Population Status and Conservation of Sterlet (Acipenser ruthenus L.) in the Transboundary Irtysh River Connecting China, Kazakhstan, and Russia: Evidence from Long-Term Monitoring and Broodstock Development

Restoration of natural sterlet (Acipenser ruthenus) stocks in the transboundary Irtysh River, shared by China, Kazakhstan, and Russia, is of critical importance for biodiversity conservation and represents a strategic priority for sustainable management of transboundary aquatic resources in all three countries. Over the past two decades, sterlet abundance and occurrence in the river have remained low, indicating an unfavorable population status. The Irtysh sterlet population is likely influenced by a combination of habitat degradation, fragmentation of spawning habitats, alterations in the hydrological regime, and increasing anthropogenic pressure. This study summarizes archival and recent data on sterlet catches, biological condition, and habitat characteristics within the Irtysh River basin. Hydrological and hydrochemical parameters, spatial distribution of catches, and the size–age structure of the population were analyzed. The results revealed a localized distribution pattern of sterlet, with the aggregation index reaching I ≥ 1 in some cases, indicating an uneven spatial distribution of individuals. Such aggregation patterns may partially reflect the presence of ecologically important habitats and provide a basis for future conservation planning and targeted monitoring, although this relationship could not be confirmed within the scope of the present study. A decline in the proportion of older age groups and deterioration of key biological indicators suggest weakening natural reproduction. The study initiated the establishment of a replacement broodstock and formed an initial broodstock of 34 sterlet individuals collected from the wild. This provides a foundation for future artificial propagation and the recovery of the endangered Irtysh River sterlet population.