Migration and habitats of diadromous Danube River sturgeons in Romania: 1998-2000

Abstract

Upstreammigrant adults of stellate sturgeon,Acipenser stellatus (10 in 1998, 43 in 1999) andRussian sturgeon,A. gueldenstaedtii (three in 1999) were captured at river km (rkm) 58–137, mostly in the spring, and tagged with acoustic tags offering a reward for return. The overharvest was revealed by tag returns (38% in 1998, 28% in 1999) and by harvest within 26 days (and before reaching spawning grounds) of the six stellate sturgeon tracked upstream. A drop-back of >50% of the tagged sturgeon, some to the Black Sea, shows a high sensitivity to interruption of migration and capture/handling/holding. Harvesting and dropback prevented tracking of sturgeon to spawning sites. Gillnetting and tracking of stellate sturgeon showed that the autumn migration ended in early October (river temperature 16 C) and identified a likely wintering area at river km (rkm) 75–76 (St George Branch). Thus, fishery harvesting after early October captures wintering fish, not migrants. Rare shoreline cliffs in the lower river likely create the only rocky habitat for sturgeon spawning. A survey for potential spawning habitats found five sites with rocky substrate and moderate water velocity, all ‡rkm 258. Drift netting caught early life-stages of 17 fish species and one sturgeon, a beluga, Huso huso, larva likely spawned at ‡rkm 258. All diadromous Danube sturgeons likely spawn at ‡rkm 258.