Growth under serial stocking density and transcriptome analysis of the brain in rainbow trout Oncorhynchus mykiss

Abstract High stocking densities negatively affect fish production, even if the water quality is managed correctly. However, the mechanisms underlying the growth inhibition at high stocking densities remain unknown. This study aimed to reveal changes in gene expression in the brain that processes sensory information and to predict neuronal response changes under high stocking densities. Juvenile rainbow trout were reared at five different densities (22–180 fish/tank) for 60 days. The 180 fish group showed lower body weights than that of the 22 fish group on the 28th day, even though environmental factors such as water quality, dissolved oxygen, and feeding conditions were the same. Final fork length and condition factor results revealed growth inhibition in the high-density group. In addition, the brain transcriptomics of the 22 and 180 fish groups were analyzed. Gene set enrichment analysis based on the Kyoto Encyclopedia of Genes and Genomes was used to detect alterations in the five pathways. Steroid biosynthesis and extracellular matrix–receptor interaction pathways were downregulated in the 180 fish group, suggesting the stagnation of myelination in a high-density environment. These results offer insights into growth inhibition mechanisms and brain responses in environments where many individuals of the same fish species are present.