Re: Why do dead fish float?

Dear Erik -

The answer to your question is two-fold. For one, fish have a swim bladder which helps them maintain different depths. When the fish dies it no longer has control over the bladder and will float on the surface if sufficient air remains in the organ.

Secondly, after death the body's tissues and immune defenses degrade. Bacteria living in the gut (and from the environment too) enter these tissues and take to feasting on everything in site. Most of these bacteria are *anaerobes* (can't live in the presence of O₂). They release gases including CO₂, H₂, and methane, among others, as they digest fats, proteins, and carbohydrates from the fish. The entrapment of gas in dead tissues can also cause the fish to float on the surface. Note however, that it make take some time for sufficient gas to build up.

While death certainly leads to dramatic changes in the body, alterations in overall density are likely to be slight. The above two reasons are the primary ones for why dead fish float.

You might try the link above (and ones beyond it) to locate pictures on the web showing the swim bladder in a fish to help explain "why fish float" to your class. I hope this helps..

-L. Bry, MadSci Admin