Introduction: Many marine fish and sturgeon larvae lack certain protein and lipid digestive enzymes when switching from live to dry food, resulting in deformities and widespread mortality. Adaptation to dry food in larvae involves more than just substituting dry food for live food in the early stages. This method involves a step-by-step shift, starting with various live foods (typically zooplankton), then gradually decreasing their presence, and ultimately substituting them with dry food, which impacts the larvae’s physiology and digestion. Food type and feeding strategy can have a significant influence on larvae growth and amino acid composition, which indirectly effects on fry survival and health, respectively. So, a variety of gradual adaptation strategies from live food to dry food have been proposed. Agh et al. (2013) were proposed a gradual feeding strategies Artemia nauplius to dry food that this way gradual adaptation was applied by Artemia reducing and dry food increasing on 5 to 7 days, and a combination of artemia and dry food from first feeding that proposed a strategy of gradual adaptation by mixture of live and planned food for larvae Huso huso, respectively. Dediu et al. (2011) investigated about effect of three feeding methods: formulated diet (100%), a combination of formulated diet and live food (50+50) and live food (100%: containing 75% tubifex and 25% daphnia) on the survival rate and growth indices of Acipenser gueldenstaedtii larvae and stated that growth indices of larvae 32 days after hatching were better than larvae fed only formulated diet (containing 58% crude protein, 12% crude fat, 0.9% fiber, and 9.8% ash). Furthermore, Mohseni et al. (2012) experimented with progressively introducing daphnia to dry food, chironomids to dry food, a combination of daphnia and artificial food, a combination of chironomid and dry food, gammarus to dry food, and a prepared diet, and they determined that a gradual shift to dry food for chironomus had a more beneficial impact on reducing mortality and improving growth in Huso huso larvae. However, no research has been conducted on the adaptation methods of Stellate sturgeon (Acipenser stellatus) to dry food.
Materials and Methods: The formulated diet was prepared at the Caspian Sea Fish Research Institute. Its composition included 87.13% dry matter, 51.95% protein, 14.12% lipid, 11.12% fiber, 58.3% nitrogen-free extract, and 7.45% ash.1500 individual 8-day-old Stellate sturgeon larvae, with an average weight and length of 0.385 ± 0.034 g and 4.95 ± 0.09 cm, were fed in 50-liter fiberglass tanks in 5 treatments included gradual adaptation (chironomid + dry food) (CH + DR), gradual adaptation (Artemia + dry food) (AR + DR), gradual adaptation (Artemia + chironomid) + dry food and gradual adaptation (Artemia + chironomid + dry food) + dry food (Mix CH + AR + DR) + DR and dry food (DR) in a period of 42 days, respectively. Fish were fed 25% live food and 5% dry food per body weight at 2-hour interval in day. Biometry were carried out in 7 stages (7, 14, 21, 28, 35, and 42 days). At the end of the experiment, 30% of fish population was harvested and sent to laboratory for amino acid composition analysis.
Results and Discussion: The highest growth index (final weight, condition factor, body weight gain percentage, specific growth rate, daily growth and protein efficiency ratio) were observed in fish fed with gradual adaptation (CH+ DR) (p<0.05), although there was no significant difference with larvae fed with (Mix CH+AR+DR) +DR and (DR) treatments (p>0.05). However, the highest survival percentage was observed in larvae fed with (CH+ DR) +DR (CH+AR) and (MIX CH++AR+DR) +DR groups. studies on Scaphirhynchus albus averaging 20.4 g, fed either pellets (1 mm, 45% protein, 19% lipid) or chironomid, showed that natural food led to a higher growth rate (12.2% per day) than dry food (0.06% per day). Also, Sturgeon that consumed chironomids exhibited a better condition factor compared to those fed dry food. According to the researchers, the higher growth index and fish acceptance of chironomids could be attributed to the fact that sturgeon larvae and fry in freshwater environments mainly consume aquatic invertebrates, especially Chironomidae and Ephemeroptera, whereas Artemia, which is grown in saltwater, is not available to sturgeon larvae naturally. In the other hand, this is agreement with findings Taati et al. (2018) who stated feeding Acipenser persicus larvae for chironomid lead to stimulating appetite and increasing growth indices due to amino acids attractant such as methionine. Also, according to Shakourian et al. (2011), a mixture of paste diet and chironomid larvae increased the final weight of Acipenser persicus larvae after 35 days. The combination live food and dry food in a paste form can improve the larvae’s ability to get adaptation and accept dry food, leading to an increase in their growth rates (Gisbert et al., 2018). In this study, the highest percentage of essential acids was observed in body fish that fed with gradual adaptation of (Mix CH+AR+DR)+DR (p<0.05). The highest arginine, lysine and methionine belonged to (Mix CH+AR+DR)+DR group, However, highest threonine amount body were recorded in (CH+DR) that no significant difference with (Mix CH+AR+DR)+DR (p>0.05), but had a significant difference with the other experimental treatments (p<0.05). Due to rapid growth and high metabolic activity (both catabolism and anabolism), fish larvae require essential amino acids; therefore, deficiencies in these nutrients can impair growth and feed conversion ratio (Saaverda et al., 2006).
Conclusion: According to the results obtained, it seems that the gradual adaptation strategy (breast milk+larval diet) (and (mixture of Artemia+breast milk+larval diet)+larval diet is a suitable nutritional strategy for transitioning to formulated food in ozone-bred larvae due to its positive effects on growth performance, survival, and better balance in the amino acid profile of the carcass.
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