groundslocated in theupperAmurRiver and in a
region 250 km longdownstream fromBlagovesh-
chensk, in the secondhalfofMay and earlyJune.
Spawningtakesplace in June. Thebiology of this
population has not beenstudied.
1960scontinues.Furtherdecrease in population
sizeshould beexpected,especially in themiddle
part ofthe Amur River. Thesize of theZeya and
Bureya population, ifevaluated on thebasis of the
very low catch in theAmurRiverwithin theAmur
district(0.09–1.03metrictons), issosmallthat the
population isonthe verge ofdisappearing.
The efficiency ofnaturalbreeding of kaluga is ve-
ry low, as can beseen from theslowrate ofrestora-
tionofthe estuary population: until the beginning
of the 1990s, its sizeincreasedonly35%, orlessthan
2% per year.Contemporary statusThestatus ofkaluga populations in theAmurbasin
has changed since 1900. At the end of the 19thcen-
tury, when the highest catches were recorded (more
than 595metrictons peryear), themiddleAmur
population was themostabundant.This is evident
from theanalysis ofabsolute andrelative catches
throughout theAmurRiver. According todata for
1891, fish from the middle Amur population consti-
tuted 87% of theannualkaluga catch in theAmur
River Basin. Fish from the estuary and lower Amur
populations, however, constituted 2% of the catch,
with remaining 11% coming from the Zeya and Bu-
reyapopulation (Kryukov1894).
At present, theestuarypopulation ismost abun-
dant. Due to the strictly limited catch since 1976, the
totalnumber offish in theestuarypopulation in-
creased by approximately onethird, and the num-
beroflargefish ofmorethan 100 kg in weight in-
creased by 2.5 times in comparisonwith theearly
1970s(Krykhtin1979),sothattherewereabout
70000 fishgreaterthan oneyear old at the end of
1980s.Approximately 5000 of thesefishweighed
more than 100kg andwerepotentiallysexually ma-
ture.
However, by 1993, as a result of an illegal fishery
inthe lowerAmurduring thespawningmigration,
the number ofsexuallymaturefish in theestuary
population was reduced by approximately 30–35%.
The current population of kaluga consists predom-
inantly of youngfish, withonly2–3% of the pop-
ulation weighing more than 100 kg and classified as
adults.Thus,accordingcalculationsusing thearea
method andbased onirregularcontrolcatchesin
the lower and middle Amur, the number of individ-
ualsolderthan 2years in the lowerAmurpopula-
tion is approximately40 000 and in the middle
Amur population, 30 000.The decrease in the kalu-
ga populationwhichappeared at the end of theAmur sturgeonBiology and population structureThe Amursturgeon(Figure 2b,d) isrepresentedin
the AmurRiverbasin by twomorphs:brown and
gray.Young andadultsofthe brownmorphinhabit
the middle and lower reaches ofthe Amur River.
Brownmorphs arerare andgrowmoreslowlythan
do the grayones,withfemalesmaturing96–117 c m
long and weights of 3.5–5.6 kg, whereas female gray
morphs are125–142 cmlong andweigh8.3–16.4 kg
by maturity at age 12 years.There are a fewsmall
localconcentrations of brownmorph in theAmur
River.
The maximum length of thegraymorph isabout
3 m, and the weight is 190 kg at an age ofmorethan
60 years. The distribution,mode oflife, and pop-
ulationstructureofthe graymorphresemblethose
of kaluga, but they do notenter thesea.Theyfeed
on benthos and cannibalism rarely occurs. Freshwa-
ter mollusks and larvae of the Arctic lamprey, Lam-
petra japonica,are usuallypresent in thestomachs
of the graymorphindividuals(Yukhimenko1963,
Svirskii1971).Maturation, spawning migration, and breedingMostgraymorphindividualsmature at an age of
10–14years, being105–125 cm inlength andweigh-
ing 6.0–18.5 kg.Femalesreproduce atleast every
fouryears.Ifprevitellogenic andvitellogenic oo-