you could make a variable called "fishing skill" start it at ten then when a player catches a certain number of fich decrease it, and in the fish event, make a percent of say
number between 1 and 10-20
number = 1: catch big fish
number = 2: catch medium fish
number = 3-4: catch nothing
then make the number decrease depending on level
Ect, level 1 the chances are
big: 1/20
med:1/20
small:2/20
but when the level increases to 5
big:1/15
med:1/15
small:2/15
and at max level (10)
big 1/10
med 1/0
and small 1/10