Growth hormone and growth
47
In goldfish, a significant daily rhythm in serum GH levels is not pre—
sent
at
any time of
the year
(Marchant, 1983; Marchant and Peter, 1985),
although previous studies indicated that circulating GH levels might change
on
a
daily basis in teleosts (Leatherland gt gl°, 1974; Carillo gt
1980)° In mammals, GH is secreted in a pulsatile manner and blood levels of
GH fluctuate rapidly in several mammalian species (Arimura and Culler,
A preliminary study in goldfish indicates that GH is also secreted in
a
pulsatile manner (Cook, 1981); results suggest that the peak in serum GH
levels during a secretory pulse occurs over a shorter period of time (20
minutes) that the declining portion (60 minutes)o In addition, the secretory
bursts of GH in individual fish appear to occur randomly(Cook, 1981)° This
finding and the results of our study on the daily profile of serum GH levels
in the goldfish (Marchant and Peter, 1985) indicate that GH is secreted as
spontaneous bursts with no temporal synchronization with respect to time
between individual goldfish° Therefore, when large numbers of goldfish are
sampled at various time throughout the day, a reproducible daily pattern in
circulating GH levels is not evident under the experimental conditions of our
study (Marchant, 1983; Marchant and Peter, 1985).:
THE
INFLUENCE OF CONSTANT PHOTOPERIODS AND TEMPERATURES ON GROWTH RATE AND
SERUM GH LEVELS
‘
Temperature and photoperiod are two environmental variables With the
potential to influence the rate of body growth in teleost fishes (Shul'man,
1974; Brett, 1979). Generally, growth rates have been shown to increase with
increasing water temperature until an optimum temperature is reached, above
which an increase in temperature results in decreased growth rates (Brett,
1979)° Environmental temperature influences a number of processes related to
body growth, including metabolic expenditures and food intake and conversion
rates°
In our
seasonal study on goldfish (Marchant and Peter, 1985), growth
rates
throughout the year were closely related to changes in water tempera—
tures; growth rates were increased in fish maintained at the higher water
temperatures, with the exception of the experiment conducted in late summer
(August). At this time of the year, the water temperature was highest (25 C)
but growth rates had declined compared to early summer. It is unlikely that
the optimum temperature for growth in the goldfish was exceeded as the
optimum temperature for growth in closely related species such as the common
carp is higher, usually closer to 30 C (Adelman, 1977).
The influence of photoperiod on body growth in teleosts is not clear as
experiments in this area have often yielded conflicting results (for review:
Brett, 1979). A relationship between the seasonal Cycle of body growth and
seasonal changes in photoperiodicity have been reported for the brown trout
(Swift, 1961), and several coregonids (Hogman, 1968). In our study in the
goldfish (Marchant and Peter, 1985), changes in growth rates were more
closely related to temperature changes, although an influence of photoperiod
such as modifying the growth response to temperature cannot be ruled out.
The use of
static or constant photoperiods to evaluate the influence of
light on body growth in teleosts has produced uncertain results. In the
common
carp, Adelman (1977) did not find a differential effect of long and
short photoperiods on growth rate or the response to injected doses of bovine
_
GH.
However, in other freshwater teleosts (for review: Brett, 1979), evidence
suggests that long daylength or increasing daylength over a period of months
may be stimulatory to growth whereas decreasing daylength may inhibit body
growth. Thus, light may influence body growth in teleosts, although the
effects are subtle and may take several months to be manifested. In addition,
the influence of other variables, such as the time of year, the previous
47
In goldfish, a significant daily rhythm in serum GH levels is not pre—
sent
at
any time of
the year
(Marchant, 1983; Marchant and Peter, 1985),
although previous studies indicated that circulating GH levels might change
on
a
daily basis in teleosts (Leatherland gt gl°, 1974; Carillo gt
1980)° In mammals, GH is secreted in a pulsatile manner and blood levels of
GH fluctuate rapidly in several mammalian species (Arimura and Culler,
A preliminary study in goldfish indicates that GH is also secreted in
a
pulsatile manner (Cook, 1981); results suggest that the peak in serum GH
levels during a secretory pulse occurs over a shorter period of time (20
minutes) that the declining portion (60 minutes)o In addition, the secretory
bursts of GH in individual fish appear to occur randomly(Cook, 1981)° This
finding and the results of our study on the daily profile of serum GH levels
in the goldfish (Marchant and Peter, 1985) indicate that GH is secreted as
spontaneous bursts with no temporal synchronization with respect to time
between individual goldfish° Therefore, when large numbers of goldfish are
sampled at various time throughout the day, a reproducible daily pattern in
circulating GH levels is not evident under the experimental conditions of our
study (Marchant, 1983; Marchant and Peter, 1985).:
THE
INFLUENCE OF CONSTANT PHOTOPERIODS AND TEMPERATURES ON GROWTH RATE AND
SERUM GH LEVELS
‘
Temperature and photoperiod are two environmental variables With the
potential to influence the rate of body growth in teleost fishes (Shul'man,
1974; Brett, 1979). Generally, growth rates have been shown to increase with
increasing water temperature until an optimum temperature is reached, above
which an increase in temperature results in decreased growth rates (Brett,
1979)° Environmental temperature influences a number of processes related to
body growth, including metabolic expenditures and food intake and conversion
rates°
In our
seasonal study on goldfish (Marchant and Peter, 1985), growth
rates
throughout the year were closely related to changes in water tempera—
tures; growth rates were increased in fish maintained at the higher water
temperatures, with the exception of the experiment conducted in late summer
(August). At this time of the year, the water temperature was highest (25 C)
but growth rates had declined compared to early summer. It is unlikely that
the optimum temperature for growth in the goldfish was exceeded as the
optimum temperature for growth in closely related species such as the common
carp is higher, usually closer to 30 C (Adelman, 1977).
The influence of photoperiod on body growth in teleosts is not clear as
experiments in this area have often yielded conflicting results (for review:
Brett, 1979). A relationship between the seasonal Cycle of body growth and
seasonal changes in photoperiodicity have been reported for the brown trout
(Swift, 1961), and several coregonids (Hogman, 1968). In our study in the
goldfish (Marchant and Peter, 1985), changes in growth rates were more
closely related to temperature changes, although an influence of photoperiod
such as modifying the growth response to temperature cannot be ruled out.
The use of
static or constant photoperiods to evaluate the influence of
light on body growth in teleosts has produced uncertain results. In the
common
carp, Adelman (1977) did not find a differential effect of long and
short photoperiods on growth rate or the response to injected doses of bovine
_
GH.
However, in other freshwater teleosts (for review: Brett, 1979), evidence
suggests that long daylength or increasing daylength over a period of months
may be stimulatory to growth whereas decreasing daylength may inhibit body
growth. Thus, light may influence body growth in teleosts, although the
effects are subtle and may take several months to be manifested. In addition,
the influence of other variables, such as the time of year, the previous