Giant Clams Aquaculture

Giant Clams Aquaculture Giant clams are slow-growing, long-living organisms, and the largest living bivalve shells in the world. They are found mostly in tropical waters of the Indo-Pacific region. Giant clams have been traditionally harvested for their flesh, especially the adductor muscle. Unfortunately, there was a surge in interest and demand for their meat between the 1960s and 1980s that saw large-scale harvesting across the Pacific Islands. This included commercial harvesters and illegal poachers, and as a result, population numbers drastically reduced. Since then, giant clam farms were developed with the intent of growing clams as a food source to relieve pressure on wild stocks. In the early 1980s, there was a joint effort within the Indo-Pacific countries to help with the declining giant clam’s population’s issue. It was reported that there are currently 34 active giant clam hatcheries across 25 countries, with most being in Southeast Asia, such as Singapore, Thailand, and the Philippines. Giant clams are well-suited for aquaculture due to their unique abilities to produce their own food through algae called zooxanthelle, which lives on them, similar to corals. The algae supply food to the clams through photosynthesis which supplies sugars and nitrogen-rich compounds. Referred to as “built-in food factories,” giant clams need only sunlight, water, and carbon dioxide to make their food. This is considered an advantage because, in mariculture, feeding is entirely omitted. There are several aspects to consider for efficient and successful giant clams’ mariculture which include farm site selection, clam seed selection, farm management as well as predators and pests management. Farm Site Selection An ideal farm site should have large, sheltered, relatively pollution-free lagoons that are perfect for giant clam farming. A typical site would be located in 1-5m of clear ocean water and should be away from point sources of pollution such as dredge sites, garbage dumps, and sewage outfalls. Giant clams prefer full-strength seawater and will quickly die if exposed to brackish water or freshwater for long periods. Farm sites should be located away from freshwater sources such as river mouths. Light is important for clam growth and only mild water cloudiness can be tolerated. A good water exchange rate is important for bringing fresh nutrients to the clams but this must be balanced against providing shelter for clams during harsh weathers. Ideal sites are those that are close to reef islands or located on the backside of barrier reef in sand or coral rubble areas, which are typically protected from harsh waves and tidal action. Depth is also an important factor to consider when placing a farm site. Shallow sites are easier to work on but are more likely to be damaged during a storm.

After 12-18 months in the nursery, giant clams of size 20 mm may be transferred to the grow- out phase. The site must first be considered according to the species used. T. gigas and H. hippopus are suited to intertidal zones while T. derasa and the others are grown best in shallow sub-tidal sites. Clam Seed Selection Clam seeds are the young clams that are obtained from a hatchery to stock the farm. The minimum size for stocking any species of clams into a sea-based farm is 2-3 cm. Clams smaller than this do not survive well and can make the farm unprofitable. Farm Management It is important to consider the best farm management practices to ensure a higher chance of survival of these cultured clams. Small giant clams are vulnerable to attack by predators such as fish, octopus, and snails and must be protected by cages placed on the ocean floor. Cages are more commonly made from vinyl-coated wire. Cages are sometimes raised off the seafloor on legs to help stop predatory snails from entering the cage and to protect the clams from sand and silt that may be washed up by wave action. Cage legs are usually made from local wood such as mangroves or steel re-bar. Clams are placed into the cages on plastic nursery trays that have been filled with a thin (1 cm) layer of concrete. The concrete acts as a base for the clams to attach to and helps to weigh down the cage structure. Cages should be constructed before seed clams are received at the farm. One 3 ft x 3 ft cage can accommodate about 150 seed clams. This will allow adequate space for the clams to grow for 6-12 months without being moved. A general rule of thumb is that clams should not be touching each other. Grow out time to harvest for aquarium clams is from 12-18 months and for food clams are 2-3 years. Maintenance of a clam farm requires very little time but must be done regularly. Every week the cages should be scrubbed with a brush to remove fouling. A build-up of fouling reduces light source to the clams that slow their growth. During cleaning, dead shells should be removed and the clams checked for predators. The cage should also be examined for tears or damage. Not visiting and maintaining the cages regularly leads to slower growth, excessive mortality, and overall lower profits for the farm. Predators and Pests Management Small giant clams have many predators and the farmer needs to protect the clams through regular maintenance. Fish with strong mouths such as trigger and puffer fish can easily bite

through a giant clamshell as well as some cage materials. It is important to ensure that the cage can withstand these attacks and that there are no holes in the cage. Octopus is also dangerous predators for giant clams and can get into cages if the mesh is too large or the cage is holed. One of the worst predators of giant clams is snails. These settle into the trays of clams from the water and crawl inside the clam to eat it. During regular maintenance of the cages, the farmer must look for dead shells or clams that look weak as these are signs of snail attack. Weak clams often lose their color or gape open. The trays of clams must be carefully inspected for snails and any found must be removed and killed. Experts say that the giant clams (nearing extinction in most parts of the world) are attractive to farm for economic, social, and ecological reasons because of their innate characteristics – self- feeding, sedentary habit, adult resistance to predation. Besides, technology for its mass production from breeding to harvest has been proven successful in many parts of the Indo- Pacific. Although mariculture or farming is a good way to ensure that populations of giant clams do not decline and hopefully increase, the high cost to sustain them in terms of time, labor and money is an issue. Also, cultured clams could result in low variability in their genes and possibly different from the ones in the wild. There is a higher chance of pathogen introduction in the hatchery as well. Research works in the mariculture or farming of giant clams would have to continue to create better and more sustainable mariculture technologies.