What does collectible fish do?

Sawzallz

I know it's for the achievement but is there anything else also do I need to hold on to them for the achievement

IRDeja

I’m pretty sure they’re just valued for selling to a vendor. Just a little something extra because you’re not able to skin them. Sell em!

Carbonised

They swim. And hide from wannabe master fishermen.

But once you catch them, yeah, just sell them unless you want to keep them as memorabila.

emilyhyoyeon

They also aren't bank-able IIRC, so keep that in mind if you plan to keep them. I planned to keep all the shark type fish until I found out that I couldn't bank them.