Also currently stuck on the regular Saltwater Fisher achievement for the last 25 hours of fishing, missing the Mantis Shrimp.
In the meantime I have caught anywhere between 1 and 8 (!!!) of all the other saltwater fish in the collection, meaning I've effectively almost completed it twice but, ya know, none of this will count for avid.
So that's a bit of a pain.
I'm quite dedicated to seeing this through, but as you said
I have a feeling that once I complete it I won't look at fishing for a long long while.