Status of Marine Turtles in the Gulf of Mannar, India
Abstract
Lepidochelys olivacea was the most frequent sea turtle documented in the Gulf of Mannar during 2000–2001, whereas during the 1970s it was Chelonia mydas. Reduction in overall proportions of C. mydas and its size classes is attributed to exploitation of eggs, turtles, and incidental catch in fishing gear.
Five species of sea turtles occur in the Gulf of Mannar off the southeast tip of India, including the olive ridley (Lepidochelys olivacea), the green turtle (Chelonia mydas), the loggerhead (Caretta caretta), the hawksbill (Eretmochelys imbricata), and the leatherback (Dermochelys coriacea). Our study focuses on the status of marine turtles in Gulf of Mannar with particular emphasis on the changes in the population of green turtles, which occur there in considerable numbers but whose actual status is poorly documented. The Gulf of Mannar extends from Cape Comorin (lat 8°4′40″N, long 77°33′4″E) to Dhanushkodi (lat 9°9′9″N, long 79°26′46″E), and has about 250 km of coastline (Fig. 1). Extensive coral and seagrass beds on the northern side and deep sea on the southern side make this area suitable for all 5 species of sea turtles found in this region. Over 20 islands are found in the Gulf of Mannar, including Krusadai and Rasmeswaram. Turtle fishing was practiced in this area for ages, and turtles were exported to Sri Lanka and other countries until 2 decades ago (Agastheesapillai and Thiagarajan 1979). Although the export of sea turtles was banned during the early 1980s by the Indian Wildlife Protection Act of 1972 (Anonymous 1991), the subsistence exploitation of turtles continues there even today.
Citation: Chelonian Conservation and Biology 5, 1; 10.2744/1071-8443(2006)5[139:SOMTIT]2.0.CO;2
Methods
We surveyed the beaches south of Tuticorin (70 km) and Rameswaram Island (15 km) bimonthly, from November 2000 to April 2001. At least 10 km was covered during a day's survey, and the sampling was restricted to 0500–0800 hours. The number of tracks and shells of dead turtles found were recorded. We also monitored the fish market of Rameswaram Island and 6 others along the Tuticorin coast. On locating a turtle, we recorded the species and its curved carapace length (CCL). The shells of dead turtles were marked with paint to avoid repeat count.
Results and Discussion
In this area, fishermen collect turtles using multifilament gill nets. These nets measured over 50 m in length and 6 m in width, and had mesh size from 15 to 22 cm. Entangled live turtles were taken for consumption, and the dead ones discarded in the sea. Considerable trawler fishing is also practiced throughout this range. Altogether, we observed 178 specimens, including 122 along the Tuticorin coast and 56 in Rameswaram Island (Table 1). In addition, 15 nests (14 on the Tuticorin beach and 1 on Rameswaram Island) were observed. Overall proportion of species showed that L. olivacea was the most frequent (56.2%), followed by C. mydas (38.2%); other species together formed only 5.6%. Based on the turtle landing data prior to 1950 at Krusadai Island, Kuriyan (1950) reported that the majority of turtles caught in the area were C. mydas. This species formed about 89% of all turtles caught in the Gulf of Mannar and Palk Bay during 1971–1976 (Agastheesapillai and Thiagarajan 1979). Even though the method adapted for data collection in the earlier studies was different from that of the present one, available data indicate a reduction in the proportion of C. mydas.
Table 2 provides some aspects of morphometry of species found in the Gulf of Mannar. Length frequency of 99 L. olivacea specimens showed that about 15% of the turtles had CCL of less than 60 cm (Fig. 2). Dash and Kar (1990) speculated that in the Bay of Bengal this species attains sexual maturity when it grows over 60 cm in shell length. Information on subadult or juvenile ridleys is scanty, and only 3 reports are available from the Bay of Bengal. Pandav et al. (1995) reported only 5 subadult ridleys during the last 20 years of research along Gahirmatha beach, which is one of the well-known mass nesting areas of L. olivacea. Along the Nagapattinam coast (southeastern coast of India) 2 subadults were recorded from November 2000 to May 2001 (Bhupathy and Karunakaran 2003). Deraniyagala (1953) reported 2 juveniles of this species from Morutuva in Sri Lanka. Our records of over 15 juveniles and subadult ridleys in the Gulf of Mannar during December–April, the breeding season for L. olivacea, partially fills the information gap on the developmental habitat for this species in this region.
Citation: Chelonian Conservation and Biology 5, 1; 10.2744/1071-8443(2006)5[139:SOMTIT]2.0.CO;2
Agastheesapillai and Thiagarajan (1979) predicted that in Gulf of Mannar, C. mydas attains sexual maturity when it grows over 81 cm in CCL. Of the 68 C. mydas observed during our study, about 70% of them had CCL below 80 cm (Fig. 3), indicating that the majority were immature. Prabavathy (1992) studied the biology of C. mydas from March 1991 to April 1992 along Tuticorin Bay. The study area (Tuticorin), and data collection methodology (i.e., based on stranded turtles along the beach and market surveys) were largely similar to the present study. Of the 31 specimens observed from November 1991 to April 1992, only 25.8% had a CCL of less than 80 cm (Fig. 3). The size class distribution of turtles in both studies was significantly different (χ2 = 186.99, p < 0.001, df = 4). Reduction in the proportion of adult turtles could be due to exploitation over several decades, especially the take of nesting females.
Citation: Chelonian Conservation and Biology 5, 1; 10.2744/1071-8443(2006)5[139:SOMTIT]2.0.CO;2
Because both the overall proportions of C. mydas and the proportions of mature turtles have decreased, we conclude that, as elsewhere (Limpus 1995), the population of this species is declining in the Gulf of Mannar, India. All of the major detrimental factors, such as exploitation of eggs and turtles irrespective of size (Frazier 1980) and incidental death due to the careless operation of fishing gear, exist in this region.
The length frequencies of both L. olivacea and C. mydas (Fig. 3), show that the Gulf of Mannar is an important development and foraging habitat for turtles in the Bay of Bengal. However, no population estimates for these species are available for this region. Unless serious conservation measures such as exploitation control are undertaken, long-term survival for sea turtles in the Gulf of Mannar is bleak. An ecological study and sea turtle population monitoring program would provide the required data to propose conservation measures for sea turtles in this region.
Map of southeast India showing the Gulf of Mannar.
Size structure of Lepidochelys olivacea in the Gulf of Mannar, India, from November 2000 to April 2001 (n = 99).
Comparison of size class distribution of Chelonia mydas in the Gulf of Mannar, from 1991–1992 (n = 31) and 2000–2001 (n = 68). Data for 1991–1992 were extracted from Prabavathi (1992). A significant difference was found between the size classes in the 2 sampling periods (χ2 = 186.99, p < 0.001).
