Characteristics of spawning behaviour in cultured greenback flounder Rhombosolea tapirina

Cultured stocks of 2nd generation 2+ year old greenback flounder Rhombosolea tapirina were maintained at low density and under conditions of minimal disturbance over 4 reproductive seasons. Natural in-tank spawning began in the second year after introduction in 2 separate tank systems and continued for 3 successive seasons until the experiment was terminated. Video records showed spawning on 5 out of 34 days on which filming occurred, with spawning behaviour consisting of approach and courting of an ovulated female by a male, followed by a circular paired swim in mid-water culminating in egg release. Spawning was concentrated in austral winter and spring in all 3 study years. In the first year, spawning episodes were strongly correlated with lunar phase (new moon), whereas in years 2 and 3, initiation of substantive spawning for the season coincided with the new moon but there was little evidence of lunar synchronisation thereafter. Volumes of eggs produced and egg fertility were highly variable and not clearly related to season. Most spawning events occurred between midnight and 07:30 h, with the majority between 04:00 and 06:00 h (∼2 h before sunrise). The results of the present study further emphasize the utility of low disturbance maintenance for the development of naturally spawning cultured broodstock, and suggest that wild stocks are likely to show pre-dawn spawning in winter and spring, possibly synchronised to lunar phase.