DREDGE

This achievement take some time to get. As you progress through the DLC story you'll start to upgrade the buildings on rig which goes for another achievement. Your main focus is upgrading the factory which has four stages. Upon reaching the stage 4 for the factory there is a new bait option which allows you to bait the exotic fish. To make it you'll need 1 regular bait which you can purchase from the pontoon merchant and 1 Dark Canister which you get for using the new siphon trawler by swimming through the oil (black substance).

Once you have one of each item you'll combine them which costs 700.00 dollars (sell fish if you need the money) and you'll get the exotic bait. Then head to one of the regions, use the exotic bait, then use the Atrophy ability and you'll get the exotic aberration and the achievement.

The easiest one in my opinion is the oarfish which is located in the Gale Cliffs behind the waterfall (which is the one I got). Thanks to DaffyHail915984 you can just used the bait outside the docks of Gale Cliffs or any section of a region to get the exotic fish not just the location where you caught it the first time. Side Note you can also get the Sleeper Shark aberration from the Pale Reach DLC which also counts