Growth, gonadal development
and spawning time of Atlantic cod (Gadus morhua) reared under
different photoperiods
T.
Hansen, Ø. Karlsen, G.L. Taranger, G.-I. Hemre, J.Ch. Holm, O.S.
Kjesbu-2001
Aquaculture, 203(1-2): 51-67
Abstract:
Individually tagged Atlantic cod (Gadus morhua
L.) (mean body weight 459±6 g) were reared in 20-m3 tanks
supplied with running sea water. During the first year the fish were exposed
to one of four photoperiod regimes: Natural light (LDN), continuous light
from June 23, 1993 (LL), natural light from June 23, 1993 and continuous
light from December 22, 1993 (LDN/LL) or continuous light from June 23, 1993
and natural light from December 22, 1993 (LL/LDN). In July 1994, the LDN and
LDN/LL groups were terminated. The LL and LL/LDN groups were reared on until
May 1995. From July 1994 to the termination of the experiment in May 1995,
the LL group was reared under continuous light and the LL/LDN group was
reared under natural photoperiod.
Cod reared under natural photoperiod spawned in the
period between January and April. Photoperiod manipulation changed the
incidence of sexual maturation, spawning time, fecundity and egg size. Cod
that were transferred from natural photoperiod to continuous light in
December spawned earlier, having lower fecundity and smaller eggs than the
cod reared under natural photoperiod. Oocytes of females reared under
continuous light were arrested in the cortical alveoli stage, and even in
their second year on continuous light very few females matured.
When transferred to natural photoperiod in December,
females ovulated within 4-5 months, approximately 3 months delayed compared
to the natural photoperiod group. These cod had a higher fecundity and
smaller eggs than natural photoperiod cod. When reared on for another year
under natural photoperiod these cod spawned again the next year at their
normal spawning time, e.g. 9 months after their first spawning.
The pattern of sexual maturation influenced the
somatic growth pattern. Differences in growth were explained by differences
in the timing of the spawning and the relative spawning investment. At an
age of 26 months, the weight of the cod reared under natural photoperiod and
continuous light were 1.5 and 2.5 kg, respectively.
From this, it is concluded that a reduction in
daylength is a vital environmental signal regulating the maturation and
spawning of cod, and that sexual maturation may be arrested or considerably
delayed in its absence.
(Institute of Marine Research (IMR), Matre
Aquaculture Research Station, N-5198 Matredal, Norway, e-mail: Tom.Hansen@imr.no)