The number of spawning fish in the Ural River is
critical for the survival of this population.
Factors affecting sturgeon stocksEffects of sea level on sturgeon growthFluctuations in water levels in the Caspian Sea and
consequent changes in salinity impacted sturgeon
stocks. Fluctuations impact accessibility to feeding
sites, the abundance of food organisms at these
sites, and the concentration and distribution of stur-
geons in the sea. We used the rate of weight gain to
estimate the effect of sea level fluctuations.
Changes were seen in beluga that returned to riv-
ers to spawn. Beginning in 1970, the sea level de-
creased until, in 1977, it reached a minimum at 29 m,
a previous lowest level which also occurred in 1936–- Starting in 1972, the relative rate of weight
gain in beluga began to decrease. The relative
weight gain decreased more in males than in fe-
males. Once sea level began to rise in 1978, beluga
spawners continued to show a decrease in relative
weight. In the early 1970s, the average weight of be-
luga females was 110 kg, and in 1990–1991, it was
only 57 kg. The sex structure of the spawning stock
of beluga also has changed, with females now (1991)
constituting 21 to 24% of the stock as opposed to
twice that percent in the 1960s.
The rise in sea water level since 1978 was correlat-
ed with an increase in the growth rate of stellate
sturgeon. A sharp drop in growth rate in 1989 may
be due to high levels of pollution in the Volga River
and the Caspian Sea. The discharge of toxic chemi-
cals caused deaths of stellate sturgeon in the river
and negatively impacted sturgeon feeding in the
Caspian Sea (Khodorevskaya et al. 1995).
Russian sturgeon are less tolerant of high water
salinity than are stellate sturgeon. Optimal condi-
tions for growth of Russian sturgeon are: salt con-
centration of not more than 10, an abundance of
brackish water prey organisms, and extended op-
portunity for juveniles to live in the river. Between
1967 and 1978, the relative rate of growth of Russian
sturgeon returning as spawners decreased sharply
with minimum weight gains from the 1970-1980s.
Lukyanenko et al. (1986) considered that the rela-
tive increase in salinity negatively impacted the
growth rate of Russian sturgeon in the 1970s.
Russian sturgeon
The tonnage of Russian sturgeon harvested from
the Volga-Caspian stock increased from 1961 to
- The increase in harvest followed the 1962 ban
on sturgeon fishing in the Caspian Sea. The number
of Russian sturgeon harvested increased from
480 000 in 1950 to 3 746 800 in 1974, and tonnage in-
creased more than 8 times. For 20 years, from 1966
until 1985, the number of spawning fish harvested
remained more than one million individuals. Dur-
ing that period, the catch was based on fish hatched
between 1935 and 1961, before Volga River flow be-
came regulated by dams. Beginning in 1978, the
number of fish harvested decreased to 766 600 (Fig-
ure 4c) and their tonnage declined to 16 300 metric
tons. The decline followed a sharp decrease in nat-
ural reproduction, because as much as 80% of the
spawning grounds for Russian sturgeon became un-
available to the fish after the Volgograd Dam was
built in 1958–1960. The number of individuals har-
vested from the 1959–1960 generations was 691 500–
730 000, and that from the 1965–1968 generations
was 461 000–600 000.
The age structure of the fish migrating in the Vol-
ga River has also changed (Table 1). In the early
1960s, the run consisted of 8–l2% first-time-spawn-
ers, 62–75% of second-time-spawners, and l6–28%
of older fish. During peak years (1966–1985), sec-
ond-time-spawners (68–78%) and older individuals
(10–18%) prevailed. This phenomena continued
until the early 1990s (Table 1). At present, older in-
dividuals dominate the run (approximately 50%),
while first time spawning fish constitute around
11%.
Natural reproduction plays an important role in
the formation of the present Russian sturgeon
stock. After the Volga River flow became regulated
in 1959, natural reproduction decreased from 7500
metric tons in 1960 to 3000 metric tons in 1981–1985.
In the early 1990s, natural reproduction decreased
to 830 metric tons because of the low number of
sturgeons reaching the spawning grounds.