Downstream migration of young fish and efficiency of natural reproduction of the sturgeon (Acipenseridae) in the Ural river
Downstream migration of young fish is an important component of the life cycle of sturgeons. Various species of anadromous sturgeons are characterized by the range on spawning migration and, thus, by the duration and range of downstream migration of youngfish from spawning grounds. This, in turn, preconditions differentiation of species by the age, size, and stages of development ofyoungfish when it comes to the Caspian Sea. The Ural River is the only river in the Caspian basin which is not dammed in its low and middle course and, thus, it becomes possible for the sturgeon to propagate in conditions which are almost the same with preservation of thefull structure of thefish population migrating to this river. Using actual data and information collected over many years we studied the conditions and specific features of downstream migration of young sturgeons in the Ural River and evaluated efficiency of their natural reproduction. Starred sturgeon and sturgeon species (64.1% and 27.9% of the total number of young sturgeons in this river respectively) prevailed in the composition of the downstream migrants. The decrease in duration of downstream migration of young fish in the river, which has been observed lately, results in decrease in the mass of fish migrating downstream and, thus, has a negative impact on its survival. We established that hydrological conditions ofthe Ural River and the number of coming to spawning grounds are the factors limiting natural reproduction of species. The number of spawners has reduced to a critical level and, as a result, at the present moment there are hardly any young great white sturgeons, barbell sturgeons, and sturgeons in control catches. Just solitary young starred sturgeons arefound in such catches. The ban on sturgeonfishing which came into effect in 2010 has been no positive effect sofar.