New occurrence of big eye thresher shark Alopias superciliosus lowe, 1841 in Gulf of Mannar, southeast coast of India. A.M. Gowthaman, P. Jawahar 1 & V.K.Venkataramani 1 Uni - president Enterprises Corporation, Chennai, India 1 Fisheries College and Research Institute, Tamil Nadu Veterinary and Animal Sciences University, Thoothukudi-628 008, India. [Email: jawaharphd@gmail.com] Received; revised The big eye thresher Alopias superciliosus Lowe, 1841 is a pelagic subtropical species with a circum-tropical distribution 1,2,3,. Its occurrence in India has been reported to be restricted to southwest coast of India 4 until this record. This is the first observation of capturing Alopias superciliosus Lowe, 1841 in southeast coast of India. Species was caught by surface gill net (locally called paruvalai) operated by fishers from Solapuram near Manapad, southeast coast of India (lat. 8 0 34 N; Long. 78 0 06 E) and the rare specimen caught was a male Alopias superciliosus whose total length was 295cm and caught on 27.07.2010. The characteristic deep horizontal groove on each side of nape (Figure 1), the very large eyes with expanded orbit onto the head s dorsal surface and the position of the first dorsal fin base, closer to pelvic bases than to the pectoral, helped to identify the landed specimen as Alopias superciliosus 2,4,5. Morphological features recorded are given in Table. 1. Various literature appraisals indicated that the species was never recorded from Gulf of Mannar, southeast coast of India 6,7,8. This is the first report about the occurrence of Alopias superciliosus from Gulf of Mannar, one of the important Biosphere Reserves located in India. Fischer and Bianchi 4 indicated that these species normally occur at the depth range of 475-500 m. The present observation indicates that the specimen was caught by surface gill net at a depth less than 20 m which might be during the pelagic migration. Preti, et. al., 9 reported that these sharks have a diverse diet and feed opportunistically on locally and temporally available prey, including epipelagic, mesopelagic, epi-benthic and deep scattering-layer species. This migration likely relates to finding prey at night and avoiding predators during the day 10. Nakano. et. al., 11 also indicated that this species undertake a diel 1
vertical migration, spending daytime in deeper water, below the thermocline and ascending above it to water less than 100 m deep during night. Thresher sharks are listed as a game fish by the International Game Fish Association (IGFA) 12. Recently all the thresher shark species are designated as Vulnerable by the IUCN 13. Ebert 14 indicated that the bigeye thresher sharks are highly susceptible to overexploitation due to its low lifetime fecundity. Fischer and Bianchi 4 revealed the availability of three species viz., Alopias pelagicus, Alopias vulpinus and Alopias superciliosus in Indian waters. Of these three Alopias vulpinus is the most commonly occurring species of thresher sharks from southeast coast 4, 15 whereas A. superciliosus was recorded only in Arabian coast 15 and it has been designated as a rare group 16. Earlier report indicates that they are mainly caught by pelagic long line fishing and drift gill net 17, 18, 19. Capture of this species was possible due to multiday fishing, as indicated by the fishermen of southwest coast of India 15. Jensen 12 indicated that the male attains maturity by approximately 8.86-9.45 feet (270-288 cm) around 9-10 years. The landed specimen length has been measured as 295cm, indicating matured male and well developed claspers has been observed. The colour of the specimen was brownish grey colour above, creamy below. Fischer and Bianchi 4 indicated that this shark is an ovoviviparous and could have a reproductive potential of up to 4 per litter. Low fecundity and slow growth make the big eye thresher vulnerable to long line and gillnet fisheries 14. Sharks, a group occupy the role of top predators in the marine food web 20. In general, overexploitation of elasmobranchs resulting in considerable decrease in population will have a long term negative influence on their stocks and also to the marine community. IUCN 13 has recently included all the three thresher shark species as Vulnerable to protect them from extinction. Present report is the first report of their extended distribution in southeast Indian waters and particularly in the biodiversity rich Gulf of Mannar region. As these species are rarely caught as by catch and considerable efforts and educating the stakeholders are necessary to maintain the biodiversity of this vulnerable group in this region and to protect the gulf ecosystem. 2
Table: 1. Morphometric characters of Alopias superciliosus captured from southeast coast of India Morphometric Characters Measurement (in cm) Total length 295.00 Snout length 13.00 Standard length 165.00 Caudal fin length 130.00 Eye diameter 9.50 Mouth width 10.50 Pre dorsal length 96.00 Inter dorsal length 31.00 Pre anal length 156.00 Pre pectoral length 49.00 3
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