The gills are the respiratory organs located underneath the ear-like flaps behind the eye. When a fish is alive, blood is pumped through the gills and absorbs oxygen from the water. During that absorption process, the blood's hemoglobin undergoes a chemical process and turns bright red.
If the fish is properly stored after it dies, the oxygen in the air will help keep the gills bright red for a few hours. Then over-oxidation gradually changes the hemoglobin from bright red to pink to brownish red and finally to grayish brown. Gills provide an excellent early warning system because they are more perishable than the flesh of the fish and will develop an off-odor before the flesh starts to acquire the telltale scent.
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