Environmental enrichment for the early larval stages of Acipenser baerii in captive environments

The Siberian sturgeon, Acipenser baerii Brandt 1869, is included in the IUCN Red Data List. Taking into account the continuous decrease of its natural populations, it is necessary to gather as much information as possible in order to better manage the populations and to enhance farming practises. Larval stages are particularly sensitive to environmental conditions, as these can affect survival and potential growth (Johnston, 2006). In fish destined to production, a suitable environment is of great importance for growth efficiency at harvest. Environmental enrichment is considered to improve biological functioning of captive animals by improving their psychological and behavioural needs. Little information is available on habitat preferences of the free embryos of Acipenser baerii upon hatch. The aim of this study was to assess the environmental enrichment without compromising larval morpho-functional aspects. After hatching and throughout the endogenous feeding period, larvae were reared in two types of substrate (Bioballs type 1 – BB1 and Bioball type 2 - BB2) vs. no substrate (CTR). Behavioural, larval growth parameters (weight and length) and histometrical analyses were studied in order to evaluate muscle development. The analysis of myogenesis and stress were carried out through the use of Real Time PCR, evaluating the relative expression of a pool of genes: Myod, Myog and Mrf4 (involved in myogenesis), Igf2 (involved in growth), Hsp70, Hsp90α, Hsp90β and Glut2 (markers of cells exposed to high stress conditions). This study was approved by the Ethic Committee of the University of Milan (OPBA_15_2018). Larvae reared with BB1, when compared to those reared with BB2 or CTR, were heavier and longer at the end of the trial, and histometrical analyses showed larger areas of total muscle area, slow muscle area and fast muscle area (P<0.05). This is in accordance with studies performed with other species of sturgeon (Baker et al., 2014; Boucher et al., 2017). Moreover, CTR larvae showed an acceleration of muscle differentiation, which can have negative consequences on its growth potential. Despite this, gene expression analysis suggests, instead, that the larvae reared in BB2 are those with greater growth potential, which may eventually occur after the start of exogenous nutrition. The highest expression of stress-related genes in BB2 larvae may indicate some sort of metabolism activation of these animals and not necessarily be a negative result. According to our results, it would seem more favourable to provide a substrate rather than a bare bottom, for Siberian sturgeon in these phases of development. Understanding the effect of the rearing environment on larval sturgeon development is vital for effective hatchery practices, particularly in conservation aquaculture, but also for habitat restoration to enhance natural propagation. (1) Baker et al. Swimming performance and larval quality are altered by rearing substrate at early life phases in white sturgeon, Acipenser transmontanus (Richardson, 1836), Journal of Applied Ichthyology, 30, 1461-1472, 2014. (2) Boucher et al. The effect of substrate rearing on growth, aerobic scope and physiology of larval white sturgeon Acipenser transmontanus, Journal of Fish Biology, 92 (6), 2017. (3) Johnston, I.A. Environment and plasticity of myogenesis in teleost fish. The Journal of Experimental Biology, 209, 2249-2264, 2006.