Effects of continuous light and light intensity on the growth performance and gonadal development of Nile tilapia

ABSTRACT Continuous illumination has been known to exert positive effects by stimulating growth and delaying unwanted maturation in seasonal-temperate farmed fish species like salmonids. However, in tropical fish like Nile tilapia ( Oreochromis niloticus ), some studies exist showing inconsistent results and even fewer data is available regarding the effects of light intensity. To clarify some of the inconsistent results in literature and evaluate the effect of different light intensity levels on growth and sexual maturation in Nile tilapia ( Oreochromis niloticus ), we reared twenty days post-hatch Nile tilapia larvae under continuous illumination at three different light intensities and compared against a control photoperiod (12L:12D) up to 118 days post-hatch. A total of 600 fry were used using 75 fry per experimental unit in a previously tested experimental aquarium setup. Fish exposed to high and medium intensity continuous illumination treatments were significantly heavier (13-20%) and longer (6-8%) than fish exposed to the control photoperiod. Importantly, however, the degree of growth enhancement did not vary significantly according to the light intensity used. Feed intake was also higher in all continuous illumination treatments than in the control photoperiod, suggesting that growth benefits might be due to an increase in feed intake, which is not affected by the light intensities used. Gonadal development on the other hand, presented differences between sexes with a delay in spermatogenesis, while an advancement towards ovarian maturation occurred compared with the control fish. These results suggest that continuous illumination can influence both growth and gonadal development in Nile tilapia with no apparent differences between the light intensities tested in this study.

摘要：持续光照已被证实可通过促进生长、延缓不必要的性成熟，对鲑科鱼类等季节性温带养殖鱼类产生积极影响。然而，对于尼罗罗非鱼（Oreochromis niloticus）这类热带鱼类，已有研究得出的结果并不一致，且关于光照强度影响的可用数据更为匮乏。为厘清现有文献中存在的不一致结果，并探究不同光照强度对尼罗罗非鱼（Oreochromis niloticus）生长与性成熟的影响，本研究将孵化后20天的尼罗罗非鱼幼鱼置于三种不同强度的持续光照条件下饲养，并以12小时光照:12小时黑暗（12L:12D）的常规光周期作为对照组，饲养至孵化后118天。本实验共使用600尾仔鱼，每个实验单元放置75尾仔鱼，采用已验证的实验水族箱装置开展饲养。与对照组相比，接受中、高强度持续光照处理的试验鱼体质量显著提升13%~20%，体长显著增加6%~8%。值得注意的是，生长提升的幅度并未随所用光照强度的不同出现显著差异。此外，所有持续光照处理组的摄食量均高于对照组，这表明生长获益可能源于摄食量增加，而该效应不受所用光照强度的影响。另一方面，性腺发育存在性别差异：与对照组相比，试验组雄性的精子发生过程被延缓，而雌性的卵巢成熟进程则被提前。上述结果表明，持续光照可同时影响尼罗罗非鱼的生长与性腺发育，且本研究所测试的不同光照强度之间未显现出显著差异。