Spawning migrations and reproductive parameters of the kaluga sturgeon, Huso dauricus (Georgi, 1775), and Amur sturgeon, Acipenser schrenckii (Brandt, 1869)

Mature specimens of kaluga sturgeon Huso dauricus (Georgi, 1775) and Amur sturgeon Acipenser schrenckii (Brandt, 1869) were previously abundant in catches throughout their range that included the Amur River basin and coastal waters of the Sea of Okhotsk and Sea of Japan. However, both kaluga and Amur sturgeon now reach maturity mainly in the Amur Estuary and sea waters. The most intensive migration from the estuary to the river takes place in spring, from mid-May to mid-June (with a peak 21-25 May), and in autumn, from mid-August to late September (with a peak 16-25 August). The migration of kaluga and Amur sturgeon spawners is anadromous. Spring spawners numerically prevail over autumn spawners in the kaluga sturgeon, whereas an inverse situation is observed in the Amur sturgeon. Average body sizes of females in both species and that of male kaluga sturgeon have decreased over the past 5-10years. The Amur and kaluga sturgeons from the Amur River mature earlier than those from the Amur Estuary. The average absolute fecundity of both species has decreased in recent years because of a reduction in the proportion of spawners in older age classes.