-10 mm) are planted at 35 million/ha, although density is based upon seed size and sediment type. Usually, clam beds are not protected with nets.
In China, clams are harvested after 10-16 months, at 30 mm or larger. In European waters and in Northern America, clams are usually harvested after 16 to 30 months at a larger size (30-40 mm shell length) to obtain an improved exchange value. In both cases, they are usually either raked or mechanically harvested. In Europe, manual harvesting of Japanese carpet shells consists of raking them out of the substrate and bringing them to the surface. Mechanical harvesting is carried out by suction or elevator dredges; a tractor equipped with a lateral conveyor belt can dig and grade clams from sandy bottom areas, covering more than 200 m²/hr (>600 kg/hr).
Handling and processing
Once harvested, the clams are stored in boxes or bags and transported for mechanical grading. Then they are usually wet stored to purge grit and sand before processing and marketing. They can be wet stored longer in the low intertidal areas using plastic mesh bags, or in ponds, or suspended from wet storage rafts until they are sold.
Production costs are highly variable due to several factors. Depending on site characteristics, clams may take 2 to 4 years to reach marketable size, therefore affecting overall yield. Local carrying capacity, bottom softness, and the cost of operational procedures such as preparing seeding plots, removing predators, cleaning to avoid the clogging of structures by mud or algae, regular grading and sorting are among the main driving factors. In addition, the chosen rearing strategy will directly affect production costs. The size at which to purchase seeds from hatchery-nursery is critical. The lower cost of buying small seed may be balanced by increased mortality rates and higher operating costs (nursery structures); in contrast, buying larger seed is more expensive and requires higher survival rates to be cost effective. Bottom type characteristics (sandy versus muddy bottom) affect harvesting procedures and efficiency; for example, mechanical harvesting is easier in sandy bottoms, providing improved yield. All these factors should be carefully considered when planning clam culture in order to optimize production costs and select and appropriate rearing strategy.