Introduction
The great sturgeon (Huso huso) is the largest and one of the most economically important sturgeon species native to the Caspian Sea (Ciulli et al., 2020). In recent years, this species has attracted considerable interest in global aquaculture due to its rapid growth rate, high tolerance to environmental stressors and pathogenic challenges, and adaptability to a wide range of culture conditions (Mohseni et al., 2021). In aqua-feed industry, environmentally friendly feed additives have emerged as a promising strategy to reduce reliance on chemical treatments while enhancing nutrient bioavailability in fish and shellfish (Ng and Koh, 2017). Acidifiers (compounds containing organic acids and their salts) are one of the common additives with many beneficial effects in aquatic nutrition (Sotoudeh et al., 2020; Zhang et al., 2022). The presence of organic acids reduces the acidity of food and prevents the growth of harmful microorganisms during storage or the secretion of toxic metabolites, especially mycotoxins produced by fungi (Metzler and Mosenthin, 2007). A-Cid Aqua Acidifier is a collection of short-chain organic acids including citric acid (15%), formic acid (15%), fumaric acid (10%), propionic acid (10%), lactic acid (5%), acetic acid (5%), and vermiculite (40%) as a carrier, which has properties such as improving digestive tract activity, strengthening the immune system, and reducing feed conversion ratio (FCR). A unique feature of this compound is the presence of fumaric acid, which increases feed intake by making it palatable, thereby improving digestion and absorption. Fumaric acid has an antimicrobial effect due to its lipophilic nature and high dissociation capacity (Martin-Dominguez et al., 2018). Given that this compound has not been evaluated in sturgeon, the aim of the present study was to determine growth, blood parameters, immunity, and digestive enzyme activity of great sturgeon fed different levels of dietary A-Cid Aqua.
Methodology
Initially, the adaptation of the great sturgeon juveniles to the experimental tanks, density, and common commercial diet (Dibaq, Segovia, Spain) of the station was carried out for one week. Then, 240 great sturgeon juveniles weighing 25.40±3.91 g were randomly distributed into twelve 500-liter circular fiberglass tanks at a density of 20 fish per tank in three replicates. All tanks were equipped with a central aeration system. During the rearing period, the photoperiod was adjusted 14 h light and 10 h dark. Dietary treatments included 0 (control), 1 (A-Cid-1), 2 (A-Cid-2), and 4 (A-Cid-4) g of A-Cid Aqua (Sepehr Saman Fartak Additives Company, Mashhad, Iran) (Hadidi and Taati, 2016; Abdel-Tawwab et al., 2019; Asgharzadeh and Taati, 2020; Zhang et al., 2022) per kg in the basal diet. According to the instruction as suggested by Mohseni et al. (2021), the required amounts of acidifier were added to the basal diet for homogenization. At the end of the feeding trial, juveniles underwent a 24-hour starvation period. Subsequently, three fish from each treatment were randomly selected and blood was collected from the caudal vein using a 2 mL heparinized syringe. Hematological indices such as hemoglobin (Hb), hematocrit (Hct), red blood cells (RBC), white blood cells (WBC), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC), and differential leukocyte counts were measured as described by Klontz (1994). According to Ellis (1990), the turbidimetric method was conducted to assay lysozyme. Plasma liver enzymes including aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were determined by kinetic colorimetric method and alkaline phosphatase (ALP) was measured via kinetic enzymatic method (Moss and Henderson, 1999). Enzymatic kinetic colorimetric method was performed to specify the activities of alpha-amylase, protease and lipase enzymes. All data were analyzed using SPSS software.
Results
Fish fed diets containing different levels of A-Cid Aqua revealed significant differences in final weight, weight gain, weight gain, specific growth rate (SGR), condition factor, and FCR compared to the control group. Final weight and weight gain in fish fed acidifier at the levels of 2 and 1g showed significant differences (p<0.05) compared to the control and A-Cid-4. A significant improvement (p<0.05) was obtained in FCR in A-Cid-2 group in comparison with those fed control and A-Cid-4 diets. In the final length index, it was seen a statistically significant difference between the A-Cid-1 group with the control and A-Cid-4. The highest value of SGR was achieved in the group that received 2g acidifier-added feed, which showed a significant difference with those fed 4g A-Cid Aqua and control diet. Moreover, no mortality occurred in the experimental groups during 60-day feeding trial. RBC count and the values of Hb and Hct recorded a similar trend, such that groups fed 2 and 4g A-Cid Aqua indicated a significant increase (p<0.05) compared to other groups, respectively. The number of white blood cells showed a significantly higher value in A-Cid-2 group than the other groups (p<0.05), while the lowest number was observed in fish fed A-Cid-4. Statistical analysis revealed no significant variations in MCH, MCHC, monocytes, and eosinophils. The number of lymphocytes increased (p<0.05) in all treated groups. Liver enzymes in the control group were significantly (p<0.05) lower than those fed acidified diets. In this regard, AST and ALT in the A-Cid-2 group recorded the highest (p<0.05) values compared to control fish and the A-Cid-1 group. The immune system was boosted. An increase in lysozyme activity was apparent in fish exposed to A-Cid Aqua levels but only the A-Cid-2 group showed a significant difference with the control (p<0.05). The A-Cid-2 group had the highest value (p<0.05). In addition, IgM levels markedly (p<0.05) elevated in the acidifier-fed groups. The highest IgM values were reported in A-Cid-2 and A-Cid-4 groups as compared with those fed the control diet and 1g A-Cid Aqua (p<0.05). The activity of all three digestive enzymes in fish fed all levels of acidifier was higher than the control. There was no significant difference between fish fed the three levels of A-Cid Aqua from viewpoint of Alpha-amylase enzyme activity, but the levels of this enzyme in the control population decreased significantly (p<0.05). The highest (p<0.05) amount of lipase enzyme activity was found in the A-Cid-2 group. In addition, the protease enzyme activity elevated in all supplemented groups (p<0.05), and those fed 2g A-Cid Aqua indicated the highest value.
Discussion and conclusion
Aquaculture feed enriched with acidifiers improves palatability and physical quality, such as durability and stability in water, and can increase feed efficiency in the host. After a 60-day feeding period, the results clearly indicate that supplementation of diets with the acidifier A-Cid Aqua improved the growth, weight gain and feeding efficiency, such as reducing the FCR in great sturgeon. According to the obtained data, suitable condition was achieved in blood parameters, immunity such as lysozyme activity, IgM, and digestive enzyme activity. The beneficial effects of organic acids including enhancing growth performance, improving blood and biochemical parameters, and enhancing immune responses, have been proven in tiger fish (Astronotus ocellatus) (Hadidi and Taati, 2016), common carp (Cyprinus carpio) (Asgharzadeh and Taati, 2020), rainbow trout (Oncorhynchus mykiss) (Mohammadi et al., 2020), yellowfin seabream (Acanthopagrus latus) (Badzohre et al., 2020), Siberian sturgeon (Acipenser baerii) (Falahatkar et al., 2020; Kazemi et al., 2023; Ashourpour et al., 2026), Sterlet sturgeon (Acipenser ruthenus) (Kakavand et al., 2021), and great sturgeon (Huso huso) (Panahi Sahebi et al., 2023). Nevertheless, because of the different results in liver enzymes compared to other parameters examined, this section requires a detailed and complementary study in the future to determine all its dimensions. Consequently, due to the undeniable role of A-Cid Aqua on growth, physiological factors and digestive enzymes, it is recommended to utilize this compound at the level of 2 g in great sturgeon diet as the main cultivated sturgeon species. Future research should focus on extracting organic acids from specific plants, developing food encapsulation technologies to increase stability in water, and inventing environmentally friendly organic acids-based feed formulation to increase mineral uptake, especially phosphorus, and prevent its excretion to realize organic and sustainable aquaculture.
Conflict of interest
The authors declare that they have no conflict of interest to report.
Acknowledgments
The authors gratefully acknowledge Mr. Ali Baghalian, aquatic research and development specialist at Sepehr Saman Fartak Additives Company (Mashhad, Iran), for his valuable assistance and support. The authors also extend their sincere appreciation to the staff and experts of the International Sturgeon Research Institute–Guilan Sturgeon Research Station for their technical assistance and support throughout this study.
