Abstract:

The effects of extended photoperiods, mimicking the longest day of the year, were studied in
1- and 2-year seabream. The photoperiod regimes started in late July, 36 and 39 days after the
summer solstice and continued for 11 months, well beyond the natural reproductive season of
December–March. Regime 1 Žlong day, 15.5L:8.5D., which used natural and fluorescent light,
reduced the incidence of maturity in both year classes and females did not spawn although some
gonadal development was observed. Among all 1-year sampled fish of regime 1, a maximum of
5% became spermiating males ŽMarch. and 5% reached the yolk granule stage of vitellogenesis
ŽVO3; 250–400 mm diameter. by May. Among 2-year sampled fish of regime 1, 45% became
spermiating males and 25% were females, which reached the advanced vitellogenesis stage ŽVO4;
400–600 mm. by April. Regime 2 Žskeleton photoperiod., consisting of natural light and a 1.5-h
pulse of fluorescent light during the period 14–15.5 h after sunrise, postponed gonadal development
and spawning for up to 3 months. In this regime, a maximum of 80% of 1-year sampled fish
were spermiating males in February and a maximum of 10% were VO3 stage females in March. In
the sampled 2-year fish, the maximum levels were 50% spermiating males in February and 25%
VO3 stage females in March. Control fish, which were exposed to the natural photoperiod
Ž29834XN., spawned during their natural season. The maximum levels for 1-year sampled control
fish were 95% spermiating males and no females in December, while among 2-year sampled fish,
maxima of 75% males in February and 45% VO4 stage females in November.