The effect of a 24-hour photoperiod on the survival, growth and swim bladder inflation of pre-flexion yellowfin tuna ( Thunnus albacares) larvae

The effects of two different continuous photoperiod regimes on survival, growth and swim bladder inflation of pre-flexion yellowfin tuna ( Thunnus albacares) larvae were investigated. Each photoperiod regime was tested twice with a different larval cohort to confirm the observed results. Trials 1 and 2 tested the effect of a reduced night-time light intensity (10 μmoles m − 2 s − 1 = 30% of the daytime intensity) and found that those larvae reared for 8 days under the 24 h lighting (24-L) photoperiod exhibited a slight improvement in survival compared to those reared under the control photoperiod of 12 h light (12-L), however these improvements were not significant. In addition, those larvae reared under this photoperiod regime were equal in length to those in the control. Trials 3 and 4 compared the same variables in larvae reared under a continuous photoperiod (24-L) with a constant light intensity of 30 μmoles m − 2 s − 1 , against those reared under the aforementioned 12-L photoperiod. Survival of larvae under the continuous photoperiods were 9 ± 1% (n = 2) and 10 ± 2% (n = 3) for Trials 3 and 4, respectively, compared to less than 1% in both control treatments; differences that in both cases were highly significant. In addition, in both trials larvae cultured under the 24-L photoperiod were significantly larger and exhibited more advanced development than those reared under the 12-L photoperiod, however swim bladder inflation was significantly lower. We suggest that the improved survival and growth achieved under a continuous photoperiod is due to the extended foraging time combined with the prevention of mortality caused by night-time sinking.