Abstract

The reproductive contribution of river-resident (freshwater) Japanese eels (Anguilla japonica) remains poorly understood, particularly following recent paradigm shifts that emphasise the predominance of ocean-resident “sea eels.” To elucidate the riverine life history and maturation strategies of freshwater residents, we investigated the age structure, morphological traits, and gonadal development of 100 river-resident Japanese eels captured over a three-year period (2007–2009: n = 33, 40, and 27, respectively) in the middle reaches (25–50 km upstream) of the Oi River, Shizuoka Prefecture, Japan. Age determination using sagittal otoliths revealed a broad age distribution from 4 to 15 years. Among the female population (n = 85), highly significant positive correlations were observed between age and growth parameters (Total Length, R2 = 0.771; Body Weight, R2 = 0.709; p < 0.001). Highly mature silver eels with markedly elevated gonadosomatic indices (GSI) were captured at the uppermost sampling site, immediately downstream of an impassable dam barrier. This spatial concentration of mature females suggests an energetic and temporal life-history strategy in which river-resident eels initiate precocious maturation in the upper reaches to compensate for the prolonged downstream migration to the river mouth. Notably, the calculated female growth rate of approximately 3.6 cm/year was significantly slower than that reported for coastal and estuarine conspecifics in Japan, indicating a distinct slow-growth history unique to river-resident cohorts. Although historical baselines exhibit significant annual variation (ANOVA, p < 0.05), they provide an invaluable benchmark for river-resident cohorts prior to the severe depletion of wild stocks. Our findings underscore that large freshwater-resident females may serve as vital somatic and reproductive reserves, and provide key recommendations for rigorous habitat conservation within river systems to sustain the species.

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