Also remember Aquariums were supposed to be expanded soon, and that seems to have disappeared from the publish notes... I'm not expecting it to be connected to fishing, at least directly, but it would be nice if we could put our catches "live" into them...
There's basically a lot of pros and cons to holding onto your fish now though;
The Con is that
you will run out no matter what. Everyone seems to have 1 or 2 fish per group which are caught slightly less and used slightly more than others. The Devs stated they thought people would trade fish instead. Thus if you're questing, and not trading or purchasing, no matter how much you fish you can't avoid some of your stocks becoming empty; I'm sitting on 200 of some crabs and zero of others. Unless the fish odds themselves change, you aren't saving yourself any of the hard work alas.
Also if you don't fish now, you're banking on new items being introduced you may want later; the changes could just be changes to the odds, so you may see the market for fishing items actually worsen as more enter the market. This is a pro if you want the things for yourself however! But a con if you want to get more bang for your fish-buck.
And one further con is that of course, the chances of doing the fishing content now, if everyone starts waiting with their fish, becomes much higher too; Not so much for Osireadon as he comes via the MiBs, but certainly you won't be seeing as many Charybdis... and as the months drag out into maybe even a year, people could become bored with High Seas entirely... which ties back into the first con as you may find no one has any stocks of the fish you do need when you get there.
The wider pro to holding onto fish is if there are new rewards, which you'll then have a head start upon. It also gives you some redundancy in case the changes aren't good; you can engage or not, and simply sell your fish stocks to any new fishers coming through. Just normal fishing should give you plenty of Deep Sea at least, and apart from cutting them up for fish steaks for the reputation there's little practical use for them outside of the quests... and you can always buy piles of 500 un-named fish from vendors anyway. So if you're fishing, but not questing, sitting on them does make practical sense, as that's where they're most useful.
You can see I listed more cons than pro; perhaps it's a flaw in my thinking, but it leads me to think I shall continue questing now; certainly any resulting scrolls will be worth more in the short term... perhaps even more if the new rewards turn out to be awesome and demand for scrolls flies up? But I'm also sitting on huge piles of most fish from chasing the legendary, so it's not much work to complete the quests now. You just have to factor in your own grind and grind tolerance I guess!