1. There is growing international concern about declines in populations of anguillid eels, resulting in their inclusion in the International Union for the Conservation of Nature (IUCN) Red List of Threatened Species. Monitoring the population dynamics of these species is often challenging, however, owing to their broad distributions and complex, catadromous life histories.2. Whether environmental DNA (eDNA) analysis could be used to monitor the spatial distribution of anguillid eels in rivers was investigated by conducting basin-scale surveys of Japanese eels, Anguilla japonica Temminck & Schlegel, 1846, in 10 rivers in Japan, and comparing the results obtained using eDNA analysis with the results obtained using the electrofishing method. Moreover, the relationship between the eDNA concentration and the abundance and biomass of Japanese eels was examined.3. The eDNA of Japanese eels was detected at 56 (91.8%) of the 61 study sites from which individuals were collected by electrofishing, and at an additional 35 sites where individuals were not directly collected. This indicates that eDNA analysis has a greater sensitivity for detecting the presence of eels, making it a powerful tool for monitoring the spatial distribution of anguillid eels in rivers. 4. A significant, but weak, positive relationship between the eDNA concentration and the abundance and biomass of Japanese eels was also found, suggesting that eDNA analysis may be useful for estimating the abundance and biomass of anguillid eels in rivers. 5. This is the first study to demonstrate the potential usefulness of eDNA analysis for estimating the spatial distribution, abundance, and biomass of Japanese eels in rivers. eDNA analysis will allow anguillid eel populations to be monitored over large spatial and temporal scales using a consistent protocol, with reduced time and effort compared with conventional techniques, providing invaluable information for managing populations of these endangered species.

We successfully discriminated wild and cultured anguillid eels without artificial tagging. For the purpose of population restoration of anguillid eels, stocking has been widely conducted in the European Union (EU) and Japan, although the net benefits of stocking remain unclear. To evaluate the effectiveness of eel stocking for population restoration, the survival, growth, escapement, and reproduction of stocked eels should be tracked. In this study, we explored the potential of using otolith oxygen and carbon stable isotope ratios as a natural tag to discriminate between wild and cultured eels, because cultured eels are often stocked into natural rivers and lakes. A discrimination model was developed based on wild (n = 95) and cultured (n = 314) Japanese eels as a training dataset. The results of accuracy estimation based on leave-one-out cross-validation were 96.8%. We then tested the applicability of the model to stocked–recaptured eels (n = 20); 100.0% were successfully identified as cultured eels, indicating that these eels spend their early continental phase in aquaculture ponds before stocking. This method could be widely applied to assess the effectiveness of eel stocking by determining the proportion of stocked eels among those captured from rivers, coastal areas, or spawning grounds.

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. Although fresh waters cover only 2.3% of the Earth's surface 1 , the number of described species per area is much higher than that of terrestrial and marine ecosystems 2. Further, they support approximately 10% of all known species, which includes 40% of global fish species and ~33% of global vertebrate species 3,4. However, declines in biodiversity are far greater in fresh waters than the most affected terrestrial ecosystems 5. Freshwater ecosystems are the most globally threatened; they also concentrate human populations that have led to widespread habitat degradation, pollution, flow regulation, water extraction, unsustainable fisheries, alien species introductions, changing climates, infectious diseases, harmful algal blooms and expanding hydropower 6-8. As a result, nearly one-third of species in fresh waters have been classified as 'Endangered' in the International Union for Conservation of Nature (IUCN) Red List of Threatened Species 9. Monitoring and managing all aspects of biodiversity is challenging, and 'shortcuts' , such as a surrogate species (i.e., indicator, umbrella and flagship species), whereby resources are focused on a limited number of focal species for broader benefit 10. Depending on the conservation goals, several concepts of surrogate species have been distinguished 11. 'Indicator species' have been used to assess the magnitude of anthropogenic disturbance and changes in habitat (health indicators), to locate areas of high regional biodiversity (biodiversity indicators), and to monito...

Fish distributions in river systems are known to be affected by large dams and other artificial cross‐river structures such as weirs that prevent their movements and migrations. However, little is known about the cumulative effect of low‐height barriers on fish distributions. Although fishes like eels and lampreys, which include endangered species, have a unique type of ability to climb or attach to obstacles, the strength of their anguilliform swimming locomotion style is generally lower than in other fishes. To investigate the cumulative effects of small barriers irrespective of their distances from the river mouth on riverine distributions of endangered eels, the individual density of Japanese eels and habitat characteristics of five model river systems having differences in numbers, size and positional patterns of small barriers were surveyed and analysed using a generalized linear mixed model approach. The results clearly showed that a continuum of small barriers and the distance from the river mouth affected the riverine distributions of Japanese eels, and that these effects were eel size‐ or stage‐dependent. The density of small eels decreased as the cumulative height of barriers downstream of each station increased, while that of large eels decreased as the distance from the river mouth increased. Small individuals dispersed in a wide range of habitats before the end of their upstream migration phase, suggesting that small barriers have an impact on their ideal distributions, life‐history patterns (becoming residents or nomads shifting growth habitats to estuaries) and sex differentiation. This study and existing information indicate that conservation/restoration actions are needed to maintain or improve riverine connectivity for migratory species, especially those having lower swimming ability and a benthic growth phase in freshwater ecosystems that are at risk even when migration barriers are not very high.