While some sharks can breathe without moving at all, most sharks, including the bull shark, have to move around so that water passes over their gills. Some fish and sharks have muscles in their necks that help to pump water over the gills, but most sharks simply rely on their movement while they swim. When they are swimming, the water passes through their gill slits on the side of their necks and passes over complex networks of capillaries that withdraw oxygen from the water. While there is a very minimal amount of oxygen in ocean water, sharks are able to take in a much larger percent of the oxygen (up to 80%) compared to the average humans 25%.
The primary reason that sharks often die when they get caught in fishing nets is because they can no longer move around and end up suffocating from a lack of oxygen.
