标题：Effect of a New Feed <i>Daphnia</i> <i>magna</i> (Straus, 1820), as a Fish Meal Substitute on Growth, Feed Utilization, Histological Status, and Economic Revenue of Grey Mullet, <i>Mugil cephalus</i> (Linnaeus 1758)
摘要：The formulator of aquatic diets is part of a continuous search for alternative protein sources instead of depreciated fish meal. The utilization of zooplankton as a feed ingredient is an interesting trend due to their high-quality protein content and abundance of essential nutrients. The current study aims to investigate the effects of partial and total replacement of fish meal (FM) by <i>Dahpnia magna</i> meal (DMM) on growth performance, feed utilization, histological, and economic status of mullet, <i>M. cephalus,</i> larvae. In addition to the control diet, 100% FM, D<sub>0</sub>, four diets containing DMM at different levels were used: 25% (D<sub>25</sub>), 50% (D<sub>50</sub>), 75% (D<sub>75</sub>), and 100% (D<sub>100</sub>) replacement of fish meal. A total of 300 larvae (0.097 ± 0.001 g) were equally divided into five groups (three replicate per each group) at a density of 200 larvae m<sup>−3</sup>. The aquariums were renewed at a rate of 30% daily. During the 60-day experimental period, all larvae were fed their respective diets at a level of 20% of live body weight, five times a day (9.00 a.m., 12.00, 15.00, 18.00, and 21.00 p.m.). The results indicated that compared to D<sub>0</sub>, fish fed D<sub>75</sub> recorded the highest significant value of growth, and feed utilization parameters, while fish fed D<sub>100</sub> achieved the lowest feed cost and incidence cost, and the highest profit index and economic efficiency ratio. A strong correlation was reported among weight gain, feed conversion ratio and fish meal replacement with DMM; R<sup>2</sup> = 0.94 and 0.91, respectively. The fit regression model representing mullet response to FM replacement with DMM is a poly-nominal regression model with maximum response at 75–100%. The histological investigation of the intestine revealed an improvement of histomorphometric indices and goblet cell number with increasing DMM inclusion levels. These findings confirmed that 75% to 100% partial substitution of FM with DMM is the ideal replacement for mullet, <i>M. cephalus,</i> larvae for improving fish growth performance and feed utilization.
