Abstract A research cruise was conducted in the eastern Indian Ocean off west Sumatra, Indonesia, in June 2003 to learn about the spawning and larval ecology of the tropical freshwater eels of the genus Anguilla in the region. A total of 43 anguillid leptocephali were collected during the cruise and they were genetically identified as 41 Anguilla bicolor bicolor, 1 Anguilla marmorata, and 1 Anguilla interioris. A. bicolor bicolor leptocephali were 44.1–55.5 mm TL and most of them were at the fully grown stage. Reexamination of the historical data of Jespersen (1942) also suggested a relatively low abundance of small size leptoceph- ali (<40 mm) of this species off west Sumatra. Although the study area has long been considered to be a spawning site of A. bicolor bicolor, the distributions of leptocephali from the two surveys and the patterns of ocean currents in the region suggest the possibility that the main spawning area of this species is located farther offshore. Keywords Freshwater eels Æ Indian Ocean Æ Larval migration Æ Distribution Æ Spawning area Introduction In the Carlsberg Foundation’s Oceanographical Expedition Round the World from 1928 to 1930, a total of 1,225 anguillid eels ranging from fairly small leptocephali to metamorphosing larvae and glass eels were collected by 326 tows of 2 m diameter plankton nets in the eastern Indian Ocean (Jespersen 1942). Most of the leptocephali collected by Jespersen (1942) were identified as the shortfin type of anguillid species. The only shortfin species known from the eastern Indian Ocean was Anguilla bicolor bicolor and therefore this species was thought to spawn near the Mentawai Trench (Ege 1939). However, almost all of the small leptocephali (10–20 mm) could not be categorized as being shortfin or longfin types, and at least eight large sized specimens were questionably identified as Anguilla celebesensis, which is distributed in the western Pacific region (Ege 1939). These problems were clearly due to the undeveloped morphological characters of the small specimens and the critical overlap of the taxonomic charac- ters among tropical anguillid species (Castle 1963; Aoyama et al. 1999). Even so, Jespersen’s (1942) J. Aoyama (&) Æ M. J. Miller Æ Y. Minegishi Æ M. Kuroki Æ T. Kawakami Æ K. Tsukamoto Ocean Research Institute, The University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan e-mail: jaoyama@ori.u-tokyo.ac.jp S. Wouthuyzen Æ S. R. Suharti Æ K. O. Sumardiharga Research Center for Oceanography, Indonesian Institute of Sciences, Jl. Pasir Putih 1, Ancol Timur, Jakarta 11480, Indonesia Environ Biol Fish (2007) 80:445–452 DOI 10.1007/s10641-006-9143-z 123 ORIGINAL PAPER Distribution of leptocephali of the freshwater eels, genus Anguilla, in the waters off west Sumatra in the Indian Ocean Jun Aoyama Æ Sam Wouthuyzen Æ Michael J. Miller Æ Yuki Minegishi Æ Mari Kuroki Æ Sasanti R. Suharti Æ Tatsuya Kawakami Æ Krunaen O. Sumardiharga Æ Katsumi Tsukamoto Received: 2 May 2006 / Accepted: 28 August 2006 / Published online: 26 October 2006 Ó Springer Science+Business Media B.V. 2006