The result of this study indicates that growth and nutrient utilization were not impacted by the fortification of diets with onion (Allium cepa) powder. The average weight gain, growth rate and feed conversion were statistically similar for all groups of experimental fish. Feed intake with the corresponding conversion to flesh was not improved, thus signifying the possible non-effect of onion powder on feed cost reduction or modulation. The digestive properties and appetite enhancing abilities of onions was not affirmed in this present study. And this contradicts the report of Saleh et al (2015) where onion powder inclusion in diets of sea bass resulted in improved feed intake, growth and nutrient utilization. Earlier works on Clarias gariepinus and Oreochromis niloticus fed onion powder as additive also resulted in improved growth (Aly and Mohamed, 2010; Agbebi et al., 2013). Variations in the results of these earlier works and this present study may be attributed to the differences in the processing methods of onions powder.
The surface area for nutrient absorption in fish gut is dependent on the size of the villi and number, and there must be a balance in cell renewal and loss for the sustenance of digestive and absorptive capacity of the intestine. A response to anti-nutritional compounds or pathogenic compounds may result in disequilibrium in cell turnover and consequently change in villus height (Azevedo et al., 2016). There exists significant differences (P<0.05) in villus height, width and area absorption among treatments, with groups fed onions fortified diets exhibiting higher values. Although this is similar to results of other studies where length and width of villi were significantly increased with addition of phytogenic extracts into fish diet (Merrifield et al., 2010) the observed similarity in growth indices in this present work is at variance with the earlier works (Mello et al., 2013; Omitoyin et al., 2019).
The survival of fish fed onion supplements was better than in the OP0 group in this work. The presence of quercetin and organosulfur, which are flavonoids, in onion is suggested to better the health standing of fish according to Griffiths et al. (2002). Results in this work is in line with the findings of several authors where survival rates were improved in fish fed onion powder (Farahi, et al., 2010; Kalyankar et al., 2013; Saleh et al., 2015). Microbial population in the gut of fish has a considerable influence on survival. This is because microflora in the digestive tracts of fish carries out very important function in the resistance of infectious pathogens by the production of materials that inhibit disease-causing microbes from gaining entry into the organism (Sugita et al., 1988). These gut microbes produce some physiologically active compounds (enzymes, amino acids, vitamins) beneficial to the host and they also help in the breakdown of nutrients (Sugita et al. 1997), while the host ensures the required metabolic activities for the sustenance of the microbes. In this study eight isolate bacteria species were obtained (Proteus sp. Flavobacterium sp., Enterobacter sp., Bacillus sp., Staphylococcus sp., Klebsiella sp., E. coli, Pseudomonas sp.), with Bacillus sp. present in all samples examined. The bacteria diversity was more in the gut of fish fed onions powder. Diets generally impact on the gut microbes of fish. According to Givens et al (2015), feed variation resulted in significant variability in the microbiota of fifteen different species. Similarly, the microbial gut components in rainbow trout and salmon fed different diets caused a shift in specific bacterial genera, for instance, an increase in the occurrence of Staphylococcus and Lactobacillus in corn-based diet (Wong et al. 2013; Schmidt et al. 2016).
Monitoring heamatological parameters in cultured species can be used to ascertain their health and physiological status (Nwanna et al, 2013). Though observations show no significant differences in parameters such as red blood cells, haemoglobin and white blood cells, the increase recorded in packed cell volume at 1.5% and above onions inclusion levels shows onion has a positive health effect on fed fish. This is more so because all the other blood indices fall within the optimal range for cultured fish (Hrubec et al., 2000). The similarity observed in the RBC, Hb and WBC levels in both treated fish and the control groups is at variance with the report of Saleh et al. (2015), where the fish blood had increased Hb and WBC when administered diets with garlic or onion powder and Kalyankar et al., (2013), where Swordtail, X. helleri showed significant increase in all blood indices when fed 1.5% garlic in diet.
The marginal reduction in the level of aspartate aminotransferase (AST) in fish fed up to 1.5% onions supplements and the significantly lower (p<0.05) values of Alanine aminotransferase (ALT) signifies that fish organs are functioning at levels devoid of physiological stress. As the release of these two enzymes are indicative of cellular damage in the presence of stressor (Soosean et al., 2010). Omitoyin et al (2019) observed a reduction in value of AST and ALT when O. niloticus were fed diets with guava leaf extracts as supplement. Following the postulation in Omitoyin et al (2019), it may be stated that the presence of alkaloids in onion protects the hepatic tissue of fish fed diets. Significant increase in the values of Alkaline phosphate is a signal that Allium cepa supplemented diets enhances the health status of O. niloticus. Omitoyin et al (2019) stated that increased ALP is an indication of better absorption, better build-up and proper functioning of cells.
High levels of reactive oxygen species (ROS) that results in damage to proteins, lipids, and changes in glutathione and antioxidant enzymes are caused by oxidative stress. Quantification of oxidative stress in cultured fish is achieved by determining membrane damage by lipid peroxidation and evaluating the antioxidant defense (Ajani et al., 2020).
Although the kidney’s Total protein (TP), hydrogen peroxide (H2O2), glutathione S-transferase (GST), glutathione (GSH), and glutathione peroxidase (GPx) were statistically similar (p>0.05) among experimental fish, there was however a reduction in malondialdehyde (MDA) values in OP2, OP3 and OP4 groups. Also, TP, MDA, H2O2 and GPx were reduced significantly in the liver of fish fed diets fortified with onions. A decrease in the products of oxygen free radical peroxidation of lipids is indicative of cellular integrity in the fish fed onion powder diets. This assertion is supported by Yagi (1984) and Abdel-Tawwaba and Abass (2017) where the mop up of superoxide anions by flavonoids in phytogenics is reported to stimulate immunomodulatory activity in fish. A similar trend of reduced MDA and H2O2 is reported in Xu et al (2015) when hybrid tilapia was fed yeast nucleotides as additive.
After the 14-day challenge with A. hydrophila, higher relative percentage survival was recorded in O. niloticus fed onions fortified diets. This suggests better innate immune system is enhanced by onion powder, which enables them resist the attack from Aeromonas hydrophila and better survival rate. The antibacterial properties in Allium cepa is reported in Bello et al (2013).