Typically, swim bladder in fish contains gas (usually oxygen) and functions as a hydrostatic organ, enabling the fish to maintain its depth without floating upward or sinking.
Nile perch (Lates niloticus) is a luxury fish with high commercial value in Africa especially when exported. Upon processing Nile perch not long ago, whether in local markets or for export, the swim bladder was like any other organ in the abdomen was thrown away.
This video was filmed in a typical fish market in Cairo, Egypt in two different occasions and in my last visit (June, 2017) I was lucky to spot a filled gas bladder. It seems that the trade in Nile perch gas bladder in Egypt began at almost the same time as occurred in Lake Victoria countries when middlemen showed up offering relatively high price for such organ which used to be neglected in the near past. The value of fresh bladder at present is now more than twice the value of fish fillets. In fact, the revenue of selling the bladder –especially- those taken from large specimens is significant; the deflated bladder by the end of the video shows the size of a large Nile perch specimen.
The bladder is sold by middlemen to traders for export to China where high demand on bladders does exist. In China, where traditional medicine is advanced, the swim bladder is on high demand in the pharmaceutical industry. According to some published articles, the gas bladders may be used in the making of bio-degradable stitches for surgery. Moreover, swim bladders are an ingredient in nutritious soups in China.