Effect of different photoperiods on growth indices of female sterlet sturgeon, Acipenser ruthenus

Introdution: Light is one of the most important environmental factors influencing the physiological activities and growth of living organisms, including various fish species. Sterlet sturgeon, Acipenser ruthenus is a sturgeon species native in northern latitudes with shorter light periods, however, due to high performance in rearing, it has been distributed in different regions of the world with different latitudes.
Materials and methods: This study was carried out to examine the effects of various photoperiods on growth indices and survival rate of female sterlet sturgeon broodstock. A total number of 36 fish, 1166.78 ± 248.35 g in average weight, were distributed in 12 concrete tanks in 6 treatments each with two replicates and 3 fish in each tank. They were reared under six different photoperiods for 12 weeks, including natural light (LDN), constant darkness (0L:24D), 8 h light and 16 h darkness (8L:16D), 12 h light and 12 h darkness (12L:12D), 16 h light and 8 h darkness (16L:8D), and constant light (24L:0D). Fish biometrical characteristics were measured and growth performance was evaluated every 4 weeks  intervals.
Results: The results showed a significant difference in weight gain (WG) among different treatments from the 8^th to 12^th weeks. The highest WG was 70.00 ± 26.34 g in treatment 8L:16D. In addition, the treatments of 16L:8D and 24L:0D exhibited significant differences with the constant darkness and 12L:12D from the 8^th to 12^th weeks of the experiment. The lowest and highest body weight increase (BWI) were observed in the constant darkness and 16L:8D, respectively. The highest specific growth rate (SGR) was observed in the 16L:8D, while the lowest was found in the constant darkness. From the 8^th to 12^th weeks of study, the lowest food conversion ratio (FCR) in different treatments was attributed to the constant darkness, with significant difference with other treatments. At the end of the experiment, even though the WG, SGR, and FCR indices displayed better conditions in the constant light, however, the differences were not significant (p>0.05). The condition factor also did not exhibit any significant difference among different treatments throughout the experiment. The survival rate was 100%.
Discussion: Based on the obtained results, the light regimes affect the growth of female sterlet sturgeon broodstocks, however, it was not significant. The lowest growth and the highest FCR were observed in fish reared in constant darkness. Although sturgeons are among benthopelagic species and do not depend much on the vision to find food, light can affect their growth performance. This study was conducted at the low temperature of the optimal feeding range for sturgeons. Obviously, at a higher temperature, the growth of fish can be increased and the possibility of a greater weight differences in various light regimes may be achieved. It seems that alterations in the photoperiod and increasing the illumination affect the secretion of hormones involving in the metabolism as well as enhance the sterlet sturgeon appetite and ultimately improve the ability of food convertion.
Conclusion: Although the effects of light on feeding and growth of female sterlet sturgeon in short periods were not significant, light and increasing the duration of illumination can enhance the efficiency of feed and growth. Further studies can be suggested at a longer period in order to optimize the rearing conditions of this species in different latitudes.