Distribution of leptocephali of the freshwater eels, genus Anguilla, in the waters off west Sumatra in the Indian Ocean

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 leptocephali (<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.

The ecology of oceanic dispersal and survival of anguillid leptocephali

“…Japanese scientists then discovered the spawning area of A. japonica in 1991 (Tsukamoto 1992) after smaller and smaller larvae were collected farther offshore . These surveys expanded out across the Indo-Pacific (Aoyama et al 2003(Aoyama et al , 2007Kuroki et al 2006) with evidence of offshore spawning by eels in the western South Pacific being found (Kuroki et al 2008;Aoyama 2009) and local spawning by eels in the Indonesian seas being discovered (Aoyama et al 2003). These collections and genetic or morphological information about anguillid population structures (Minegishi et al 2008;Watanabe et al 2008Watanabe et al , 2011 and silver eel tagging studies (Schabetsberger et al 2015;Jellyman and Tsukamoto 2010) led to ideas about the locations of Indo-Pacific anguillid spawning areas ( Fig.…”

“…None of the surveys had sampling stations distributed with appropriate geographic coverage to be able to understand the larval dispersal patterns of each species, but they showed that anguillid leptocephali are widely distributed in the deepwater areas of the region. Anguilla bicolor bicolor leptocephali were abundant off West Sumatra in SeptemberNovember 1929, however (Jespersen 1942), and were present in June 2003 (Aoyama et al 2007), indicating spawning occurs in that area. It is unclear though how widely the larvae from that spawning area may be dispersed by the South Equatorial Current or by equatorial jets (Fig.…”

“…Anguillids in the western South Pacific, such as the two subspecies of Anguilla australis from Australia and New Zealand, the New Zealand longfin eel (Anguilla dieffenbachii) and the Australian longfin eel (Anguilla reinhartii), migrate far offshore to spawn in the South Equatorial Current according to collections of their larvae (Jellyman 2003;Kuroki et al 2008;Aoyama 2009) or data from satellite pop-up tagging studies of migrating silver eels (Jellyman and Tsukamoto 2010). There is also evidence about the location of a separate tropical eel spawning area in the western South Pacific from tagging studies (Schabetsberger et al 2015), and larval collections have indicated there is an anguillid spawning area in the eastern Indian Ocean (Jespersen 1942;Aoyama et al 2007). The spawning areas that are known or inferred are located in westward flowing currents that transport the larvae towards boundary currents or are closer to landmasses or islands (Fig.…”

Ecology of Anguilliform Leptocephali: Remarkable Transparent Fish Larvae of the Ocean Surface Layer

“…Spawning seasons of temperate anguillids have been roughly estimated from the timing of the downstream migrations of silver eels into the ocean (Jellyman 2003;Tesch 2003) Kleckner and McCleave 1988;Tsukamoto 1992;Tsukamoto et al 2003). The timing of spawning of tropical anguillids is much less understood, however, even though seasonal differences have been observed in the presence or size of leptocephali in the central Indonesian Seas (Wouthuyzen et al 2009) and off west Sumatra in the eastern Indian Ocean (Jespersen 1942;Aoyama et al 2007).…”

Ariosoma-type Leptocephali (Congridae: Bathymyrinae) in the Mentawai Islands region off western Sumatra, Indonesia

“…Sampling at each station consisted of two tows of an Isaacs-Kidd midwater trawl (IKMT) with a mouth opening or 8.7 m 2 and a 0.5-mm mesh size. At each station, the IKMT was deployed at night in an oblique tow and a step tow (five horizontal sampling layers), both of which sampled the upper 200 m (see Aoyama et al 2007 andMiller et al 2011a for details). An extra tow was made at stn.…”