Why do perch pulled from deep water come up bloated?

Perch are physoclistous: the swim bladder is a sealed sac with no duct to the gut, adjusted slowly by gas exchange through the blood — there is no way to "burp" on a fast ride up. Hauled up from 10 m (33 ft), the bladder's gas roughly doubles in volume; the fish arrives with a swollen belly, sometimes the esophagus pushed into the mouth, and eyes starting to bulge. This is barotrauma, and it is a depth problem, not a handling problem: it happens before you ever touch the fish. From about 10 m (33 ft) and deeper, plan on keeping what you pull up.

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