Ed,
You can get fish to live longer by growing them in colder water and/or breeding fish that live longer. Granted they won't grow as fast in colder water but sometimes they will at least be as large as a fish that grew faster in warmer water and died early.
Case in point: The Seeforellen browns come from colder water than the Great Lakes where they are planted. Even though some of them were reaching large size, the concern was they would die before they reached world record potential due to the relatively warmer water.
