Downstream migration of young-of-the-year sturgeons (Acipenseridae) in the Lower Danube River, Bulgaria

Abstract

ABSTRACT The downstream migration of young-of-the-year (YoY) of four sturgeon species (Acipenser ruthenus, A. stellatus, A. gueldenstaedtii and Huso huso) was monitored in the Danube at river km 396 in 8 continuous years (2014–2021). A total of 713 YoY specimens were registered for the whole period. The most frequent migrant on an annual basis was A. ruthenus, observed in 6 out of 8 years. The most abundant young sturgeons during the first downstream migration were A. ruthenus with 599 specimens, followed by A. stellatus. For the species H. huso, only 7 specimens were registered for the entire period. The natural hybrids A. ruthenus × A. stellatus were quite frequent. One YoY specimen of A. gueldenstaedtii was registered after 10 years with no detections in the Lower Danube River. The catch per unit effort (CPUE/10 ha) was 0.346 for the whole period for all sturgeon species. This paper suggests a method for calculating the CPUE taking into account the studied area, with the goal of ensuring comparability of monitoring data between the countries of the Lower Danube. Recommendations are also made for better protection of the identified nursery site.