What are the accessory respiratory organs in vertebrates like fish?

In some fish species, the air bladder is one of the accessory respiratory organs.

Fish have a number of respiratory structures that aid in the extraction of oxygen from water and the release of carbon dioxide. 

In addition to gills, which serve as the primary respiratory organs in fish, some species have air bladders, which serve as secondary respiratory organs. 

Air bladders are sac-like structures that function in gas exchange as well as controlling the buoyancy of the fish. 

The fish can regulate its position in the water column and extract oxygen from the air by adjusting the amount of air in its bladder. The skin and mouth lining of fish are also accessory respiratory structures that can help with gas exchange.