How do fish achieve buoyancy in freshwater?

Fish achieve buoyancy in freshwater primarily through the use of a swim bladder. This internal gas-filled organ allows them to control their buoyancy and maintain their position in the water column without expending energy. When a fish adjusts the volume of gas in the swim bladder, it changes its density relative to the surrounding water. If the fish increases the volume of gas, it becomes less dense and will rise, and if it decreases the gas volume, it becomes denser and sinks.

The other options do not effectively address how fish maintain buoyancy in freshwater. Pumping water through their gills is crucial for respiration but does not contribute to buoyancy control. Regulating water salinity is more relevant to osmoregulation than buoyancy, as freshwater fish deal with different osmotic pressures than their saltwater counterparts. While fins do play a role in movement and maneuverability, they do not directly assist in achieving buoyancy. Thus, the swim bladder is the correct mechanism by which freshwater fish maintain buoyancy.