See some of my other threads-
Mark stated that all quests go through a series of checks, all starting at 1 line.
The chance of a 1 line quest becoming 2 lines is based on the percentage of possible fish you have done at least one quest for successfully.
IF it gets promoted to 2 lines, a check is made based on the number of possible fish you've completed at least two quests for.
and so on...
The numbers get you pretty close to having all 4-6 line quests, before you start getting quests for the dungeon fish.
Hitting the dungeon fish area, though, really screws around with the numbers, and the number of possible fish increase by 12, or effectively any 100% of all fish types you had suddenly become 77.777% (or worse, if they accidentally included non-quest types).
As a result, you drop to getting mostly 1-3 line quests (with the rare 4-6) when you cross that threshhold, and have to slowly work your way back up by doing dungeon quest.
To give you an idea, I dropped down into that area shortly after using my 110 PS at 105 skill (I got my 110 from a quest I turned in at 104.9).
I've been mostly getting 1-4 line quests ever since, with the average number of lines per quest slowly improving.
My first 6-line quest after the change was a nightmare. FIVE different crustaceans (each 10-count) and a 10-count shallow water fish.
I just turned that one in, a few hours ago.
I went out to get 7 new quests, since I am stuck on one (needing 20 of a crustacean that I fished for 4 hours to fill another quest for). I was able to immediately fill and turn in, without moving my ship, two of them. So, I effectively went out and got 9 quests (two of which were turned in).
OF the 9 (*1 was turned in):
4 2-lines*
2 3-lines*
1 4-line
1 5-line
1 6-line
Of the previous 10 or so, they were mostly 3 & 4 lines, with 1 1-line, a 5-line and that evil, evil 6-liner I described above.