it IS a swim bladder
It is a swim bladder. It is used for holding air to suspend the fish at different levels within the water column. It acts somewhat like balast tanks do on ships. If I am not mistaken ALL fish have them, but it is just more pronounced in others. It will be more pronounced in species that do not particularly stay in one place in the water column. For instance redfish have their mouths pointing down as they are more of a bottom feeder. They primarily stay along the bottom of whichever water body they are in and therefore do not have as big of a need to be suspended at other areas within the water column and therefore have a smaller swim bladder. Trout tend to not occupy one particular area within the water column and are much more active feeders. They rely much more on a their swim bladder as they search out bait they try to stay suspended lower in the water column in hopes of being able to ambush it's food in the middle to upper layers of the water column. You can also tell that the trout use their swim bladder to actively feed this way by their body design with a pointy head and more slender build.
Sorry for any spelling errors. I just woke up and it's 3:30 am so I really don't care about them right now.
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