Salmons decay after spawning, then die. Their offspring eat the nutrients presents in the water, including their parents leftovers.

Octopi is equally correct because we're speaking English and frequency of use trumps etymological cromulence.

Octopuses works too. I quite like "Octopods" because -podes is unusual in English, though not entirely absent (see Antipodes).

(I'm not sure "an Octopi" is correct in any of these cases!)

I'd go, fully expecting to be eaten for the good of mankind.