Effects of Increased Temperature and Lower Oxygen on Skin of Chinook Salmon (Oncorhynchus tshawytscha)–Histology and Gene Expression

Rising temperatures and the associated lowering of oxygen concentrations are some of the challenges facing fish farming in open pens. Skin is one of the barriers between the external environment and fish, which is essential for fish health. The aim of our study was to investigate the effects of exposure to high temperature (19–20 °C) and lower oxygen (5.8–7.3 mg L−1) on Chinook salmon skin health. Following the 66–71-day experimental exposure, individual fish growth was calculated, and fish skin samples were used for histology and gene expression analysis. Mean epidermal thickness was reduced from 47.75 µm (SD 8.47, n = 13) to 41.48 µm (SD 10.60, n = 60) in Chinook salmon exposed to high temperature/lower oxygen. However, this difference was only significant using the secondary Welch’s t-test (p = 0.031), whereas primary two-way ANOVA was not significant (p = 0.060). The density of skin mucous cells was not significantly affected by treatment. After Benjamini–Hochberg false discovery rate correction, the down-regulation of il1β and hsp70 was associated with exposure to high temperature/lower oxygen. Our findings indicate that abiotic factors (combined high temperature and low oxygen levels) have adverse effects on the skin health of Chinook salmon farmed in net pens.