Microarray-predicted marker genes and molecular pathways indicating crowding stress in rainbow trout (Oncorhynchus mykiss)

Stocking density is considered as a key factor determining the productivity of fish aquaculture systems. The transcriptomic response to crowding stress is, however, still poorly investigated. We aimed to identify potential biomarker genes via microarray analyses to gain insight into molecular pathways modulated by density-induced stress in farmed rainbow trout Oncorhynchus mykiss. Transcriptome profiling in the liver, kidney, and gills was complemented by analysis of classical plasma parameters and behavioural observations. Individuals of two trout strains were exposed for nine days to definite stocking densities, ~ 10 kg/m³ (low density); ~ 30 kg/m³ (moderate); ~ 60 kg/m³ (elevated); and ~ 120 kg/m³ (high). Only the PYGMA gene, encoding the key enzyme of glycogenolysis, was upregulated in gills and kidney, while about 90 genes were differentially regulated in the liver of trout exposed to high density compared with the moderate-density group. Pathway enrichment analyses predicted the upregulation of HIF1A signalling in liver, contributing to glucose homeostasis during stress conditions. This is consistent with the increased plasma glucose levels detected in crowding-stressed trout. The mTOR and PI3K/AKT signalling pathways were impaired in liver at higher stocking densities indicating stress at the cellular level. Contrary to our expectations, protein ubiquitination and cholesterol biosynthesis pathways were found to be underexpressed. Cholesterol is a precursor of the stress hormone cortisol, whose plasma concentrations were indeed within the normal physiological range in all treatment groups. In addition, we did not observe any aggressive interactions, which are known to be triggered through cortisol release. Taken together, we identified a relatively modest number of rainbow trout-specific crowding markers suggesting that farmed rainbow trout have gained enhanced stress-tolerant phenotypes through decades of selective breeding.

Statement of relevance. The comprehensive recording of animal-based biomarkers is becoming increasingly important for the further development of the aquaculture industry with importance for fish well-being and consumer opinion. Previous studies on crowding stress in rainbow trout have used single or multi-gene approaches to assess the relevance of putative biomarkers, but transcriptomic approaches to define crowding stress-induced pathways in salmonids are overdue. The present study compares the transcriptomic profiles of rainbow trout exposed to different stocking densities and correlates these data sets with classical plasma parameters and behavioural observations.