So long as they remain capable of reproduction in those later years
Maybe healthier older fish protect their offspring more effectively.
I’ve heard of territory defense helping with fish, but no idea about the fish mentioned specifically.
Also to the previous comment, I think there’s already a fair bit of evidence that older larger fish do continue to breed, (hence throwing back the bigger fish breeders).
But if they stop breeding, and there is no kin selection; then yes, natural selection wouldn’t play a role.
(Not saying fish do this, though I wouldn’t be surprised if giant elder fish clear out predators. A lot of people forget this bit but it’s suspected of being a big factor in insect and lobster longevity, which both capable of recognizing their offspring and not being territorial to their presence.)