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

Eight silver-phase male Indo-Pacific eels (giant mottled eels) Anguilla marmorata were collected from three small rivers on subtropical Amami-Oshima Island, Japan, and their morphological and gonadal-histological characteristics were examined. The total length, body mass and age of the silver eels were 543.8 ± 49.3 (496-659) mm, 376.0 ± 116.0 (282-660) g and 14.6 ± 2.3 (11-19) years old, respectively. The silvering-related characteristics (eye index, pectoral-fin index, gut-somatic index) and reproductive characteristics (gonadosomatic index, histological stage) of the silver eels were significantly advanced compared to those of yellow eels, which is similar to other anguillid species.

Slower growth of farmed eels stocked into rivers with higher wild eel density.

Farmed anguillid eels are frequently stocked into natural fresh waters to enhance eel resources, but little is known about what happens to these eels or their interactions with wild eels after stocking. A recent study observed a depressed survival and growth rate of farmed Japanese eels when they were reared with wild eels, which indicated that wild eels might interfere with the survival and growth of farmed-and-stocked eels through intraspecific competition. To contribute to improving eel stocking efficiency, the growth of farmed-and-stocked Japanese eels was compared among four rivers with different wild eel densities using mark-and-recapture studies. Based on the 2-year recapture survey after stocking, it was found that the density of the farmed-and-stocked eels was not significantly different among rivers. The daily growth rates of farmed-and-stocked eels in the rivers with lower wild eel density were significantly higher than those of the eels stocked into the rivers with higher wild eel density. The farmed-and-stocked eels moved significantly greater distances downstream than wild eels that showed sedentary behaviour. This and previous studies indicate that significant questions remain about the effectiveness of stocking farmed eels into water bodies where naturally recruited wild eels are present.

Agonistic behaviour of wild eels and depressed survival and growth of farmed eels in mixed rearing experiments.

To contribute to improving eel stocking procedures, the survival, growth and behaviour of farmed Japanese eels reared together with wild individuals were evaluated to learn about possible encounters of farmed- and stocked eels with wild eels. Two experiments were conducted to evaluate the (a) effect of the presence of wild eels on survival and growth of farmed eels in experimental ponds and (b) behavioural differences between wild and farmed eels placed in a small tank. In the survival and growth experiment, significantly lower survival and growth rates of farmed eels reared with wild eels were observed compared with farmed eels reared without wild eels (control). In the behavioural observations, the authors observed significantly higher occupation rates of a refuge and more frequent biting for wild eels, indicating a dominance of wild eels over farmed eels. Because wild and farmed eels used in the current experiments belong to the same genetic population, these differences resulted within one generation through the aquaculture rearing process.

Estimations of Riverine Distribution, Abundance, and Biomass of Anguillid Eels in Japan and Taiwan Using Environmental DNA Analysis.

Although populations of anguillid eels have declined remarkably in recent decades, monitoring data on the spatial and temporal variation in their dynamics are often limited, particularly for tropical eel species. As there are often sympatries of multiple eel species in tropical rivers, identifying eel species based solely on morphological characteristics is challenging. Basin-scale surveys were conducted in rivers of southern Japan and northern Taiwan to investigate (1) whether the spatial distribution, abundance, and biomass of the tropical eel species, the giant mottled eel (Anguilla marmorata), can be monitored in rivers by comparing the results obtained from environmental DNA (eDNA) analysis with data from electrofishing and (2) the riverine distribution of the sympatric A. marmorata and the temperate eel species, the Japanese eel (Anguilla japonica), in this region using eDNA analysis. Although we found an much lower abundance of A. marmorata in the study region, we identified the eDNA of the species from all of the study sites (21 sites) where it was collected by electrofishing, in addition to 22 further study sites where it was not collected directly. This indicates that eDNA analysis has a greater sensitivity for detecting A. marmorata, making it a powerful tool for monitoring the spatial distribution of the species in rivers. We found a significant positive relationship between eDNA concentration and both the abundance and biomass of A. marmorata, and eDNA concentration seemed to better reflect the abundance of the species than did biomass. eDNA of both A. japonica and A. marmorata was identified from almost all rivers, indicating the sympatry of these species in this region, although the degree of sympatry differed between rivers. Though the eDNA concentration of A. japonica decreased significantly with increasing distance from the river mouth, no significant relationship was found for A. marmorata. This study is the first to demonstrate the potential usefulness of eDNA analysis for estimating the spatial distribution, abundance, and biomass of tropical eels in rivers and to further apply this method to investigate sympatry among anguillid species. eDNA analysis can help in obtaining data on the population dynamics of tropical eels, providing invaluable information for managing these species.

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.

Anguillid eels as a surrogate species for conservation of freshwater biodiversity in Japan.

To monitor and manage biodiversity, surrogate species (i.e., indicator, umbrella and flagship species) have been proposed where conservation resources are focused on a limited number of focal organisms. Using data obtained from 78 sites across six rivers in the mainland Japan and the Amami-Oshima Island, we demonstrate that two anguillids - the Japanese eel (Anguilla japonica) and the giant mottled eel (A. marmorata) - can act as surrogate species for conservation of freshwater biodiversity. Anguillid eels were the widest topographically-distributed species ranging from near the mouth to the upper reaches of rivers. Moreover, stable isotopic analyses indicated that eels are likely one of the highest-order predators in freshwater ecosystems. A significant positive relationship was found between the density of eels and the number of other diadromous species collected. However, the optimal models revealed that both the density of eels and the number of other diadromous species were significantly negatively correlated with distance from the river mouth and cumulative height of trans-river structures from the river mouth to each site. This suggests the positive relationship between eel density and number of other diadromous species was indirect and related to river-ocean connectivity. Given their catadromous life-cycle, and global commercial and cultural importance, as a taxa, anguillid eels can act as indicator, umbrella and flagship species, and a comprehensive surrogate for conservation of freshwater biodiversity.

Evaluation of downward movements of Japanese eel Anguilla japonica inhabiting brackish water areas.

This study evaluated the size and age distributions and otolith microchemistry of the Japanese eel Anguilla japonica in freshwater and brackish water areas in the Aki and Tsuchikawa rivers for 1 year, and in brackish water areas in the Asahi River for 3 years to understand the movements of Japanese eels between continental habitats of different salinity after recruitment (n = 759). For all three rivers, the total length (LT ) and age distributions were consistent; yellow eels captured in the upper brackish water (Aki River: 353.5 ± 77.4 mm and 3.0 ± 0.8 years; Tsuchikawa River: 287.7 ± 87.3 mm and 3.7 ± 1.3 years; Asahi River: 418.2 ± 112.1 mm and 4.2 ± 1.7 years) were smaller and younger than not only those in the fresh water of the two rivers but also those in the lowest brackish water sampling areas (Aki River: 436.0 ± 71.6 mm and 3.8 ± 1.1 years; Tsuchikawa River: 370.9 ± 121.7 mm and 4.9 ± 2.3 years; Asahi River: 558.5 ± 85.9 mm and 5.7 ± 1.7 years). In the Asahi River, these tendencies were found throughout the 3 years. Otolith analysis indicated that the majority of the eels captured in the lowest brackish water areas had moved down from upstream. These results suggest that Japanese eels inhabiting saline water generally move from the upper estuary as they grow. The upper estuary can be an important area for the management of this species because these eels spend their early continental growth life there.

Age, Growth, and Sex Ratios of the Giant Mottled eel, Anguilla marmorata, in Freshwater Habitats Near Its Northern Geographic Limit: A Comparison to Tropical Regions.

To understand the latitudinal difference in the basic ecology of the giant mottled eel Anguilla marmorata, we measured its age, growth, and sex ratios in freshwater areas of Amami-Oshima Island, Japan (28.223°N-28.332°N and 129.329°E-129.439°E), near the northern geographic limit of its range and compared these biological characteristics with those observed in the tropical regions of Indonesia. A total of 109 A. marmorata were captured from three rivers on Amami-Oshima Island. The total length (TL) and age of sampled individuals varied across an order of magnitude (TL range: 119-1320 mm, mean: 385.5 ± 172.6; age range: 3-30 years, mean: 12.8 ± 4.9 years). Neither TL nor age differed among rivers. Male A. marmorata accounted for 88.5% of sexually differentiated individuals, which it contrary to previous results for males living in Indonesian watersheds where males were only found in small rivers and areas close to estuaries. Moreover, we found no males larger than 700 mm in TL and few males that were older than 20 years. The growth rates of male A. marmorata were significantly lower than those of females, and growth rates of sexually undifferentiated fish were significantly less than those of males. The mean growth rate of all individuals was 25.9 ± 6.6 mm/y, which is considerably lower than what has been found at lower latitudes, suggesting that growth differences occur along a latitudinal cline.

Whole-Genome Sequencing of 84 Japanese Eels Reveals Evidence against Panmixia and Support for Sympatric Speciation.

The Japanese eel (Anguilla japonica), European eel (Anguilla anguilla), and American eel (Anguilla rostrata) are migratory, catadromous, temperate zone fish sharing several common life cycle features. The population genetics of panmixia in these eel species has already been investigated. Our extensive population genetics analysis was based on 1400 Gb of whole-genome sequence (WGS) data from 84 eels. It demonstrated that a Japanese eel group from the Kuma River differed from other populations of the same species. Even after removing the potential adapted/selected single nucleotide polymorphism (SNP) data, and with very small differences (fixation index [Fst] = 0.01), we obtained results consistently indicating that panmixia does not occur in Japanese eels. The life cycle of the Japanese eel is well-established and the Kuma River is in the center of its habitat. Nevertheless, simple reproductive isolation is not the probable cause of non-panmixia in this species. We propose that the combination of spawning area subdivision, philopatry, and habitat preference/avoidance accounts for the non-panmixia in the Japanese eel population. We named this hypothesis the "reproductive isolation like subset mapping" (RISM) model. This finding may be indicative of the initial stages of sympatric speciation in these eels.