Production of Clams

Among the bivalve resources of India, clams are undoubtedly the most widely distributed and abundant. The clams of the estuarine and backwater regions provide livelihood to those who exploit them for meat and shell. Being a rich and cheap protein source when compared to other aquatic food varieties, clam is regularly harvested and meat is sold in local as well as export markets for consumption. The clam shell also holds commercial importance being the raw material for the manufacture of cement, calcium carbide and sand lime bricks. They are also used for lime burning for construction, in paddy field and fish farms for neutralizing acid soil and as slaked lime.

Black clam & Short-neck clam

The black clam, Villorita cyprinoides and short-neck clam, Paphia malabarica are the commercially important clam species found especially in Vembanad Lake and Ashtamudi Lake respectively. The black clam is the most important clam species landed in India which contributes about two-thirds of the total clam landings of Kerala. It spawns twice a year, from May to August, and from January to late March.The short-neck clam is a fast-growing species with a peak spawning season of December to February and has a maximum lifespan of around 3 years.

Seed Production

The hatchery technology for the large-scale production of their seed has been developed by ICAR-CMFRI.

Broodstock management

The black clam and short-neck clam attain sexual maturity in its first year at a shell length of 15-20 mm and 30 mm respectively and mature ones are procured from wild for hatchery seed production. Adult clams are conditioned at a density of 30 no./m² in a tank of 50-70 l capacity having unfiltered seawater at 22-24⁰C and fed intensively with mixed microalgae reared in outdoor tank. After about 3 weeks of conditioning, the clams attain full gonadal development and are subjected to thermal stimulation by slowly raising the water temperature to 32⁰C. If spawning does not occur, they are transferred back to 22-24⁰C seawater and the process is repeated every two hours. Spawning can also be induced by placing the clams in the buffer solution of 9.0 pH for 1-2 hours and later transferring them to normal seawater. The optimum salinity for spawning is 10-12 ppt for black clam and 25-30 ppt for short-neck clam.

Rearing of larvae

The fertilised eggs settle at the bottom and are reared in FRP tank (70-100 l capacity). After a series of cell divisions, they develop into veliger larvae. The unicellular microalgae, Isochrysis galbana is given as food to the larvae from day 2 onwards. In the clam hatchery, biological filtration of the sea water which allows the nanoplankter and smaller algae, measuring upto 10 um is found to be beneficial since supplementary feed is available to the larvae and spat. After passing through the umbo and pediveliger stages, the larvae settle on the tank bottom as spat in 7-10 days depending on the clam species. The larvae are reared at a density of 5 no./ml of seawater. Spat settlement at 20- 30% of the initial stock of veliger larvae is considered as satisfactory. The freshly set spat measures about 300 pm and reach 2-5 mm length in the next 4-6 days. The spat are fed with mixed microalgae.

Transportation

The spat of clam is transported in wet condition under shade.

Clam Farming

Clam can be cultured on the bottom of protected coastal waters such as backwaters, bays, creeks and estuaries scientifically by adopting proper site selection, relaying, stocking and monitoring.

Site selection

The occurance of natural clam population in the vicinity generally indicates the suitability of the site for its farming. Water quality parameters like salinity, temperature, pH, DO, chlorophyll-a, TSS etc. and sediment characteristics like percentage of sand, silt, clay etc. of the sites are analysed. Clam farms are located in areas having 70-80% sandy substratum. The shallow waters with moderate water flow and little wave action is preferred. Strong water currents may dislodge the clam from the burrow. Areas prone to frequent changes of the contour and vulnerable to pollution are avoided. Tidal exposure at low tide for 1-2 hours is desirable as it helps in the management of the farm, particularly to remove the predators, but the prolonged exposure of the clam farm during the tidal cycle results in poor growth due to reduced feeding and in summer there may be mortality due to dessication. Also, the usual fishing grounds should be avoided.

Water quality parameters

The salinity tolerance limits and type of substratum preferred varies with the clam species. Black clam prefers low saline waters and occurs in salinity range of 3-16 ppt while that of short-neck clam is 20-34 ppt.

• Water current : 1-5 m/s
• Temperature : 23-34°C
• DO : 3-5 ppm

Farming structure

Clam farming with on-bottom pen system is the best for both species in which usually 2 ha area is demarcated with bamboo poles or floats with net as markers. At first the ground is levelled and cleared from predators. Eventhough the movement of the clam is limited and fencing is not necessary, synthetic fibre net pen can be erected to hold the clams within the farming area.

Seeding

In the commercial culture of clams, seed requirement is mostly met by collection from the natural bed, because the heavily accumulated wild baby clam during breeding season may naturally get destroyed during a period of time due to overcrowding and stunted growth on one hand and the production cost of hatchery produced seed is expensive on the other hand. The spat fall season of black clam is mainly during June and November and that of short-neck clam is January to February.

The baby clam of 10-12 mm size (1 g) is collected during early morning with a hand operated scoop net or a dredge {Kolli) having 2-5 mm mesh size, kept in country craft under wet condition, tugged to the site and relayed immediately or by the late afternoon of the same day. Optimum stocking density for both species is 500-600 g/m². Seeds are planted in the farm by evenly dispersing them as far as possible.

Care & Monitoring

After seeding, 10 mm synthetic netting is laid on the bottom and is held in stretched position by stakes; this net cover offers protection against predation and strong water current. As clams are filter feeders which thrives on natural food available in the water, no artificial feeding is required.

Harvesting

It reaches a size of 28-30 mm (10-12 g) within a culture period of 7-8 months and is harvested either by handpicking or by a hand-operated dredge. The anticipated production is 3.5-5 kg/m² with an expected survival rate of 70%.

Source : Department of Fisheries, Government of Kerala