Figure 4.Approximate length and seasonality of the testicular
and ovarian cycles in domestic white sturgeon. based on observ a-
tions of iteroparous broodfish raised in outdoor tanks. The bien-
nial ovarian cycle is show n for odd and even y ears
tal portion not shown). The egg envelope reaches
maximum thickness and consists of three PAS-posi-
tive layers: zona radiata interna, zona radiata exter-
na and gelatinous coat. The cortical alveoli form a
distinct layer beneath the oolemma.
During the vitellogenic phase of ovarian devel-
opment, plasma concentrations of vitellogenin sig-
nificantly increase. Hepatic synthesis of vitelloge-
nin is stimulated by estrogen (Moberg et al. 1991a),
which causes expression of the vitellogenin gene in
the hepatocytcs (Bidwell et al. 1991, Bidwell & Carl-
son 1995). Oocyte growth and changes in plasma vi-
tellogenin level (measured as total plasma calcium)
in females raised at the university facility are shown
in Figure 3. Mean plasma vitellogenin concentra-
tion reaches 7 mg ml–1(200μg ml–1calcium) during
the late phase of vitellogenic growth and decreases
significantly before spawning. Vitellogenesis in
these females was first observed in November of
1990, and ovulation was successfully induced in
April 1992. Two putative vitellogenin proteins were
detected in the plasma of vitellogenic females by
SDS-PAGE, with estimated molecular masses of
190 and 210 kDa (Figure 3, inset).
Final ovarian maturation in white sturgeon (oo-
cyte maturation and ovulation) appears to be simi-
lar to that described by Dettlaff et al. (1993) inA.
gueldenstaedtii.Captive white sturgeon females can
be induced to ovulate by administration of GnRHa
or carp pituitary extracts when they reach a respon-
sive stage (Lutes et al. 1987, Doroshov et al. 1994).
At spawning, the number of ova removed by caesa-
rean section ranged from 100 000–200 000, and the
average fecundity measured in fish sacrificed for
caviar production was 209 000 eggs at a mean body
weight of 29 kg(n= 67). At ovulation, the ovary
contains post-ovulatory follicles, previtellogenic
oocytes, and atretic follicles with dark pigmentation
(Figure 2f). The late phase of ovarian development
is highly sensitive to environmental temperature
and a prolonged exposure to water temperature of
19º C induces ovarian atresia before completion of
germinal vesicle migration and acquisition of the
meiotic response by the oocytes (Webb et al. 1994).
These observations and studies with Russian stur-
geon,A. gueldenstaedtiiand Siberian sturgeon,A.
baerii, suggest that exposure to low temperature
during the late phase of vitellogenesis is required
for some sturgeon species to complete a normal
ovarian cycle (Williot et al. 1991, Dettlaff et al.
1993).GonadalcyclesRepeatedly spawned males of cultured sturgeon ex-
hibit annual seasonal cycles of spermatogenesis,
with meiosis occurring during the fall and winter,
and spermiation during the spring (Figure 4).
Sperm that was successfully used for insemination
was obtained each spring from the same males dur-
ing 4 consecutive years. Additional evidence for an
annual cycle in males was provided by observations
on seasonal changes in the pituitary content of go-
nadotropins correlated with histological stages of
gametogenesis (Moberg et al. 1995).
The duration and seasonality of the ovarian cy-
cles are currently being investigated. We collected
data from 147 virgin females that were sampled on
farms each fall for several years. The majority
(67%) exhibited patterns of a biennial cycle. similar
to that in Figure 3, but some (29%) had a shorter
duration of vitellogenesis (the stages as depicted in
Figure 2c the first fall and in Figure 2e the following
fall). Furthermore in small numbers of fish (4%) vi-
tellogenesis continued for a period of 3–4 years. In
approximately 23% of all females the onset of vitel-
logenic growth was arrested, and the advanced oo-
cytes with differentiated envelopes and granulosa