I had a similar question. My husband and I used to do the MMM fishing for skillups because we were guaranteed at least .2 skills per run. We stopped doing it for awhile when we became busy with other things.
Last night we went in to give it another run, and had difficulties. The large fishes, namely giant catfish and monke-onkes, were literally impossible to pull up. Once we had whittled them down to nothing and hit enter, every single time without fail we got the message 'You lost your catch due to lack of skill.' We are higher skill now than we were when we were doing this previously, and we could reel them in without difficulty back then.
Also, a point of confusion-- I thought the Angler's senses message was only for fish that were lower level than the fisher. My husband, several times, got the message that his keen angler's sense told him he had a giant catfish on his line. So, he has to be above the cap for giant catfish, right? But then every time without fail he would lose them 'due to lack of skill.' He can't be both high enough level to see the message and yet below the cap for them, can he? That seems counter-productive, to say the least.
The NPCs for the competition were also having trouble. We timed out for the first time ever because the NPCs could also not pull up the big fishes and were stuck with crayfish and copper frogs and the like.