[Cuthols non-existent for me]

I've been at Twin Horns (Geyser Pond) trying to fish Cuthol for the next Ostron offering. So far, I've used 23 baits in total at night, used countless luminous dyes and have not seen a single Cuthol, only hordes of Mortus Lungfishes. 

 

I feel like I'm going to lose it. Where are these fish? Am I missing a vital key to this?