How Different Stocking Densities Affect Growth and Stress Status of Acipenser baerii Early Stage Larvae

SIMPLE SUMMARY: Siberian sturgeon is a freshwater fish species currently at risk of extinction. Few studies have been performed on the environmental conditions during early larval stages, a phase of development where mortality rate is still relevant. In this study, we focused on assessing the impact of different rearing densities during the endogenous feeding stage of the Siberian sturgeon on growth, stress status, and muscle development, using a multidisciplinary approach. Results indicate that lower densities seem more advantageous in this stage of development in terms of growth rate and stress levels. However, rearing larvae at such low densities is not economically feasible in commercial hatcheries but it could be interesting, instead, in larvae production for repopulation purposes. ABSTRACT: In the present study, a multidisciplinary approach was used in order to evaluate growth, muscle development, and stress status in Siberian sturgeon Acipenser baerii larvae at schooling (T1) and complete yolk sac absorption (T2), reared at three stocking densities (low, medium, and high). Larvae growth, morphological muscle development, and whole-body cortisol levels were assessed. The expression of genes involved in the growth process (igf1 and igf2), in the myogenesis (myog), and in the regulation of cellular stress (glut1, glut2, glut4, and hsp70) was analyzed using a quantitative PCR. Larvae reared at lower densities showed a higher Specific Growth Rate and showed a physiological muscle development. Cortisol levels were low and did not differ significantly, both in different time sampling and across densities, suggesting that either the considered densities are not stressors in this species in the early stages of development or the hypothalamus-pituitary-adrenal (HPA) axis is not yet fully mature. Gene expression of glut1, igf1, and igf2 showed an up-regulation in both developmental stages at all the rearing densities considered, while myog significantly up-regulated at T1 at the highest density. Considering all of the results, it would seem that lower densities should be used in these stages of development, as these showed a higher growth rate, even if it is not economically feasible in commercial hatcheries. Therefore, choosing an intermediate stocking density could be a good compromise between larval performance and economical feasibility.