I made this guide because this achievement is a pretty tricky one to get given how not many things are explained and you need to revisit some areas to re-fish on waters you're almost certain you've caught everything there is.
Needless to say I will not be explaining how to catch the very basic fish since those you will automatically get by just fishing on the spots you find in the locations you visit. I will go into detail about the ones you probably miss aka those that you dont get automatically and have to ''try'' . BEWARE SPOILERS AHEAD.

In order to get the achievement you'll need the top fishing equipment which you get access to late in the game from the black mark vendor in Bligewater main hub right behind the Fishmonger. The equipment is the one following:

So the ones you are probably missing are the following:


It is extremely unlikely you have those and you've missed something else since the rest is extremely common to get.

Friendly reminder:
--TO CATCH WHAT IS MENTIONED ABOVE AND BELOW YOU NEED THE ROD AND BAIT I MENTIONED ON INTRO--

Moving on:
The Small Eel:

To get this you need to go to Angler's Envy which you select from Charming Lady



Simply fish everything you see with your Legendary Rod and Bait and it will be one of them.
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Long Whiskered Catfish:

To get this you need to go to Calm Waters which you select from Charming Lady



Same thing as above applies, simply fish everything there is with your Legendary Rod and Bait and it will be the one furthest away.
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Poison Blowfish, Baby Berserker Shark, Young Berserker Shark, Lucky Squid.

All of the above take place in the Vault of the Vesani fishing spot in the Shadow Isles which is why they are easy to miss. The fishing spot will spawn a combination of the above, ONLY two at a time everytime you visit it, including a level 27 treasure.

So for example you may have Baby Berserker Shark + Treasure OR Young Berserker Shark + Poison Blowfish etc.
In order to get them all you have to RESET the fishing spot, that is possible by RESTING in the res point at the entrance of the Vault. After you rest the fishing spot resets rolling a new combination of the above. Keep resetting until you get everything.

The Sharks and the Treasure will ALWAYS be the ones at the VERY far back, Poison Blowfish and Squid are slightly closer usually.

The Spot:



PS: Lucky Squid can be fished elsewhere as well, but it can spawn in the Vault of the Vesani fishing spot as well.

The Baby Berserker Shark and the Young Berserker Shark sell for 32 and 36 Black Marks respectively, Use the resetting method to farm them in order to get as many marks as you want in a very time efficient manner.

That will be it, i have every achievement in the game so if you'd like me to do another guide just let me know and should i get the time i will. Most of the stuff is fairly self explanatory anyway or there are guides for them already.

Dont forget to take very good care of yourselves and each other.

Cheers