First record of the Pacific bluefin tuna Thunnus orientalis (Temminck & Schlegel, 1844) from the coast off Sur, Sultanate of Oman

— A single specimen of the Pacific bluefin tuna Thunnus orientalis was caught on 11 May 2017 in a long-line operated about 40 nautical miles off the coast of Sur atconfluent of Sea of Oman and Arabian Sea coast of Oman. This first record of its occurrence indicates the extension of distributional range of the species to the Arabian Sea coast of Oman.


INTRODUCTION
Oman has a very long coastline of 3165 km with connections to three seas namely the Arabian Gulf, the Gulf of Oman and the Arabian Sea. The country has rich fish biodiversity (Al-Jufaily et al., 2010). The biodiversity of fish fauna of Omani coasts had been reported by several earlier workers (Boulenger, 1887;Steindachner, 1902;Regan, 1905;Norman, 1939;White & Barwani, 1971;Randall, 1995

II.
MATERIAL EXAMINED One specimen, Fig. 2, is 250 cm total length (TL), (204 cm standard length-SL; 224 cm fork length-FL); caught by long line; 11 May 2017. Atconfluent of Sea of Omanand Arabian Sea coast of Oman. It is difficult to say precisely because the fisherman did not have GPS (global positioning system). The sample was brought to the Marine science laboratory at the Marine Science and Fisheries Centre, Ministry of Agriculture and Fisheries Wealth, Sultanate of Oman for analysis. After weighing,the morphometric and meristic characters of the fish were recorded following standard procedures (John& Schaefer, 1949). Then, the fish was cut open to identify the sex, maturity stage of the gonad and feeding intensity. The liver and air bladder structure were observed. Otoliths were extracted from head for subsequent age analysis. The muscle tissue was collected for genetic analysis. The fish sample was buried to preserve and display the bones in the museum.

Description
The fish weighed 237 kg and was a mature male. The feeding intensity was 1/2full stomach and the food was in fully digested state. Ventral surface of liver was striated with blood vessels. Air bladder was irregular. There were 36 gill rakers in the first left gill arch (Fig. 3)

Fig.3: First gill arch-Total gill rakers-36.
The morphometric characters (Table-1) and meristic counts (Table-2) of the specimen agree with the descriptions of the species T. orientalis which was further confirmed by the genetic analysis of mitochondrial DNA with D-Loop (NCBI. Accession No. JN631213.1). Body: Fusiform, almost round, very robust in front and tapering towards caudal peduncle. Head lengthless than body depth. Mouth large,teeth small and conical in a single series. The first dorsal height less (10.7% of FL) than the second dorsal fin height (16.1% of FL) in the present specimen; pectoral fin short and less than head length and about 17% of FL. In T. orientalis, the short pectoral fin and heights of 1st and 2 nd dorsal fins are considered as the prominent morphometric characters to identify the species (Nelson, 2006). Coloration: Lower side of belly was silvery white but no other colour pattern was visible. This might be due to the delay in bringing the specimen to the laboratory after three days. However, in fresh specimens lower sides and belly are with faint colourless transverse lines alternated with rows of faint colourless dots (Collette et al., 1983;Nelson, 2006;Tamura & Takagi, 2009).The first and second dorsal fins were dark bluish with dusky yellow tips. Dorsal and anal Finlets were yellowish, base reddish-brown towards middle and tips. Median caudal keel was black. Pectoral fin rays 30 5 Anal fin rays? 12 6 Anal Finlets 8 7 Gill rakers 36 8 Keels 3

III. DISCUSSION
Thunnus orientalis is an epipelagic oceanic species that forms school by size and performs wide horizontal and vertical migrations and seasonally moves close to the shore (Magnuson, 1973;1978;Kitagawa et al. 2007). This species is distributed in depth range of 1 to 550 m (Froese & Pauly, 2009;IUCN, 2014). The fish has great tolerance of sea-surface temperature ranging from 17 o C to 23 o C and be able to dive deep waters as cold as ~3°C(fish Base). It is a voracious predator feedingprimarily on a variety of small schooling fishes such as anchovies, sardines, herrings, menhaden and mackerels and also on squids, crabs and other less sessile organisms (Collette and Nauen, 1983;Allain, 2005;Swada et al., 2005).
The record of T. orientalisfrom the Arabian Sea coast of Oman is significant as the species has been found distributed in north and south Pacific (Ashida et al. 2015;Collette et al. 2014, Lewis, 2012Itoh et al. 2003;Tanaka et al. 2006). Although, this species moves in schools, only single specimen was caught presently. This would lead to speculations such asincidental occurrence owing to disorientation or be linked to the ecological changes that occurred due to rise in water temperature in its distributional ranges causing a decline in prey organisms and hence, migrated to more abundant prey related regions or to unknown biotic and/or anthropogenic factors (Sharp & Dizon, 1978;Kimura et al., 2010). The species T. orientalis is represented by a single Pacificwide stock that is found primarily in the North Pacific Ocean (Kitigawa et al., 2007). Hence, it may be presumed that the migration of the species from the north Pacific to the Arabian Sea coast of Oman might have occurred through the Bay of Bengal. Hence, it would be interesting to know whether the extension of the species distribution is very recent to the countries bordering the Bay of Bengal coast or already established populations are available in the region. Intensive research in this line is needed to better explain the occurrence of a solitary Pacific blue fin tuna from Oman.Tagging studies wouldreveal the possible routes of movement pattern of T. orientalis if any, from north Pacific to the western Indian Ocean.
ACKNOWLEDGEMENT I would like to thank the fisherman for taking the effort to report to Ministry of Agriculture and Fisheries. Thanks are due to the Director General of Fisheries Research and Director of Marine Science and Fisheries Centre for arranging to purchase the fish sample. Thanks to all staff of the centre for their effort in handling such huge fish specimen.