Cumulative effects of low-height barriers on distributions of catadromous Japanese eels in Japan

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.