Discovery of mature freshwater eels in the open ocean


All freshwater eels of the genus Anguilla are catadromous, in that they perform long-distance migrations to offshore spawning area after years of growth in freshwater and nearshore habitats [1]. Finding younger and smaller leptocephali indicated the spawning area to be located in the Sargasso Sea for European and American eels (Anguilla anguilla and A. rostrata, respectively) [2, 3] and in the western North Equatorial Current (NEC) area of the western North Pacific to the west of Mariana Islands for Japanese and giant mottled eels (A. japonica and A. marmorata, respectively) [4–6]. Recent investigation could further narrow down the area and timing for spawning of the Japanese eel (A. japonica), where a large number of small pre-leptocephali (n = 130, 4.2–6.5 mm in total length) were collected during the new moon in June 2005 around 14 N, 142 E to the west of the Suruga Seamount of the West Mariana Ridge [7]. No matured adult eels have been captured or observed to date in the spawning area, not only for the four species mentioned above, but also for any species of freshwater eel. The Fisheries Agency of Japan concerned recent declines in catches of glass eel (small juveniles migrating to freshwater area), and dispatched R/V Kaiyo Maru to capture adult Japanese eel for collecting biological information. Here, we report the first discovery of fully matured anguillid eels in the open ocean. We used a large mid-water trawl net (maximum mouth opening 65 m wide and 55 m height with 7 mm mesh cod end) (Nichimo Co., Ltd., Tokyo), which could be towed at 4–5 knots. A total of 21 nighttime tows were performed for 0.5–4 h at a depth mainly of 170–300 m in the area adjacent to the southern part of the West Mariana Ridge (Fig. 1a) during 25 May to 9 June 2008 (Fig. 1, closed circles). Eventually, three anguillid eels were caught in two tows on 3 and 4 June operated at 13 000N to 13 050N, 142 080E to 142 250E (Fig. 1c, red circles), approximately 130 km south of the Suruga Seamount. Two individuals (48.5 and 62.3 cm total length, 112.0 and 313.5 g body weight) were caught in a single set on 3 June (Fig. 1c, larger red circle), where stepwise towing was performed at a depth of 230–280 m for the first 2 h followed by a shallower depth from 175 to 225 m for next 2 h. The water temperature at 170–300 m ranged from 13.6 C to 25.4 C. Since the largest individual was alive, drifted or vigorously swam around in the holding tank for 4 h without staying on the bottom until sacrificed for dissection (Fig. 2a, d); we assumed that the eels were caught in the shallower layer. However, a subsequent set towed in the opposite direction following the same course at a depth of 160–210 m for 2 h S. Chow (&) H. Kurogi National Research Institute of Fisheries Science, Nagai 6-31-1, Yokosuka 238-0316, Japan e-mail:

DOI: 10.1007/s12562-008-0017-5

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@article{Chow2008DiscoveryOM, title={Discovery of mature freshwater eels in the open ocean}, author={Seinen Chow and Hiroaki Kurogi and Noritaka Mochioka and Shunji Kaji and Makoto Okazaki and Katsumi Tsukamoto}, journal={Fisheries Science}, year={2008}, volume={75}, pages={257-259} }