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

Abstract

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.

Influence of environmental parameters on habitat use by sympatric freshwater eels Anguilla marmorata and Anguilla japonica on Yakushima Island, Japan

“…Catadromous anguillid eels, among the top predators in freshwater ecosystems, are another group that segregates their habitats when more than one species occurs in the same river system (e.g., Glova et al 1998;Hagihara et al 2018). Several studies have reported the habitat use of allopatric freshwater eels (e.g., Itakura et al 2018;Itakura and Wakiya 2020;Kume et al 2020). However, few studies have collected detailed information on the environmental differences in habitats in sympatric freshwater eels, although these habitat features could play key roles in habitat segregation for anguillid species.…”

Residence, activity patterns and behaviour of the giant mottled eel Anguilla marmorata in two freshwater protected areas in the Philippines

“…The telemetry array was confined to the FPA boundaries, so the extent of upstream movements was unclear. In the only (to our knowledge) previous freshwater telemetry study of A. marmorata , recaptured marked eels in the Oganeku River on the Amami‐Oshima Island of Japan had moved an average of 46.5 ± 72.5 m (median 20 m; Itakura & Wakiya, 2020). The Japanese eel has also been shown to have small home ranges using similar methods (Itakura et al, 2018).…”

“…Given the long distances from the estuary (>100 km), it would be expected that eel population structure in the two FPAs studied would comprise larger individuals at lower density relative to the middle reaches (Itakura & Wakiya, 2020). All captured eels exceeded 300 mm length, but fyke nets of the mesh size used (10 mm) would not be expected to capture smaller individuals efficiently.…”

“…Although there have been satellite tracking studies of oceanic migration patterns (Schabetsberger et al, 2013), freshwater habitat use and movement patterns have largely been inferred from point sampling of individuals (Shiao et al, 2003; Shuai et al, 2021). One exception is a study that used mark–recapture and a variety of sampling methods to study the growth patterns, habitat preference and movements of A. marmorata on the Amami–Oshima Island, Japan (Itakura & Wakiya, 2020).…”

Morphological and gonadal histological characteristics of the silver‐phase male Indo‐Pacific eel Anguilla marmorata

“…In recent years, commercial exploitation of A. marmorata has increased (Shiraishi & Crook, 2015), so a better understanding of its life history and ecology has become important for conservation and sustainable use of the species. Although the ecology of most tropical anguillids is still poorly understood, various research studies on A. marmorata have been conducted in recent years, and consequently the ecological knowledge on its genetic population structure (Minegishi et al ., 2008), spawning areas (Kuroki et al ., 2009, 2020), growth of leptocephali (Kuroki et al ., 2006), recruitment of glass eels (Aoyama et al ., 2015; Arai et al ., 2001), riverine distribution (Hagihara et al ., 2018a; Itakura & Wakiya, 2020), migratory history (Arai & Chino, 2018; Hagihara et al ., 2018a), age and growth (Hagihara et al ., 2018b; Wakiya et al ., 2019) has accumulated. Studies have also been conducted on their morphological and physiological changes in relation to downstream migration (Hagihara et al ., 2012), seasonality of downstream migration (Hagihara et al ., 2018c), swimming behaviour of migrating eels in a freshwater lake (Watanabe et al ., 2016) and the ocean (Schabetsberger et al ., 2013), and biological characteristics of adult eels in an offshore spawning area (Tsukamoto et al ., 2011).…”