Cockle culture was practiced by native at Mauk from 1950-1969. The activity was halted in 1975 due to lack of seed (Unar et al, 1982). Culture practices are simple, but vary among farmers. The pattern is starting plant the seed at high density in a portion of the farm set a side as a nursery, and thin out as they grow bigger size.
Culture grounds are not normally prepared in any special way. For old grounds that have been cultured for years, empty shells and predators such as boring gastropods should be removed as much as possible.
Seed for culture operation can be obtained from seed collectors or collect it from natural ground for own uses. The spat are collected by means of a wire basket-shaped device illustrated Fig. 2). Most of the wire used is very light gauge being only 1–2 mm in diameter. The gap between the wires (mesh size) is approximately 2 mm. The wooden cross-piece is served as handle. The basket is swept through the surface mud down to a depth of about 3cm. The mud is sieved out by vigorous agitation in the water. and the spat retained are tipped into the sled.
The collectors normally proceed to the mud flats at high water of a neap tide in motorized shallow-drift boats 6–10m long to the spatfall areas. They start to work when the water covering the mudflats is 60 cm deep. They takes a wooden mud sled (Fig. 3) to provide support for them in the difficult mudflat environment as well as forming a repository for spat collected. Collection continues until no standing water remains on the mudflat. When the water is shallow the sled is used to move across the mud. The operator kneels inside on one knee and uses the other leg to propel himself along. Skill collectors can get about 7–8 tins (1 tin=18 liters) during two to three hours of works.
The price of spat varies depending on the size of spats in each tin and impurities as well as on the demand for spats. The survival rate of spats depends largely on size. The larger the size the better survival rate. Spat counts approximately 5000 pcs/kg with size ranging between 6–10mm, weighing about 16 kg/tin is considered suitable for culture purposes.
Stocking density per unit area depends on the size of the spat. For spat counts about 5000 pcs/kg, it is initially stocked at 2 000 pcs/sq.m. As the cockle grow, the population will be thinned down. The large specimens will be removed and resown at density of 200–300 pcs/sq.m in another portion of the culture lot. The thinned out can be started when the cockle reached a count of about 480/kg which take about 3–4 months. For old bed with poor fertility, it may require 6–9 months before the cockle can be transplanted or thinned out.
Spats are sown immediately after arrival at the culture sites. Boat loaded with spats is manned by at least three persons, one of whom steers the boat slowly over the culture site while others sow the spats. The spats should be distributed as evenly as possible. There is no specific gear for sowing the spats, a plastic scoop or shovel can served the purposes. A day or two after sowing the spats, the culture sites are inspected for any uneven distribution of the spat so that it can be rectified.
Time required for cockle to reach market size varies from area to area and is influenced by the age of the culture bed. Growth rates decline as the culture bed are ages. Initially only a part of the culture area is stocked. After four to six months the young cockle are redistributed over the whole culture area. Redistribution of the cockle can be achieved by use of the mud sled as practiced in Thailand or with the use of motorized boat as in Malaysia. Harvest of cockle takes place about 12–18 months after seeding. The cockle are marketed when they reach a length of approximately 3.2–4.0 cm or a count of 50–60/kg.
Growth rate of culture cockle is affected by environment conditions and population density. Pathansali (1966) found that on culture beds, at a density of 525–1 050/sq.m a size of 18–32 mm was attained in 10–12 months. Pathansali and Soong (1958) found that on natural beds where the density was less than 10.5/sq.m, A. granosa grew from 4–10mm to 18–32mm in nine months. Broom (1982) also confirmed that both density and exposure affect the growth of the culture cockle (Fig. 4).
The predators found heavily prey on cockle are starfish (Asterias amuvensis), moon shell (Notica maculosa), snail (Thais carinifera), and marine catfish (Plotonus anguillaris). Toyo et at. 1987 reported that a single starfish measuring 76 mm can eat 22 cockle in one month. Broom (1985) also presented evidence that a moon shell can consume one cockle every two days, irrespective of the size of the predator, and a snail can eat one cockle every 2.4 days. Tookwinas (1985) showed that the marine catfish also prey directly on small cockle in large quantity.
At present, large quantities of industrial effluents are discharged daily into a coastal waters constituting a source of numerous pollutants and may pose a threat to culture operation. The effect may have a directly toxic on the organism itself and may, by virtue of contamination of the organism, render it unsafe for human consumption. Even through the results outlined by Philipps and Muttarasin (1985) and Huschenbeth and Harms (1975) showing that anadarinids does not accumulate either organochlorines or heavy metals, but their feeding habit is likely to increase their risk potential, particulary those are cultured in polluted area and marketing without purification.
During the culture period, the cockle may congregate in a certain spot due to current or animal moving itself. The dense population have to thin down and redistributed to the less dense area of the culture plot. Predators of cockle may also be removed at this time. To preventing pilferage of cockle, it is necessary to have guards or watch men on duty 24 hours basis.
Form initial stocking density of 2 000 psc/sq.m, and 300 pcs/sq.m as final density, a production of 21 mt/ha/yr (on shell) are expected based on 35 per cent mortality rate.
Both harvesting and thinning done from a motorized, shallow-draft boat using a dredge similar in design to that for collecting spat except that it is larger and more robust (Fig. 4). The wire used in its construction is heavy duty, 3–4mm in diameter. The spacing between the wires (mesh) depends on the use to which the dredge is put: for final harvesting it is wider than it is for thinning. The dredge is affixed to the end of a long pole. The boat is usually manned by two men one of whom maneuvers it while the other thrusts the dredge into the mud and holds it there as it is dragged along. To ensure that the dredge is maintained in position while being dragged along, a rope secures the distal end to a point forward on the vessel thus relieving the operator of the near impossible task of holding it upright.
The cockle are washed to remove the mud and brought aboard. Two persons working five to six hours can harvest on average 12–15 bag (70kg) of cockle. The harvest is continued until the density is reduced to 1–3 pcs/sq.m.
Culture trial on cockle culture have been carried in Tg. Balai Asahan, Medan in 1 ha culture plot from 25 February – 5 December 1987. The seed was obtained from local seed bed adjacent to the culture lot. Cockle spat size ranging between 6–10mm was stocked at 60 pcs/sq.m. After 10 months, 70 per cent of the cockle attained a size about 3.5–4.5cm, 29 percent are between 2.5–3.5cm and 1 per cent are between 4.0–6.0cm. Poaching by competing fishermen is a problem. Local granting of culture plot and enforcement of property rights is only way in which this problem can be kept within reasonable limits. Providing guards and watch men on the culture site is another solution when the culture operation is started.
