Incremental spacing before the check, corresponding to freshwater growth, is smaller than after the check, corresponding to salt water growth. On-going research has indicated a less dramatic transitional zone on the otoliths of these fish with a distinct difference in increment spacing before and after habitat transition. FISH Are salmon, sturgeon, etc. Swimbladders allow precise control of buoyancy because the volume of gas they contain can be regulated with ease. Two types of swimbladders exist.
Some fish have swimbladders with a connection or duct to the gut.
The swimbladder is inflated by gulping air at the water's surface and forcing it through the duct into the swimbladder. Defaltion occurs by a reflex action initiated when reduced external pressure makes the fish too light, releasing gas by way of the duct into the esophogus. Special structures associated with the circulatory system are used for inflating or deflating. The structure is a tight bundle of capillaries. This vascular arrangement provides an efficient, countercurrent exchange of blood gases, allowing gas pressure to increase within the swimbladder.