In practice, do fishes die from old age?
Yes, at least some fishes have intrinsic lifespans and deaths that are related to their own life-history and not to external forces such as predation or disease.


  Fishes show three types of senescence. Lampreys, eels and pacific
  salmon exhibit rapid senescence and sudden death at first spawning.
  The guppy, red panchax, medaka, platyfish, Indian murrel and many
  other teleosts undergo gradual senescence, as observed in most of the
  vertebrates. A number of fishes (e.g. sturgeons, paddlefish, female
  plaice, flatfish, rockfish) show indeterminate growth, the occurrence
  of senescence in them is supposed to be very slow or negligible.


Patnaik BK, N Mahapatro, and BS Jena. 1994. 'Ageing in fishes' Gerontology 40:113-32