Habitat preference, movements and growth of giant mottled eels, Anguilla marmorata, in a small subtropical Amami-Oshima Island river

Although anguillid eel populations have decreased remarkably in recent decades, few detailed ecological studies have been conducted on tropical eels such as the giant mottled eel whose range extends across the whole Indo-Pacific. This species was studied throughout the entire 0.5 km mainstem reaches of Oganeku River on the subtropical Amami-Oshima Island of Japan over a two-year period using four sampling periods to understand its habitat preference, early life-stage dispersal process, movements, and annual growth using a mark-recapture experiment conducted with quantitative electrofishing. A total of 396 juvenile growth-phase A. marmorata eels were caught and tagged, with 48 individuals being recaptured at least once. Their density irrespective of size of eels was most strongly determined by distance from the river mouth, followed by riverbank type according to random forest models. Eel density decreased with increasing distance from the freshwater tidal limit located about 100-150 m from the river mouth. Eels preferred vegetated riverbanks, while they avoided those of concrete and sand. The density of small eels (total length: TL < 240 mm) was also associated with depth and velocity, with small eels tending to prefer riffle or run habitats. In contrast, large eels (TL ≥ 240 mm) were found in habitats of any depth and velocity. The TL of eels had a minimum peak at around the tidal limit, and it increased with increasing distance from the tidal limit. The observed density and size gradients of eels in relation to the distance from the river mouth suggested that A. marmorata initially recruited to freshwater tidal limit areas and then dispersed in both downstream and upstream directions. The growth rate of eels varied greatly among individuals that were at large for various periods of time and ranged from 0 to 163.2 mm/year (mean ± SD of 31.8 ± 31.0 mm/year). Of the recaptured eels, 52.1% were recaptured in a section that was different from the original capture section, and their mean ± SD distance travelled was 46.5 ± 72.5 m (median = 20 m). 47.9% of the eels were recaptured from the original section of capture (i.e., <10 m distances travelled), suggesting that they had strong fidelity to specific habitats with limited movements. The distance travelled of eels that had moved was greater for small eels (range = 10-380 m; mean ± SD = 84.4 ± 121.9 m) than large individuals (range = 10-120 m; mean ± SD = 30.9 ± 31.0 m), which indicates that the mobility of the eels declines as they grow. This is the first clear detailed documentation of the spatial distribution, growth, and movements of tropical eels in a small river system in relation to environmental conditions that provides an example of how future studies can be conducted in other areas to understand how conservation efforts can be most efficiently targeted for maximum success

九州の河川におけるニホンウナギの生態学的研究

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Spawning behavior of garden eels, <i>Gorgasia preclara</i> and <i>Heteroconger hassi</i> (Heterocongrinae), observed in captivity

<div><p>The behavior and reproductive ecology of typically nocturnal marine eels is poorly known but garden eels (Congridae, Heterocongrinae) are familiar tropical marine eels. They live in colonies and protrude their bodies from burrows during the daytime to feed on zooplankton. Apparent daytime courtship/spawning-like behavior has been observed within the colonies but actual spawning and fertilized egg production has not been confirmed. This study describes observations of splendid garden eels (<i>Gorgasia preclara</i>) and spotted garden eels (<i>Heteroconger hassi</i>) spawning in low light at night and producing fertilized eggs in a large multispecies public-display tank in the Sumida Aquarium. Video recordings of 5 of 17 detected spawning or egg or sperm release events documented the spawning behavior of <i>G. preclara</i>, and positively buoyant fertilized eggs of both species were collected. Their spawning behavior while protruding from widely spaced burrows confirms that garden eels likely spawn within their colonies with their pelagic eggs drifting away with currents.</p></div