Abstract

As climate changes worldwide, aquaculture is increasingly exposed to stressors such as warming, altered ocean chemistry, increased CO2 “ocean acidification”, and reduced dissolved oxygen (DO) (i.e., hypoxia). Hypoxic events may also occur naturally during upwelling, when nutrient-rich but low-oxygen deep waters reach the surface, a process that may intensify with climate change along coastal areas such as the U.S. West Coast and Chile. The aim of this study was to determine the effects of hypoxia on several osmoregulatory organs and tissues of Oncorhynchus kisutch, including gills, kidney, intestine (foregut, midgut, and hindgut), muscle, red blood cells, and brain. Fish were exposed for 28 days to four hypoxic conditions (60, 50, 35, and 25% DO) and a normoxic control. Plasma chloride increased significantly at 25% DO, while plasma pH decreased significantly only under this condition. Plasma osmolality and calcium showed only non-significant tendencies. NKA activity decreased significantly in gills at 50, 35, and 25% DO and in kidney under all hypoxic conditions; however, kidney H+-ATPase remained unchanged. Gill H+-ATPase decreased significantly only at 35 and 25% DO. Intestinal responses varied among segments, whereas muscle and red blood cells showed higher NKA activity at 35% DO, with no significant changes in H+-ATPase activity. Overall, prolonged hypoxia affected osmotic regulation in a tissue-specific way, with reduced NKA activity in key osmoregulatory organs and compensatory NKA responses in peripheral tissues, which may indicate a shift in energy use toward oxygen transport under severe hypoxia.

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