Many animals (including fish) have camouflage color schemes that work correctly when looking up (for fish, light) or when looking down (for fish, dark). When they die & stop being in the correct orientation, they become very obvious & say to their predators: 'dinner is served'. At 07:04 AM 6/28/2013, Bill Gosper wrote:

And while I am this far off topic, why do nearly all fish bother to swim upright? This supposedly requires rocks in their heads--"otoliths", of which my father had a collection. Wouldn't bird|dinosaur-style "gastroliths" (well anyway, belly ballast) be simpler? Do fish go belly-up the moment they die?