Spatial and temporal trends in the relative abundance of Caspian sturgeon during 2006–2022

We used the bycatch of Caspian sturgeon to quantify the species composition, length structure, spatial and temporal distribution, temporal dynamics of relative abundance (indexed as catch-per-unit-effort, CPUE), effects of fingerlings released, and to forecast future sturgeon stock status. Dynamic factor analysis of temporal changes in species CPUE was used to quantify relationships to environmental variables (sea surface temperature, chlorophyll-a, sea surface level, and river discharge) during 2006–2022. We found that the Guilan subarea had the lowest relative abundance and differed from all other subareas. Fish abundance was highest in October and November, particularly in Mazandaran and Golestan coastal waters. Relative fish abundance declined significantly over time. Acipenser persicus was the most abundant species, followed by Aciperser gueldenstaedtii, Aciperser stellatus, Aciperser nudiventris, and Huso huso. The annual number of fingerlings released decreased during 2000–2021. Over the years, environmental variables such as sea surface level, sea surface temperature, and chlorophyll-a showed different trends, with sea surface temperature generally increasing, chlorophyll-a generally decreasing, sea surface level decreasing nearly linearly, and river discharge varying erratically without a trend in the southern Caspian Sea during 2006–2022. The relative abundance of all species decreased and was significantly related to sea surface level. Fish abundance was forecasted to decline by 2025, as sea surface temperature increases and sea surface level decreases. Illegal, unreported, and unregulated (IUU) harvest of sturgeon must be effectively regulated to ensure the survival and recovery of sturgeon populations in the Caspian Sea.