Barometric Pressure Affects on Fishing Part II

There are two kinds of gas-filled structures in animals. One is the lung and the other is the swimbladder. However, in some species of fish, the swimbladder may function as a lung. There are two types of swimbladders: closed-system swimbladders which are called physoclistous (bass, crappie, bluegills)that have a "gas gland" to regulate bladder pressure, and open-system swimbladders which are called physostomous (carp, goldfish) with a pneumatic duct connecting the esophagus which regulates bladder pressure. The lungs are for respiration, and the primary function of the swimbladder is to provide buoyancy. Both develop from the embryonic forgut.

Physoclistous swimbladders (gamefish) are completely closed without a pueumatic duct. The volume of gas inside the bladder is increased or decreased entirely through secretion or transport of gases in or out of the blood. Species that have this type of swimbladder are called physoclists. In closed-system swimbladders, this additional gas is mainly pure oxygen secreted into the bladder by organs called "gas glands." Inflation of the physostomous swimbladder (carp, goldfish) requires gulping and swallowing air at the surface and forcing the air into the bladder by way of the pneumatic duct and then adjusting bladder gas volume by belching bubbles until neutral buoyancy was achieved. Species that have this type of swimbladder are called physostomes. Physostomous swimbladders are emptied via the duct to the esophagus. Physoclistous swimbladders are emptied via a "perfused patch" on the bladder called an "oval."

The swimbladder is a gas-filled sac that is flexible to contact or expand according to ambient pressure. The walls have very few blood vessels and are lined with guanine crystals which make them impermeable to gases. The pueumatic duct has muscular bands, sphincters, and nerve supplies. It has a sphincter at the exit of the bladder and a nerve complex at the esophagus junction. The pneumatic duct also has a pneumatic bulb which acts as a pneumatic pump. The entrance to the pneumatic bulb is guarded by taste buds and a sphincter. The function of the taste buds is to guard the pnematic duct from food obstuction much like our epiglottis that guard our trachea to the lungs. The taste buds all the opening into the pneumatic duct to be closed when the fish swallows food thus protecting the air duct. Its main cavity is lined with mucous epithelium and powerful striated muscular walls.

So what is hydrostatic pressure, and what is its relationship to buoyancy? Hydrostatic pressure is the pressure at a given depth in a static liquid and is a result of the weight of the liquid acting on a unit area at tha depth plus any pressure acting on the surface of the liquid. Hydrostatic pressure in relation to buoyancy may be described as a swimbladder immersed in a fluid of a net positive vertical force originating from the depth-dependent liquid pressure. This vertical force is termed buoyancy and is equal in magnitude, but opposite in direction, to the weight of the displaced fluid.

The swimbladder is primarily a hydrostatic organ that contains the exact quanity of gas necessary to make the specfic gravity of the whole fish equal to that of the environment in which the fish is swimming so that it can rest, thus neither to rise or sink, which is called "neutral buoyancy." Since gas is compressible and water is not, any increase of external or atmospheric pressure will reduce the volume of gas in the swimbladder and cause the fish to sink, which is called "negative buoyancy." When in negative buoyancy, the fish will compensate by adding additional gas into the swimbladder. In Part III, we will look at swimbladder physiology and buoyancy.