Yes, you’re 100% correct. You’ve described kin selection. So if grandparent fish can somehow help their grandchildren procreate more, then that can be selected for.
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.
Or if they care for their descendants in their later years.
(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.)
