3.7 Estuarine shellfish

An estuary is a semi-enclosed coastal body of water which has a connection to the open sea one side and a river with fresh water on another side. Due to the presence of freshwater and seawater together, a, wide spectrum of flora and fauna can be found in estuaries. Land runoff, coastal nutrients and contributions by rooted vegetation like seagrasses and mangroves support high biological productivity. The resulting wide range of fish and shellfish supports a productive fishing industry.

Western Ghat estuaries

All the west-flowing rivers of Maharashtra, Goa and Karnataka rise in the Western Ghats hills. None is longer than 150 km. Where they meet the Arabian Sea, they form extensive estuaries.

The high rainfall in the Western Ghats means that large amounts of freshwater flow into the estuaries. Water temperatures are generally low during the monsoon and high during the summer. Most of the estuaries are shallow and predominantly sandy. The meteorology, physico-chemical conditions of the estuary and the substratum are ideal for shellfish to grow and propagate.

Shellfish

Shellfish include all species with either shells or exoskeletons. These are mainly molluscs and crustaceans. Molluscs such as clams, mussels and oysters are important in the Western Ghat states.

Molluscs

Molluscs have a soft unsegmented body which is usually covered by a shell. The shell is formed by an outer body wall (mantle) which secretes the calcareous substances.

Microhabitat of shellfish

The estuaries in the Western Ghat region support a variety of fauna. The distribution of clams illustrates how microhabitats vary.

Clams show a definite settlement pattern and breeding cycles based on changes in the environment such as salinity. The substratum plays a prominent role in the formation of clam beds.

The clams occupy characteristic zones within the estuary from the bar mouth to the interior.

Distribution of shellfish in Goa’s estuaries

Estuarine fisheries

Five species of clams occur on the west coast of India: Meritrix meritrix, Meritrix casta, Katelysia opima, Paphia malabarica and Villorita cyprinoids. They have an annual yield of 12,000 tonnes a year.

Red tide

Red tide is an algal bloom that can poison shellfish and the people eating them. The growth period of shellfish is January-March. Meritrix casta (Khube) affected in Karnataka and Tamil Nadu has been responsible for deaths of people. However, such incidents are rare.

Uses of shellfish

A total of 15,000 tonnes of shellfish are consumed in the Western Ghat states each year. An average of 10% wet weight of the meat is protein. Shellfish also contain substantial amounts of vitamins and minerals.

Overexploitation of fish in the coastal waters has increased pressure on shellfish resources. Besides being used as food, mollusc shells are also used in paper, rayon, leather, carbide, cement, lime and fertilizer industries as well as for shell grit for poultry and aquaculture.

Conservation and culture

A constant watch on the population structure and growth of shellfish in the intertidal region is the best way to monitor the biodiversity of shellfish in the estuarine region. The shellfish can be easily transplanted and grown on rafts, ropes, cages and on rocks during the lean season. These approaches may help increase the availability of shellfish products and protect shellfish biodiversity.

Threats from people and industry

· Dredging
· Increased turbidity
· Reclamation, changing bed profiles
· Organic wastes, using up dissolved oxygen
· Hot water
· Discharge of chemicals such as phenols
· Organochloride compounds, e.g., DDT, PCB (polychlorinated biphenols)
· Heavy metals, e.g., mercury, lead, zinc and cadmium
· Sewage
· Hydrocarbon pollution
· Runoff from iron ore mining
· Sand
· Chemical fertilizers

Influences on shellfish habitat

Prepared by S.G.P. Matondkar, Nakul Mhamal and D. P. Kavlekar

Conservation and culture