Introduction: Among the sturgeon species cultivated in Iran, the Beluga (Huso huso) is of great importance due to its faster growth and adaptability to farming conditions, and its farming is economically viable (Mohseni et al., 2008). Protein is the most expensive component of the diet and plays the most important role in the growth, appetite, and food price of fish, especially carnivorous fish (Mohseni et al., 2016). Finding the optimal amount of protein according to the specificities of each species leads to increased protein efficiency and ultimately growth. (Hosseinpour Aghaei et al., 2018; Zhao et al., 2021). The best solution to reduce the amount of dietary protein without negatively affecting growth is to provide protein based on the amino acid requirements of the species in question. This strategy has been used in some species and has led to improved nitrogen utilization (Peres and Oliva-Teles, 2008). Lysine is an essential amino acid and its dietary supply is essential for fish growth. Its adequate amount not only enhances growth (Ahmed and Khan, 2004), but also causes nitrogen retention, reduces body fat (Berge et al., 1998), reduces oxidation of other amino acids, and consequently reduces the need and use of essential amino acids in synthesis (Kerr and Easter, 1995). However, there is no information on the use of the amino acid lysine in low-protein diets in Beluga . Therefore, the current study aimed to investigate the effects of different levels of lysine supplementation in Beluga fed low-protein diets on growth performance, body composition, and food digestibility.

Material and Methods: For this purpose, 200 belugas with an average initial weight of 68.00±0.45 g were prepared and reared in Shahid Rajaee farm in Sari. After acclimatization, the fish were randomly transferred to 1000-liter polyethylene experimental tanks. Ten fish were distributed in each of the 18 tanks for 8 weeks. Six experimental treatments included a diet containing 42% protein and 24% fish meal (control 1), 37% protein containing 24% fish meal (control 2), 37% protein containing 12% fish meal (control 3), and diets containing control 3 as basal along with 0.25, 0.5 and 1% lysine supplementation. At the end of the 8-week period, the length and weight of the fish in each tank were measured individually using a scale and caliper. The growth indices calculated, including weight gain, specific growth rate, fatness coefficient, feed conversion ratio, feed efficiency ratio, protein efficiency, fat efficiency, visceral index, liver index and fish survival rate, were obtained through the relevant formulas. After 56 days of the rearing period, 2 fish from each treatment were randomly caught and, together with samples of experimental diets and feces, their approximate amounts of protein, fat, moisture, ash and dry matter were calculated using the AOAC (1995) method.

Results and Discussion: According to the growth results, the final weight, body weight gain and specific growth rate reduced significantly in low-protein and fish meal diet (p<0.05), while these values were compensated by adding 0.25% lysine to the diet (p<0.05). Also, fish fed with 0.25 lysine had higher protein efficiency than that of 1% lysine (p<0.05). Beluga carcass protein fed 0.5% lysine was similar to control 1 and was significantly higher than control 3 (p<0.05), while it was not considerably different from other groups (p>0.05). Also, addition of lysine (0.25, 0.5 and 1%) to low-protein diets, led to significantly decreasing carcass fat (p<0.05). Protein digestibility was significantly diminished by the reduction of protein and fish meal in the diet (control 3) (p<0.05), whereas it improved by supplementation of lysine and reached the control diet at levels of 0.25 and 0.5% lysine. Fat digestibility showed no significant difference between different treatments (p>0.05). Jiang et al. (2016) reported that supplementing the amino acids lysine and methionine to a canola meal-based diet in Amur carp (Ctenopharyngodon idella) enhanced nutrient digestion and absorption by increasing the capacity of digestive enzymes. In a similar study on crucian carp, adding 4.5 g/kg lysine to the diet improved the protein digestibility of a diet without fishmeal (Ji et al., 2021). Fishmeal replacement and fishmeal reduction were also compensated for by adding 0.4 and 0.5% lysine to the diet, respectively, in Nile tilapia (Oreochromis niloticus) and rainbow trout (Oncorhynchus mykiss) (El-Saidy and Gaber, 2002; Cheng et al., 2003).

Conclusion: The present study was consistent with these findings, although the optimal level in studies may vary depending on the species and experimental conditions. In general, it can be stated that the addition of 0.25 and 0.5% lysine can compensate for the negative effects of the combined reduction of protein and fish meal in the diet of Beluga .