When a fish dives, its swimbladder is compressed as a result of the increase in pressure at greater water depths.  So in order for the fish to return to its former, higher level in the water, the swimbladder must be reinflated.  Therefore, the most essential function of the swimbladder is the regulation of gases coming in and out of the fish's body.  Thus it is necessary for fishes to be efficient controllers of their buoyancy (Marshall 1966).  I have already mentioned how fish with open swimbladders maintain the gas concentration within their bladders (gulping and burping through the pneumatic duct), so now we will  mainly look at the function of the closed swimbladder. 

Closed Swimbladder- Deflation
Closed Swimbladder- Inflation