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In the present study serum cortisol level was estimated by radioimmunoassay (RIA) method in the
freshwater fish Notopterus notopterus in a single day and also during four reproductive phases. The
results indicates that during morning hours 8.30 am (15.75 ± 0.383) cortisol level is low while
afternoon 13.30pm (21.08 ± 0.71) and evening 20.30pm (25.50 ± 0.76) hours serum cortisol level is
higher. The fish N.notopterus, fed in the morning hours and metabolic activity increases during
afternoon and evening hours probably resulting in the increase of serum cortisol level. The
reproductive phases were categorized based on the morphology of the gonads observed during
different periods. The cortisol estimation during four phases of the reproductive phases indicates
that in control fish cortisol level increases during preparatory pre-spawning and spawning. The
cortisol administration during preparatory pre-spawning and spawning phases may be because of
energy requirements for spermatogenesis and spermiation activities. The cortisol administration
during four reproductive phases and estimation of serum cortisol level a day after administration
indicate that during preparatory and pre-spawning phases serum cortisol decreases’ from that of
control. The decrease in serum cortisol indicates its utilization for energy release needed for
reproductive activity.
Key words: Notopterus notopterus, cortisol, hormone, reproductive phases