GEOGRAPHIC DISTRIBUTION OF OYSTERS AND STUDY AREA

Transcription

1 GEOGRAPHIC DISTRIBUTION OF OYSTERS AND STUDY AREA

2 Oysters are widely distributed from tropical to temperate areas ranging from 64 N to 44 S (Harry, 1985; Hedgecock, 1995). They are benthic marine species inhabiting near shore areas, shallow waters, bays, and estuaries widely distributed throughout tropical and subtropical regions (Hedgecock, 1995). Oysters are recorded worldwide throughout the Indo-West Pacific, from East and South Africa, including Madagascar, the Red Sea, recorded in the Suez Canal (Moazzo, 1939) and the Persian Gulf, to eastern Polynesia; north to Japan and south to New South Wales and New Zealand. In the Mediterranean, Oysters were recorded first in 1999 from Turkey at Erdemli and Yumurtalik, Tasuçu as Saccostrea commercialis (Çevik et al., 2001); successively from Egypt they also occur in the tropical eastern Atlantic, from Cameroun to Angola. In East Africa, its range includes Somalia, Kenya, Tanzania, and the Seychelles. In India, it is one of a number of commercially exploited oyster species. It is also found in Australia, New Zealand, and the Red Sea. The hooded oyster is found in the Indian Ocean and tropical west Pacific Ocean (Mitra and Choudhury, 1993). Among the several genera which exist in Malaysia, oysters of the genus Crassostrea (Sacco, 1897) are distributed worldwide and are important commercially (Lam and Morton, 2003). Among indigenous Oysters in Thailand, Crassostrea belcheri, Crassostrea iredalei, and Saccostrea cucullata are economically important species widely cultured over vast geographic locations on either side of peninsular Thailand (Jarayabhand et al., 1985, 1994; Yoosukh, 1988). 69

3 Generally small oysters distribute over larger geographic areas than do large oysters. For instance, S. cucullata and Striostrea mytiloides are found in tropical Indo- Pacific regions, whereas C. belcheri and C. iredalei are found mainly in the South China Sea, Andaman Sea, and Gulf of Thailand (Yoosukh and Duangdee, 1999). Up to 30 nominal species have been recorded in China alone (Qi, 1989, Bernard et al., 1993), with as many as nine of those species in the genus Crassostrea (Lam and Morton, 2003). A number of these species represent significant aquaculture industries including C. gigas, C. sikamea, C. ariakensis (Qi, 1989) and the newly described C. hongkongensis (Lam and Morton, 2003). Crassostrea ariakensis is of particular interest to scientists and managers in the United States, because of the proposed introduction of this species into the Chesapeake Bay as a means of replacing native populations of C. virginica ravaged by overfishing, habitat degradation, and disease. Saccostrea cucullata, the hooded oyster or Natal rock oyster, is a species of rock oyster found mainly in the Indo-Pacific Ocean. It was first described by the Czech mineralogist, metallurgist, and malacologist Ignaz von Born in Saccostrea cucullata has a wide Indo-Pacific distribution, ranging longitudinally from East (Day, 1974; Branch and Grindley, 1979) and South Africa (Lasiak, 1986; Dye, 1990) to the Pacific Islands, e.g., Guam (Stojkovich, 1977; Braley, 1982), and latitudinally from Japan (Torigoe, 1981) to Australia (Roughley, 1933) and New Zealand (Dinamani, 1974, 1976). S. cucullata occurs all along the Chinese coast (Tchang and Tse-kong 1956) and that of the Philippines (Rosell, 1991) and is the most widely distributed of the species of Saccostrea (Angeli, 1986). The most widely distributed species is S. cucullata, which is found throughout the Indian Ocean and tropical western Pacific 70

4 (Rabesandratana, 1971; Ray, 1977 in Angell, 1986). However, the original description of the species was from the Atlantic, raising doubts about its applicability to Oysters in the Indo-Pacific (Glude, 1971 in Angell, 1986). S. cucullata has been commercially cultured in French Polynesia (Aquacop, 1982) and the Philippines (Blanco, 1956). It is also harvested from natural stocks throughout its range (Ray, 1977; Glude, 1984). S. commercialis was found from Victoria in Australia to Thailand (Thomson, 1954; Saraya, 1982), although there is some question about its correct identity in Thailand (Mr. Kosol Mutarasint, Dept. of Fisheries, Bangkok, pers. comm.). S. commercialis is farmed extensively in Thailand, particularly along the coast of the Gulf of Thailand (Saraya, 1982) and from Victoria to Queensland, Australia (Thomson, 1954). S. morda, the pink or coral oyster, ranges from northern Australia through the western tropical Pacific (Thomson, 1954; Glude, 1971) and has been experimentally cultured in Queensland, Australia, by Bryson (1977), although he designated it amasa. Ranson (1967) reported the range of S. lugubris from the South China Sea region eastward to Papua New Guinea. It is commercially farmed in Thailand (Bromanonda, 1978) and harvested from wild stocks in the Philippines (Lopez and Gomez, 1982). One of the species which has shown promise for development from experimental culture is S. echinata (Aquacop, 1982; Angell, 1984; Braley, 1984; Glude, 1984). It may have potential for culture in high salinity water throughout its range; in the Philippines it is commonly harvested from wild stocks (Lopez and Gomez, 1982). S. echinata is widely distributed from the tropical waters of Australia (Thomson, 1954) through the Philippines and the western Pacific Islands (Carreon, 1969; Glude, 1971 in Angell, 1986). 71

5 Several other Saccostrea species are cultured commercially in the Philippines, including S. malabonensis and S. palmipes (Blanco et al., 1951 in Angell, 1986). These species were described by Carreon (1969), but no record of their occurrence outside the Philippines was obtained. The description and comparison of soft parts as used by Carreon (1969) should be extended to a wider range of species. In addition to these, S. tuberculata and S. manilai are also found in the Philippines (Carreon, 1969; Buroker et al., 1979). S. tuberculata ranges as far south as Australia (Thomson, 1954), but its commercial status is unknown. The tropical species of Saccostrea with their range and aquaculture status are mentioned in Table 4.1 (Source: C. L. Angell, 1986). Table 4.1: Tropical Species of Saccostrea with their range and aquaculture status.species Range Aquaculture Potential Experimental Commercial Harvest Wild Stocks Location or Taxonomic reference S.cucullata Indo-Pacific Ray 1977 Southern S.commercialis (Australia to + + Thomson 1954 (glomerata) Thailand) S.mordax (amasa) S.lugubris S.echinata Nothern (Australia to Western Tropical Pacific South China region east to New Guinea Philippines, Indonesia and Western Pacific Islands + Potential Thomson 1954 Glude Ranson Thomson 1954 Carreon 1969 Glude 1971 S.malabonensis Philippines + Carreon 1969 S.palmipes Philippines + Carreon 1969 S.tuberculata Western Australia, Philippines Unknown Thomson 1954 Carreon 1969 S.manilai Philippines Unknown Buroker et al.,1979b 72

6 Saccostrea cucullata (Born, 1778) is an edible oyster found in brackish water environments where it cements onto mangroves, rocks and other hard substrates including man-made structures (jetties, ship hulls, beacons). According to Lewis (1964) and Hartnoll (1976), these oysters are abundant in the upper eulittoral zone where they grow to a maximum shell length of 65mm (van Someren and Whitehead, 1961) but experience stunting in crowded conditions. Oysters are found throughout the tropics and subtropics and are commonly harvested from wild populations. The most widely distributed species is Saccostrea cucullata, which is found throughout the Indian Ocean and tropical western Pacific (Rabesandratana, 1971; Ray, 1977; Richmond, 1997). In spite of its adaptability to cultivation, Saccostrea cucullata farming has developed in only a few tropical countries. It has been commercially cultured in French Polynesia (Aquacop, 1982), the Philippines (Angell, 1986), Sulawesi, Indonesia (Latama, 1997), and Mauritius (Joseph, 1998). Among the mass-cultured oyster species in the tropics are Crassostrea virginica and Crassostrea spp. in Mexico, Ostrea chilensis in Chile, C. gigas in Mexico, C. rhizophorae in Cuba and Jamaica, C. belcheri in Malaysia and Crassostrea spp. in Senegal (Joseph, 1998). In addition to these, a number of other countries grow local species of oysters on a small scale, for example, C. brasiliana in Brazil, C. madrasensis in India and C. rhizophorae in Venezuela and Panama (Joseph, 1998). Figure 4.1 shows the distribution of tropical species of Saccostrea. (Source:

7 Fig.4.1. Distribution of Tropical Species of Saccostrea Dey, (2006) mentions about 100 species of molluscs from mangrove areas of Indian subcontinent. The assemblage of oysters were to occur on the mud banks, mud flats, mangrove forest, sandy muddy area swamps, prop-roots and pneumatophores and mussel were found attached to wherever hard substratum is available such as prop-roots and pneumatophores and oyster beds (Arularasan et al., 2012). Crassostrea madrasensis and Saccostrea cucullata occur throughout the Indian coast whereas C. gryphoides and C. rivularis are restricted to the north-west coastal regions (Mahadevan and Nagappan Nayar, 1987). In Karnataka, Nethravati, Sharavati, Kali, Mulky river estuary and estuaries at Uppunda, Bhatkal, Venkatpur and Kundapur possess oyster beds ranging from 1 ha to 5 ha (Mahadevan, 1987). Among the edible bivalves, oysters have a prime place due to the nutritive value and palatability of the meat. Crassostrea madrasensis, C. gryphoides, C. rivularis and Saccostrea cucullata are the commercially important oysters of India. The contributions of Rai (1928, 1932), Awaiti and Rai (1931), Subrahmanyam et al., (1949), Durve and 74

8 Bal (1962), Jones (1968), Alagarswami and Narasimham (1973), Rao (1987) and Mahadevan (1987) give information on the oysters of Maharashtra. Rao (1987) stated that S. cucullata inhabits shallow coastal waters and creeks along both the coasts of India including Andaman and Lakshadweep. Mahadevan (1987) reported that the population of this oyster found along Maharashtra and Gujarat coasts is exploited for food on a small scale. He further stated that our information on the extent of beds, population density and magnitude of the recruitment of Oysters of is still fragmentary. James (1987) emphasized the need to assess the oyster resource potential and collect information on the suitable areas for the culture of edible oysters for the benefit of enthusiastic entrepreneurs and agencies in states like Kerala, Goa and Maharashtra where there is no data base for the same at present. Saccostrea cucullata occur along the rocky littoral zone of Bombay and form beds. A preliminary study on the population of this little known resource was conducted during January-March 1986 in order to enhance our knowledge in the management and development of this food species for the benefit of the coastal fisher folk (Sundaram, 1988). The density of S. cucullata recorded from Bombay appears to be very high when compared to their occurrence in some other locations along east and west coasts of India observed by Rao and Sundaram (1972). Along the Indian coasts, the occurrence of oysters of only two genera viz., Crassostrea and Saccostrea has been reported. The Indian oysters were originally referred to the genus Ostrea (Awati and Rai, 1931) but later included under the genus Crassostrea (Rao 1956, 1958, Durve 1968).A number of oyster species occurs in Indian waters (Rao, 1974); those that are commercially exploited are Crassostrea madrasensis, C. gryphoides, C. rivularis and Saccostrea cucullata. The first three 75

9 species occur mainly in estuaries, backwaters and creeks and are all exploited to some extent by local fishermen. On the other hand S. cucullata, a purely marine form, is predominantly found in shallow areas with rocky substratum. It is part of the fouling community and is found on harbour walls, pilings, and other underwater structures (Barua et al., 2011).The hooded or rock oyster is found in the Indian Ocean and tropical west Pacific Ocean (Mitra and Choudhury, 1993). Six species of oysters namely the Indian backwater oyster Crassostrea madrasensis, Chinese oyster C. rivularis, West coast oyster C. gryphoides, Indian rock oyster Saccostrea cucullata, Bombay Oyster Saxostrea cucullata, and Giant oyster Hyostissa hyotis are found in India. The first four species mentioned above are of commercial value. Of the six species of oysters, the Indian backwater oyster C. madrasensis is the dominant species, more widely distributed, is euryhaline and inhabits backwaters, creeks, bays and lagoons and occurs in the coastal areas of the States of Orissa, Andhra Pradesh, Tamil Nadu, Kerala, Karnataka and Andaman. C. gryphoides is also euryhaline and occurs along north Karnataka, Goa and Maharashtra coast. C. rivularis is found along Gujarat and Maharashtra coast while Saccostrea cucullata is found all along the main land coast and Andaman and Lakshadweep islands. Culture of these species is possible at places where the seed is easily available. The native Indian oyster (C. madrasensis) occurs throughout the coast of India, whereas the other two Crassostrea species are restricted to the northwest coast regions and have not been reported so far anywhere on the east coast (Alagarswami and Narasimham, 1973). C. madrasensis dominates the entire east coast and Kerala. It is 76

10 also found in various localities of Karnataka State. C. gryphoides is mainly present along the Maharashtra coast and in several localities of Goa State. C. rivularis occurs along the coast of Gujarat State and to a lesser extent along the coast of Maharashtra State. S. cucullata occurs all along the Indian coast, however only the settlements along Maharashtra and Gujarat coasts are large enough for exploitation. Oyster fishing grounds along the west coast include Dahanu creek, Jaytapur, Versova, Satpuri, Alibag, Boiser, Marve, Ratnagiri, Cuff Parade, Mahd, Utsali, Navapur, Aramra creek, Gagwa creek, Gomati, creek and Azad island. Small oyster beds exist in Kerala and Karnataka, but little exploitation occurs. Along the east coast, exploitation concentrates in the backwaters of Orissa, in Gokulapalli (Andhra Pradesh) and in Courtallayar and Adyar estuaries and Tuticorin (Tamil Nadu). Oyster culture in India is not as well developed as in other Indo-Pacific countries; however it has received considerable interest within the last decade or so. The Central Marine Fisheries Research Institute (CMFRI) under the Indian Council of Agricultural Research (ICAR) is the main institution involved with bivalve culture. Through applied research aimed at developing suitable and low-cost culture methods and through training and demonstration courses they are trying to popularize oyster farming. Most of the productive oyster beds are distributed near the bar mouth region of the estuary where water turbulence, lush growth of mangroves and silty bottom restricted the accessibility (Naik and Gowda, 2013). It is very important to document and monitor the oyster beds in the estuary because changes in oyster abundance and distribution can be used as indicators of environmental changes. Therefore, documentation of the currently existing oyster beds in the Mulky estuary may enable the future researchers to 77

11 evaluate future changes to the oyster population. Similar survey was carried out in Ishahaya Bay, Japan to assess the distribution of oyster beds that revealed number of oyster beds suitable for oyster exploitation and culture (Jinno, 1998). The survey of North Carolina s oyster beds in 1992 reported 8,327 acres of oyster beds having highdensity population and 20,553 acres having low-density population (Jones, 1994). Intertidal benthic community survey revealed the aggregations of oyster beds along intertidal mud banks of Skardon River, Queensland, USA (Grizzle et al., 2005). In a survey, more than 70,400 acres of oyster beds were identified in Delware Bay estuary, USA (Grizzle et al., 2008). During 1984, in Virginia, USA, a survey was conducted in estuarine waters to locate and map the naturally productive oyster beds. In this survey, 20,000 acres of oyster beds suitable for harvesting were reported (Fullford et al., 2007). In a survey by the Chesapeake Bay Programme, fresh oyster beds were found in the Chesapeake Bay suitable for harvesting (Kimmel and Newell, 2007). A survey has been carried out in Caloosahatchee estuary, Florida, USA to assess the status of Eastern oyster, Crassostrea viriginica (Aswani et al., 2008). In India, Crassostrea madrasensis, C. rivularis, C. gryphoides and Saccostrea cucullata are considered as commercially important (Rao et al., 1983). It has been reported that C. rivulais occurs all along the Gujarat coast and in some regions of Maharashtra coast. Crassostrea madrasensis is a major oyster species in India and occurs all along the Indian coasts and Andaman Island. It is referred as the native oyster of India. The oysters, C. gryphoides and C. rivularis are restricted to the northwest coast regions of India (Mahadevan, 1987). Joseph and Joseph (1987) reported two species of oysters, Crassostrea madrasensis and Saccostrea cucullata in the Mulky 78

12 estuary. The distribution and main culture areas of commercially important oyster species in India is seen in Table 4.2. (Source: Table 4.2: Distribution and main culture areas of commercially important oyster species in India SPECIES STATE LOCALITY NOTES West Bengal Little information available. Crassostrea madrasensis Orissa Andhra Pradesh Tamil Nadu Kerala Bahuda estuary Sarada estuary Bhimunipatnam Upputeru canal Godavari and Krishna estuaries Gokulapalli Pulicat Lake Courtallayar and Adyar estuaries Mudasodai and Chinnavaykal Muthupet swamp Vaigai estuary Tuticorin Tamparaparni estuary Ashtamudi Lake Approximate area of 5 ha. Three distinct beds observed. Oyster beds reported. Oyster beds subjected to annual depredation due to fresh water influx. Approximate area of 2.25 ha of rich oyster beds. Low density beds. Rich oyster beds regularly exploited. Rich and extensive oyster beds regularly exploited. Approximate area of 50 ha in each locality. Regular fishing occurs. Small areas. Limited exploitation. Patchy settlements. Approximate area of 2 ha. No exploitation has been reported. Approximate area of 20 ha. Exploitation occurs. Approximate area of 2.5 ha. No exploitation has been reported. Approximate area of 5 ha. Oysters are sparsely distributed & exploited. Anchengo backwaters Highly populated oyster beds. Karnataka Nethravathi, Kali &Sharavathi estuaries Venkatpur, Bhatkal, Mulky, Uppunda and Coondapoor estuaries Oyster beds of limited extent. Oyster beds of same extent. Regular exploitation occurs. 79

13 Crassostrea gryphoides Crassostrea rivularis Saccostrea cucullata Maharashtra Goa Gujarat Maharashtra Maharashtra Gujarat Goa Dahanu creek, Mahim creek, Purnagab, Malwan, Alibag, Palghar, Satpuri, Boiser, Malad, Worli, Ratnagiri, Jaytapur, Versova, Gobbunder, Cuff Parade, Marve, Madh and Bandra Utsali, Navapur and Kelwa Ribanden, Siolim and Curca Aramra, Poshetra, Port Okha, Porbander, Sikka, Gagwa creek, Singach creek, Beet Kada, Khanara creek, Laku Point, Gomati creek, Navibander, Harsad, Balapur and Azad island Mahim, Ratnagiri and Jaytapur areas All along the coast All along the coast Vasco, Mandovi and Zuari estuaries, Chorao Island and Nerul Creek Oyster beds of same extent. Regular exploitation occurs. Oyster beds of same extent. Bottom culture is practiced. Oyster settlements reported. Oyster beds of same extent. Regular exploitation occurs. This species is found along with C. gryphoides. Oyster beds of same extent. Regular exploitation occurs. Oyster beds of same extent. Regular exploitation occurs. Oyster beds of same extent. Regular exploitation occurs. The systematics of the genus Saccostrea has been very confused in the past and there is still no general agreement among authors about the number of species living in the Indo-West Pacific. However, as Saccostrea cuccullata is often considered to be the only species occurring in that area, this conservative opinion is adopted here. India has a vast coastline of over 7000 km and the major Indian cities, Mumbai, Kolkata and Chennai are situated along these coasts. There are more than 100 major and medium estuarine channels along the east and west coast of India. The estuaries 80

14 along the east coast of India are long and wide whereas the estuaries along the west coast are smaller. Ganges-Brahmaputra Delta region along the east coast of India is the largest estuarine network in India. Since ancient times, estuaries in India have been a focal point of activities for human settlement, development of ports and harbours, transportation of men and material and for trade and commerce (Qasim, 2003). The estuaries along the west coast of India are unique in their physical and biogeochemical features. Kochi Backwaters is one of the largest estuarine systems along this region and the other important estuarine systems along this coast are Ashtamudi estuary, Kali River, Mandovi and Zuari estuarine system, Mumbai Harbour and Thane Creek system and a number of small estuaries like Sabarmati, Tapti and Narmada etc. Monsoon is a typical feature of this region. The south-westerly winds blowing from the Indian Ocean bring heavy rainfall into the Indian subcontinent during southwest monsoon (June to September) and these estuaries receive heavy river discharge during this period hence they are often called as "monsoonal estuaries". Salinity distribution in these estuaries undergoes intra-seasonal variations during southwest monsoon. Rainfall and subsequent river discharge during this period flushes out salinity very fast from these estuaries into the Arabian Sea. Thermodynamics of the water column of these coastal regions of the Arabian sea is greatly affected by the rainfall and subsequent freshwater inflow (Shankar and Shetye, 1999, 2001; Shenoi et al., 2002; Shankar et al., 2004, 2005; Suprit and Shankar, 2008). 81

15 Fig.4.2. Map of the west coast of India (Source: Manoj, 2008) Fig.4.3. Map of Goa showing the location of different river estuaries (Source: Mani et al., Aquatic Biosystems 2012, 8:30) The state of Goa, India's smallest state by area has a coastline of about 100 kilometers and lies between Lat 14 54' N ' N and Long ' E ' E. It is located in West coast of India in the region known as the Konkan, it is bounded by the state of Maharashtra to the north, and by Karnataka to the east and south, while the Arabian Sea forms its western coast (Fig. 4.2). The state is intersected by numerous rivers (Fig. 4.3), which can be navigated for different purposes. Most of the major rivers, 82

16 which cut across hinterland formations, originate in the Western Ghats across the border. The two main rivers are Mandovi (61.6 km in length), and Zuari (92.4 km), with their interconnecting Cumbarjua Canal (15 km) form a major estuarine complex. The other rivers are run for short distance as Terekhol (22.4 km), Chapora (28.8 km), Baga (5.4 km), Sal (16.1 km), Talpona (11.2 km), and Galgibag (3.8 km) in length (Esteves, 1966; Fonseca, 2001). Among the seven rivers flowing through the plains and hills of Goa, Mandovi and Zuari are of major importance and are called the lifeline of Goa. River Mandovi originates from the Parva Ghat of the Karnataka part of Sahyadri Hills and after traversing a stretch of about 70 km joins the Arabian Sea through the Aguada Bay near Panaji. Its width at the estuary mouth is about 3.2 km, while upstream it narrows down to about 0.25 km. Large number of tributaries join this river along its course which is characterized by a number of deltic islands. It is fed by monsoon precipitation from the discharge of a catchment area of about 1150 km 2. The Mandovi basin constitutes about 42% of the land area and covers about 1530 km 2 of the entire state. The occurrence of sandbar near the entrance of the Mandovi in the Arabian Sea has been known for centuries. The mechanism of sand transport, wave action and circulation of Mandovi estuary has been studied by Murty et al., (1976). River Zuari originates in the Dighi Ghat of the Karnataka part of the Sahyadri Hills and after flowing a zigzag stretch of about 67 km joins the Arabian Sea at Mormugao - Dona Paula point. It's width at the mouth of the estuary is about 5.9 km while upstream it narrows down, and at the upper reaches the width is less than 1 km. 83

17 Zuari basin covers an area of about 973 km 2 and receives discharge from a catchment area of about 550 km 2. These two rivers, Mandovi and Zuari are joined by Cumbarjua canal giving rise to a major estuarine system. The area covered by these two river basins is about 69% of the total basin area of the riverine system in Goa. There are a number of iron and manganese mines located along the banks of these two rivers from where bulk of iron ore is transported to Mormugao harbour through these rivers. The banks of both these rivers are provided with thick vegetation of mangrove forest. Geologically speaking, the Mandovi and Zuari river estuaries could be classified as drowned river valley estuaries formed due to the Holocene rise in the Sea level (Anon, 1978). The two river estuaries are rich in resources and are used for fishing activity, practically throughout the year and particularly during the monsoon months when sea -fishing gets suspended. The state of Goa is located between and N, and and E, along the Central West Coast of India. Its extreme length from north to south is 105 km, its greatest breadth from east to west is 65 km, and its entire area is 3,702 km 2 (Gomes, 1996; Fonseca, 2001). It is bounded by the Terekhol or Araundem River to the north, which separates it from the Maharashtra state, on the east by the Western Ghats, on the west by the Arabian Sea, and on the east and south by the Karnataka State. The region is divided into three main regions (De Souza, 1979; Wagle, 1982): i. The eastern Sahayadris - The Goa sub region of the Western Ghats, and covers ~ 43% of the state total area. 84

18 ii. The central uplands - It is the tract between the coast and the Ghats. It consists of a chain of rolling hills with gentle to moderate slopes and long, narrow intermediate valleys, and covers ~ 35% of the state area. iii. The western coastal plains - The coastal belt accounts for ~ 22% of the total area of Goa. Goa enjoys a typical tropical warm and humid climate with three seasons throughout the year, the cold/dry season or post-monsoon (October January), the warm or hot season or pre-monsoon (February May), and the rainy season or monsoon (June September). The pre-monsoon season is the warmest period of the year and experiences occasional showers towards the end of May. The maximum temperature for the whole year is about 36ºC, and the minimum is about 18ºC; the warmest days in the year are generally in May and the coolest mostly in February. The mean daily temperature varies slightly, from around 25ºC to about 30ºC, due to the maritime nature of the climate. The average annual temperature remains to 26ºC. The Sahayadris range (Western Ghats) prevents the cold, dry winds of the inland from sweeping down Goa and hence the state does not experience a normal winter (De Souza, 1979). However, temperature variation remains wider in the eastern region due to the mountainous topography, particularly in December to January, during which the Indian subcontinent experiences the winter. The state of Goa receives 90% of its rains from the southwest monsoon (during the wet season) as it lies along the coast and on the windward side of the Sahayadris. The average annual rainfall ranges from 2,800 85

19 mm to 3,500 mm. About 36% of the annual rains lash Goa in the month of July (Esteves, 1966; De Souza, 1979). The average relative humidity is 80%. This season is followed by the southwest monsoon (hereafter referred to as the monsoon season), during which the state receives most of its rainfall, with an average of about 3000 mm. The post-monsoon season is a fair and stable season. Normally, atmospheric temperature shows two peaks, one in October when warm and humid conditions exist and second in May, which is usually the hottest month of the year. The temperature of seawater varies between 26.5 and 31.0 C. Heavy rainfall, freshwater runoff, sandbar formation in the mouth region of estuary occurs during monsoon and is followed by recovery during post-monsoon and stability during pre- monsoon (Qasim and Sen Gupta, 1981). Salinity decreases considerably (3psu) during monsoon due to freshwater runoff and rainfall. The average annual freshwater runoff is 7 km 3 for Mandovi and 9 km 3 for Zuari estuaries (Anon, 1979). The estuaries may be classified as stratified during the monsoon season which gradually evolves towards a well-mixed one during the post monsoon period, according to the definition given by Pritchard (1952). Its pre and postmonsoonal flows are regulated by the semi- diurnal tides having amplitude of 2-3 m (average of 2.3 m) during spring tides. The currents are mainly influenced by tides during monsoon. Maximum distance of penetration of seawater (0.9 x 10-3) is about 67 km away from the mouth in May, which comes down to a minimum distance of about km in July-August. The estuarine complex is fringed with extensive mangroves (Wafar et al., 1997), which are biologically productive nursery grounds for a variety of commercially important fin and shellfish (Parulekar et al., 1980; Qasim and Sen Gupta, 86

20 1981). Earlier work on benthos (Parulekar et al., 1973; Parulekar and Dwivedi, 1974; Parulekar et al., 1975, 1980; Harkantra and Parulekar 1981, 1985) revealed high benthic production consisting of clam beds, polychaetes, other molluscs etc. These estuaries are extensively used for fishing, aquaculture, ore transport, harbor development, water recreation, waste disposal and adjacent land for human settlements (Parulekar et al., 1980; Qasim and Sen Gupta, 1981; Parulekar et al., 1986). In Zuari estuary, tides of mixed semidiurnal type with a minimum range of about 2-3 m are encountered causing the exchange of appreciable amount of saltwater into the system from the adjacent sea, the rate of which varies considerably with season (Cherian et al., 1975). During the pre and postmonsoon period the flow is regulated by the tides of semi-diurnal type like that of Mandovi. The freshwater discharge into this estuary during the monsoon season is high. However, the amount of freshwater received by Zuari is less as compared to Mandovi estuary. During post and premonsoon period, the estuary, to a distance of about 14 km is primarily tide- dominated due to meagre or negligible freshwater runoff. Maximum distance of penetration of water of 0.9 x 10-3 salinity is reported to a distance of about 65 km away from the mouth during the month of May. It gets reduced to a minimum of about 20 km during June-July following the onset of monsoon. The tidal influence has been recorded upto 41 km. A survey of oyster beds in association with mangroves was conducted along the estuarine system of Goa, for selection of sampling stations from January to June Oyster beds are under severe exploitation and only empty shells are found in many places (Fig Fig. 4.6). 87

21 Fig. 4.4: Oyster beds in Chorao Island and Nerul Creek of Mandovi - Zuari Estuary Two sites, Chorao Island and Nerul Creek of Mandovi - Zuari Estuary rich in mangroves and their associated oyster beds, were selected based upon the preliminary observations and published literature on oyster beds from the state. Fig.4.5: Collection of Oysters by fisherwomen from Oyster beds 88

22 Chorao Island (St-1): The Chorao Island is located between the Mandovi and the Mapusa Rivers in Goa. The western side of the island is taking a peninsular shape and occupied by a thick mangrove forest of about 1.78 km 2. This mangrove forest has been declared by the Government of Goa for the purpose of conservation as Dr. Salem Ali Bird Sanctuary in About 14 species of mangrove, 6 species of algae, 13 species of fishes, 34 species of resident birds and 14 species of migratory birds, in addition to reptiles and mammals, etc. were reported in Chorao island (Anon, 2003). The geographical location of the station is N latitude and E longitude. Fig.4.6. Location of sampling sites along Mandovi - Zuari Estuary, Goa (Source: Nagi, H.M., 2008) Nerul Creek (St-2): The Nerul creek is opened into the Aguada Bay of Mandovi Estuary. It extends inside the land in U-shape to a length of about 8.5 km. It is navigated by small fishing boats, and bounded by fringing and patchy mangrove habitats. The geographical 89

23 location of the sampling station is located in N latitude and E longitude, below the Nerul bridge. Based on the diagnostic characters of Saccostrea cucullata the samples were collected. The shells are stout, very variable in shape and size, usually medium sized to small and very inequivalve. Left (lower) valve is generally deep (occasionally very shallow), with large attachment area and strong radial ribs towards the periphery. The right (upper) valve is flattish, fitting down into the plicate margins of the wider opposite valve. Outer surface of right valve smoothish, sometimes with distinct radial ribs and, when not too much eroded, with concentrically arranged, imbricating plates of horny material. Irregular tubular spines rising vertically from surface of right valve sometimes, present on juveniles living in sheltered conditions form echinata (Quoy and Gaimard, 1835). In specimens not exposed to wave action and living crowded together, right valve may be small and operculiform and left valve with a greatly elongated, partly coiled ligamental area, giving the shell a slender conical or cornucopia shape (form cornucopiaeformis Saville-Kent, 1893). The adductor muscle is scar kidney-shaped, more or less posteroventral in position. Interior of valves is with a crescent-shaped series of small disjunct pallial imprints between the posterior adductor scar and the anterodorsalmargin. Umbonal cavity of left valve is generally deep. Chomata present all around the internal shell margins, forming one row of distant ridgelets in right valve, and corresponding pits in left valve. Saccostrea cucullata (Born, 1778) shell with unequal valves is variable in shape, trigonal, sometimes oblong, and irregularly circular to oval. The extremely hard left valve is more thick, deep and large than the right valve which is sometimes cup like or 90

24 sometimes flat. The outer margin with a number of sharp folds interlocks with each other. The sculpture of oppressed lamellae becomes spiny, worn or smooth. The muscle scars are kidney shaped and darker than the surrounding shell area. Nodular chomata are usually present around all margins. The shell colour is usually gray-white in colour with deep purple tinge towards the margin (Dey, 2006). The colour of the exterior of shell is dirty white to greyish brown, often with shades of purple and interior creamy white, with deep purple margins and frequently subnacreous iridescence. Adductor scar coloration may be darker than the surrounding shell area. The maximum shell height is 20 cm and commonly to 10 cm. Based on shell morphology, natural samples of Indian Saccostrea oysters were collected from 2 locations along the coasts of India namely Chorao Island of Mandovi estuary and in Nerul creek (Tributary of Mandovi towards confluence zone) down the Nerul bridge in Goa Oysters are found attached to various hard substrates, such as laterite rocks and boulders, in marine, estuarine and mangrove areas, often in dense colonies in Intertidal and shallow subtidal levels to a depth of 5 m. The oysters are from about mid-tide to low tide marks. The beds get exposed at low tides, and hence samplings were carried out at ebb. Collections of oysters (n = 25) were hand-picked from mangrove associated oyster beds at monthly intervals. Density of oyster colonies were assessed using quadrant of 25 cm x 25 cm from each site of collection, which distributed randomly over transects taken perpendicular to the shore line. 91

25 Fig.4.8. Dimensional terms applied to oysters (Source: Quayle and Newkirk, 1989) Saccostrea cucullata occur along the rocky littoral zone of Bombay and form beds. A preliminary study on the population of this little known resource was conducted during January-March 1986 in order to enhance our knowledge in the management and development of this food species for the benefit of the coastal fisher folk (Sundaram, 1988). The density of S. cucullata recorded from Bombay appears to be very high when compared to their occurrence in some other locations along east and west coasts of India observed by Rao and Sundaram (1972). Variations in the shape of the oyster shells are likely within the same species from the same area due to overcrowding, orientation, substratum and ecological conditions (Quayle and Newkirk, 1989). Due to the morphological similarities, it is difficult to distinguish between the two genera Crassostrea and Saccostrea externally. In determining the genus of oysters, the structure chromata is most important. Those which do not possess chromate will be grouped in the Crassostrea. In contrast, chromata are present in the Saccostrea group (Morris, 1985). The internal adductor muscle scar was used to distinguish between the two species (Siddiqui and Ahmed, 2002). Therefore, the preliminary identification of species based on morphology was carried out in the laboratory after shucking the specimens. 92