45
T.A. Marchanî et al.
authors have suggested that changes in circulating levels of GH may influence
the
seasonal cycle of growth in teleosts (Pickford, 1957; Gerking, 1966;
Adelman, 1977; Kayes, 1977a; Brett, 1979)° However, circulating levels of GH
have not been measured throughout the year, and virtually nothing is known
about
the relationship that changes in circulating GH levels may have to the
seasonal growth cycle in teleosts°
As
part of a study examining daily changes in serum GH levels in the
goldfish (Marchant, 1983; Merchant and Peter, 1985), we were able to deter—
mine
seasonal changes in circulating GH levels in relation to the somatic
growth cycle in this species. Results from this study show that both serum
GH levels and growth rates vary on a seasonal basis in the goldfish… The
highest mean daily serum GH levels were found in fish sampled in March and
June, whereas the lowest levels were found in fish sampled in November. The
The highest serum GH levels occurred several weeks before a significant
increase
in growth rate was observed in the goldfisha A significant
increase
in the growth rate based on changes in somatic weight (SCR) was
not
found until May, whereas a signficant increase in the growth rate
based on changes in body length (LGR) was not found until June° Therefore,
there
is
a
lag period of approximately six weeks between the seasonal
increase
serum
GH levels and the seasonal increase in growth rate in
goldfish held at ambient environmental conditions in Canada.
Swift and Pickford
(1965) found that the growth—promoting activity of
‘
perch pituitaries was highest in June, about one month prior to the period of
Ï
maximal growth in the perch. These authors suggested that increased water
'
temperatures during the spring may influence growth in two ways: by increas—
ing tissue responsiveness to circulating GH levels, and by stimulating
-
increased production and secretion of GH. Results from other studies have
also suggested that temperature may influence the responsiveness of tissues
to
GH in teleosts
(Adelman, 1977; Kayes, 1977b)° In our study (Marchant and
Peter, 1985), changes in the growth rates were closely correlated to changes
;
in water
temperature. The hypothesis that increased water temperatures during
?
the spring also serve to increase responsiveness to GH would explain the lag
period between seasonal highs in growth rates and serum GH levels in
goldfish; at a lower water temperature early in the spring, goldfish are
unable to respond to the increased levels of GH, whereas tissues are more
responsive to the higher GH levels later in the spring and early summer as
water
temperatures also increase. In the late summer (August), growth rates
decline in spite of higher water temperatures° This decrease in growth rate
is probably due to the seasonal decrease in circulating GH levels.
Although Swift and Pickford (1965) suggested that temperature may
influence the secretion of GH throughout the year, seasonal changes in serum
GH levels in the goldfish appear to be more closely correlated with seasonal
changes in day length than with changes in water temperature° On this basis,
we
hypothesized that changes in the duration of the scotophase or photophase
serve
as
the environmental cue that regulates seasonal changes in serum GH
;
levels (Marchant and Peter, 1985). Photoperiodic manipulations have been
'
shown to influence the growth of several teleosts (for review: Brett, 1979),
and GH has been implicated as the mediator of a "light—pituitary axis" in
3
the growth of Atlantic salmon, Salmo salar (Komourdjian gg al., 1976). The
results from our study on goldfish (Marchant and Peter, 1985) suggests that
{
GH may be altered by seasonal photoperiodic conditions, although further studies investigatimg the effects of accelerated or decelerated phot0period
cycles on GH levels and growth rate throughout the year are required. Studies
of the effects of photoperiod cycles on these two parameters in goldfish kept
Ÿ
under constant
temperature are also required to clarify the individual roles
of light and temperature in seasonal changes in serum GH levels and, ultimately, somatic growth rate.
T.A. Marchanî et al.
authors have suggested that changes in circulating levels of GH may influence
the
seasonal cycle of growth in teleosts (Pickford, 1957; Gerking, 1966;
Adelman, 1977; Kayes, 1977a; Brett, 1979)° However, circulating levels of GH
have not been measured throughout the year, and virtually nothing is known
about
the relationship that changes in circulating GH levels may have to the
seasonal growth cycle in teleosts°
As
part of a study examining daily changes in serum GH levels in the
goldfish (Marchant, 1983; Merchant and Peter, 1985), we were able to deter—
mine
seasonal changes in circulating GH levels in relation to the somatic
growth cycle in this species. Results from this study show that both serum
GH levels and growth rates vary on a seasonal basis in the goldfish… The
highest mean daily serum GH levels were found in fish sampled in March and
June, whereas the lowest levels were found in fish sampled in November. The
The highest serum GH levels occurred several weeks before a significant
increase
in growth rate was observed in the goldfisha A significant
increase
in the growth rate based on changes in somatic weight (SCR) was
not
found until May, whereas a signficant increase in the growth rate
based on changes in body length (LGR) was not found until June° Therefore,
there
is
a
lag period of approximately six weeks between the seasonal
increase
serum
GH levels and the seasonal increase in growth rate in
goldfish held at ambient environmental conditions in Canada.
Swift and Pickford
(1965) found that the growth—promoting activity of
‘
perch pituitaries was highest in June, about one month prior to the period of
Ï
maximal growth in the perch. These authors suggested that increased water
'
temperatures during the spring may influence growth in two ways: by increas—
ing tissue responsiveness to circulating GH levels, and by stimulating
-
increased production and secretion of GH. Results from other studies have
also suggested that temperature may influence the responsiveness of tissues
to
GH in teleosts
(Adelman, 1977; Kayes, 1977b)° In our study (Marchant and
Peter, 1985), changes in the growth rates were closely correlated to changes
;
in water
temperature. The hypothesis that increased water temperatures during
?
the spring also serve to increase responsiveness to GH would explain the lag
period between seasonal highs in growth rates and serum GH levels in
goldfish; at a lower water temperature early in the spring, goldfish are
unable to respond to the increased levels of GH, whereas tissues are more
responsive to the higher GH levels later in the spring and early summer as
water
temperatures also increase. In the late summer (August), growth rates
decline in spite of higher water temperatures° This decrease in growth rate
is probably due to the seasonal decrease in circulating GH levels.
Although Swift and Pickford (1965) suggested that temperature may
influence the secretion of GH throughout the year, seasonal changes in serum
GH levels in the goldfish appear to be more closely correlated with seasonal
changes in day length than with changes in water temperature° On this basis,
we
hypothesized that changes in the duration of the scotophase or photophase
serve
as
the environmental cue that regulates seasonal changes in serum GH
;
levels (Marchant and Peter, 1985). Photoperiodic manipulations have been
'
shown to influence the growth of several teleosts (for review: Brett, 1979),
and GH has been implicated as the mediator of a "light—pituitary axis" in
3
the growth of Atlantic salmon, Salmo salar (Komourdjian gg al., 1976). The
results from our study on goldfish (Marchant and Peter, 1985) suggests that
{
GH may be altered by seasonal photoperiodic conditions, although further studies investigatimg the effects of accelerated or decelerated phot0period
cycles on GH levels and growth rate throughout the year are required. Studies
of the effects of photoperiod cycles on these two parameters in goldfish kept
Ÿ
under constant
temperature are also required to clarify the individual roles
of light and temperature in seasonal changes in serum GH levels and, ultimately, somatic growth rate.