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Section II, Chapter II – 6
All Gassed Up
When I a youngster, my father and I caught a lot of yellow perch. Usually, they were hooked near the bottom in deep water and quickly reeled in. When I removed the hook, I frequently noticed a bulging pink lump that protruded into the mouth cavity. You have may have seen the same thing while you were landing some fish. What is that thing? And what is happening?
What we were seeing was actually the inside of the fish’s stomach. Huh. How can that be? Well, the stomach has been turned inside out by the overly expanded gas or swim bladder. What’s that all about? You see, when a fish is living deep in the water, the bladder volume is adjusted and compressed for that depth and water pressure. When that fish is abruptly brought to the surface, the gas in the bladder expands as the water pressure decreases. As the bladder expands it presses against the stomach, everts it, and forces it up the gullet and into the mouth cavity.
Why do fish have a gas bladder anyhow? The gas bladder has two primary functions. First, this gas-filled organ gives the fish the ability to control its buoyancy and hover easily at a preferred depth. Second, it acts as a stabilizer to keep the fish in an upright position in the water because the bladder is located above the center of mass. Also, in some fishes, it is used as a resonator to produce or receive sound.
There are two different arrangements for gas bladders. Among some groups of fishes, the gas bladder has a tube that connects the bladder to the esophagus. This configuration is called physostomous. Thus, the fish has the ability to allow gas to escape from the bladder or to add gas by gulping air. Trout, salmon, and herring are among the physostomous fishes.
Among other fishes, the gas bladder is simply a balloonlike structure. This configuration is called physoclystic. The connection with the esophagus is lost early in life. Thus, soon after becoming free swimming, the young fish must swim to the surface and take a gulp of air to fill the bladder before the connection is lost. Yellow perch, rockfish, and pollock are among physoclistic fishes.
Have you noticed that I have not used the term air bladder? That’s because this organ truly is a gas bladder. These organs come equipped with a special gland that can adjust the concentrations of different gasses in the bladder. This allows the fish to have greater control over buoyancy than merely the volume of the bladder.
Some fish, of course, do not have a gas bladder. But why would some fishes not have a gas bladder? How about those that prefer to live on the bottom? Like halibut and other flatfishes? These fishes do not have a gas bladder. It would simply be a disadvantage. They have no desire to spend much time up in the water column. With no gas bladder, they are negatively buoyant.
Interestingly, sharks and their relatives also do not have a swim bladder. But most sharks do not live on the bottom. They swim around higher in the water column. How do they manage? Sharks swim. And they must keep swimming. They can control their position in the water column only by swimming and creating dynamic lift by making their pectoral fins work as if they were wings on an airplane. If they stop swimming, they sink.
Okay, back to the beginning. Consider those yellow perch I was catching, or the bottom-dwelling rockfish we may catch in Alaska. These fishes have a gas bladder without a connection to the gut and the gas is adjusted for life near the bottom in deep water and it cannot escape. When they are hauled to the surface, the water pressure is diminished and the expanded gas bladder has to go someplace. Some anglers and scientists have tried various tactics to relieve the pressure of distended gas bladders and release fish unharmed, but none has proved successful, including the most obvious idea of puncturing the body wall with a hollow hypodermic needle.
Herring are just the opposite. Herring swim in large schools. As they slowly move up and down in the water column they can adjust their buoyancy. If they are threatened, however, and must dive rapidly, the change in pressure on the gas bladder can be vented through the tube that leads from the bladder to the gut. In some cases, when the school is big and the dive is fast, so much air is released that they create a mass of tiny bubbles.
One more thing. Some fishes produce sounds through the use of the gas bladder. The mechanism is a special muscle outside the bladder that vibrates much like our vocal cords and produces a sound that is magnified by the gas bladder. The freshwater drum and marine drums are among the most famous sound makers of fishes.
There you have it. There is a lot more to fish gas bladders than meets the eye.
Reference: Helfman et al., 1997.