Long-Day Photoperiod Improves the Growth and Muscle Quality of Grass Carp (Ctenopharyngodon idella)

To investigate the effects of photoperiods on the growth and muscle quality indicators of grass carp (<i>Ctenopharyngodon idella</i>), 225 fish (109.65 ± 3.62 g) were randomly assigned into five different photoperiod groups (0L:24D, 8L:16D, 12L:12D, 16L:8D, and 24L:0D). The experiment spanned a 75-day period, after which sampling and analysis were performed. Compared with the 0L:24D and 8L:16D groups, the 12L:12D and 16L:8D groups significantly promoted the growth of grass carp (<i>p</i> < 0.05). The texture parameters of the muscle in the 0L:24D and 16L:8D groups were significantly greater than those in the 12L:12D group (<i>p</i> < 0.05). The crude protein content was significantly higher in the 12L:12D and 16L:8D groups (<i>p</i> < 0.05). The amino acid content and muscle fiber characteristics, as well as the mRNA levels of myostatin (<i>mstn</i>), myogenic factor 5 (<i>myf5</i>), type I collagen α1 (<i>col1α1</i>), and α2 (<i>col1α2</i>), along with the hydroxyproline and collagen contents, were all significantly influenced by the photoperiod (<i>p</i> < 0.05). The lysine (Lys), aspartic acid (Asp), and alanine (Ala) contents in the muscle and muscle fiber density of grass carp reached the highest levels under the 16L:8D treatment (<i>p</i> < 0.05). Collectively, these results indicate that a 16L:8D photoperiod is optimal for enhancing both the growth and muscle quality indicators of grass carp. The findings of this study offer valuable scientific references for the precise regulation of grass carp quality when using a photoperiod, and they are anticipated to foster the further development and optimization of strategies for improving grass carp quality.