Why do trout gulp air at the surface, and why do lake trout "burp"?

Same duct, two directions. Because the swim bladder opens into the esophagus, trout adjust buoyancy the mechanical way: gulp air at the surface to inflate, burp it out to deflate — no slow gas-gland chemistry required. This design lets a trout change depth quickly without injury, which suits the rainbow's rise-to-the-surface feeding style and the lake trout's huge vertical range: tracking studies show lakers making regular vertical excursions of tens of meters (a hundred-plus feet) that would incapacitate a closed-bladder fish. The burp you hear from a netted laker is that valve doing its job.

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