How Salmon Breathe in the Sea: Gills & Oxygen Extraction

Salmon, like all fish, respire underwater using gills—specialized organs that extract dissolved oxygen from water. As a salmon swims, water enters its mouth, passes over the gill filaments (rich with tiny blood vessels), and exits through the gill covers (opercula). Oxygen diffuses from the water into the blood; carbon dioxide diffuses out.

This counter-current exchange system is highly efficient, allowing salmon to thrive in cold, oxygen-rich marine environments. During their ocean phase (1–5 years depending on species), salmon range widely across the North Pacific or North Atlantic, feeding and growing before returning to freshwater to spawn. Their gills adapt to both saltwater and freshwater osmoregulatory demands—a remarkable physiological feat.