In fish, what is the advantage of countercurrent exchange in the gills?.

It removes a high percentage of oxygen from water. (More than 80% of oxygen is removed from the water as a result of countercurrent exchange.)

the principles and empirical processes of discovery and demonstration considered characteristic of or necessary for scientific investigation, generally involving the observation of phenomena, the formulation of a hypothesis concerning the phenomena, experimentation to test the hypothesis, and development of a conclusion that confirms, rejects, or modifies the hypothesis.