El says: For a current skill-up table, see Fishing Skill. Original topic below:
I'm currently grinding fishing on an alt, and I noticed a change in the way the game grants skill-ups now.
Under the old method, fishing would take an average of X casts to get a skill-up, e.g. around 10 casts to get from 345 to 346. It appears that now it grants a skill-up after anywhere from 1 to X casts, e.g. to get from 345 to 346 it could still take 10, but you could get it after 5 casts or even 1 cast. And it looks like it's weighted toward less casts, rather than more.
To give you an example, here's the past few skill-ups I've gotten:
340-341: 2 casts
341-342: 3 casts
342-343: 2 casts
343-344: 4 casts
344-345: 3 casts
345-346: 7 casts
346-347: 5 casts
347-348: 4 casts
348-349: 2 casts
349-350: 1 cast
Happy fishing!