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I was watching sea birds eating fishes and wondered whether any fishes died from old age or whether they are all eaten before this can happen?
I imagine that some big predator fishes must be able to live long enough to die from their age. However, do any fishes that are lower in the food chain reach an age such that they die naturally, or are they all destined to end their lives as food for a bird or another fish?
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
Fish do not die from old age, nor do any other animals. Rather, the older the animal gets the weaker it becomes which means that it is an easier target for predators or can easily succumb to disease.
