The photoperiods with multiple light/dark cycles affect growth and relate to oxidative-stress for largemouth bass (Micropterus salmoides) larvae in recirculating aquaculture systems

A 28-day study was conducted to examine the effects of photoperiods with multiple light/dark cycles on growth performance and oxidative stress for largemouth bass ( Micropterus salmoides ) larvae in recirculating aquaculture systems (RAS). Larvae fish were exposed to four different photoperiods [light/dark (L/D), h] as follows: 6L:18D; 12L:12D; 18L:6D and 24L:0D. The feeding number was 8 times a day. The light duration of each treatment was 45min, 90min, 135min and 180min respectively in one light/dark cycle (feeding interval) of 180min. A total of six thousand fish (0.5229 ± 0.0180 cm, 0.0021 ± 0.0002 g) were randomly kept in the 12 cages (4 treatments × 3 replicates × 500 individuals). The results showed that the body weight, weight gain rate (WGR), specific growth rate (SGR), thermal growth coefficient (TGC), Fulton's condition factor (K) and mRNA expressions of growth hormone ( gh ) and insulin-like growth factor 1 ( igf-1 ) were all increased gradually with the increased extension of photoperiods, and they were highest in the 24L:0D treatment and were second in the 18L:6D treatment. The variation coefficients for body length (CV _fl ) and body weight (CV _fw ) were lest in 18L:6D treatment. There was no significant difference in survival rate (SR) among four treatments. The levels of Malonaldehyde (MDA), total antioxidant capacity (T-AOC) and mRNA expressions of heat shock protein 70 ( hsp70 ), corticotropin-releasing factor receptor 1 ( crfr ₁ ) and caspase-3 ( cas-3 ) were all significantly higher in 6L:18D and 24L:0D treatments than those of 18L:6D and 12L:12D treatments. This indicated the photoperiods of 6L:18D and 24L:0D regimes caused an oxidative-stressed status in the larvae when the photoperiods were excessively compressed and extended. Overall, 18L:6D treatment is the optimal light regime for larvae largemouth bass culture in RAS. But an experimental comparation between the cyclic dark: light regime and continuous dark: light regime would be needed to further study from the perspective of animal welfare and practice.