Almost as if there were a pre-set maximum weight...
Some plants, incidentally, play the same sort of trick as cold/deepwater fish: they cope with limited growth due to overshading, nutrient deficiency or drought by having a very low growth rate necessary to stay alive - but put them in the light, water and feed them and they grow much faster, though still (iirc - it was a long time ago) not as fast as the competitors whose dominance they've adapted to circumvent.
Is this longevity study based on monitoring individuals, or otolith/scale readings, for the latter might miss any time spent maintaining good health without actually growing, mightn't it?
I googled Kennedy and Miller + Pike, and top of the search was this:Pike: Biology and Exploitation - John F. Craig - Google Books
which paints a more complicated picture, with exploitation and age at maturation emerging as big factors. Exploitation, in this paper, seems to mean removal of trophy fish; in the UK, with predation of smaller pike unnaturally high due to cormorants etc, it might exert the reverse pressure.
All fascinating, and bafflingly complex. And, I'm relieved to find, twenty years later than my last attendance at a freshwater biology lecture, which saves me a blush or two!