The true genius of the gill lies in a principle known as . For gas exchange to occur, oxygen must diffuse from the water into the blood. However, diffusion is a passive process driven by concentration gradients. If blood and water flowed in the same direction (concurrent flow), the blood would quickly become saturated, and the gradient would flatten, stopping oxygen transfer. Instead, fish have evolved a system where blood flows through the lamellae in the opposite direction to the flow of water. Consequently, the blood constantly meets water that has not yet given up its oxygen. This allows fish to extract up to 80–90% of the available oxygen from the water, a far higher efficiency than human lungs achieve with air (roughly 25%).
Efforts are made to minimize "fishing pressure" by organizing individual fishing units into cooperative systems.