Why do fish float when they die?

Asked by: Niall Woodbury, Bury

Most fish are slightly denser than water, so sink immediately after death. However, like a drowned human, they become more buoyant over time as bacterial decomposition produces gases inside the body. Usually, enough gas builds up in body cavities to make the corpse float, like an inflated balloon. That’s not always so, as I can testify from my own fish tank: they occasionally just decompose unnoticed on the bottom.

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• Do all fish and shellfish contain mercury?

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