The present study investigated in young-of-the-year rainbow trout, the effects of stocking density on growth parameters, plasma biochemistry and the expression of muscle stress- and growth-related genes including heat shock protein 70 (HSP70), insulin-like growth factor-1(IGF-1), myosin heavy chain (MHC), myostatin-1a (MSTN-1a) and follistatin (FST). Animals were exposed to three different stocking densities: 12, 24 and 44 kg/m3, termed as low (LD), moderate (MD) and high (HD) density treatments, respectively, in an open culture system for 60 days. Also, on day 40, the culture density of a group of fish were reduced from 44 to 12 kg/m3 (reduced density; RD treatment) for a further period of 20 days. The results showed significant changes in growth parameters, so that the final weight and condition factor were significantly decreased and feed conversion ratio was increased in HD compared to LD treatments. On the other hand, no significant alterations were found in fin, hepatosomatic and viscerosomatic indices as well as plasma glucose, total protein, triglyceride, cholesterol and cortisol contents among different experimental groups. The expression of HSP70 was upregulated in HD compared to LD on days 20, 40 and 60, being the expression of MHC decreased and MSTN-1a increased significantly on day 60 only. The expression of IGF-1 and FST did not show any significant variations among treatments. After 20 days of RD treatment, no significant changes were observed in growth parameters between RD and HD, being final weight and condition factor lower and feed conversion ratio higher than LD values. In addition, the expression of HSP70 and MSTN-1a was significantly downregulated and MHC was upregulated in RD groups compared to HD ones. In conclusion, increase of stocking density, reduced the growth and changed the expression of stress- and growth-related genes in muscle. Finally, the 20-day period of RD treatment could not recover the fish to LD conditions and a longer treatment period or, alternatively, experience of adjusted stocking densities would probably be required for a full recovery.

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