Temporal Dynamics and Evidence of Declining Fish Diversity in Lake Sarysu, Azerbaijan (1967–2021)

Lake Sarysu, the largest natural lake in Azerbaijan, is a key freshwater ecosystem in the Kura-Aras Lowland. This study quantified long-term changes in fish species richness and community composition across a 54-year period, from 1967 to 2021, by integrating published historical records from three survey periods with recent field surveys conducted in 2020–2021. A total of 30 fish species were recorded across all periods, nine of which were consistently observed. Species richness increased from 15 species in 1967–1972 to 25 species in 2009–2015, followed by a 44% decline to 14 species in 2020–2021. The partitioning of Sørensen dissimilarity indicated that earlier temporal differences were primarily associated with species turnover, whereas nestedness-resultant dissimilarity dominated the most recent change, which is consistent with net species loss. Hierarchical clustering separated the two most recent periods from the earlier periods, indicating a marked temporal change in the documented assemblage. Overall, these results document a recent contraction in fish diversity and provide a baseline for future monitoring and management of Lake Sarysu.