you fish. you gather stacks of fish. then you fillet all of them. it is RNG.. on average you get one per 100 fish.. on average. sometimes it can be less, sometimes more.
PC / NA - 2100 CP
PvE Tanks
L50 Nord Arcanist (US/EP) "Now Thats a Huge Witch"
L50 Imperial DK (US/DC) "Rampant Rabbit"
And I also always fillet the fish in another zone then where I caught them.
One of the first times I did that, I got about 5 roes on about 60 pulls.
So it just became a (luck/superstition) thing for me and has surprisingly continued to work in my favor (although not as well as that first time).
It's kept me in plenty of Roe for whatever my needs and/or a quick pop of gold if/when I vendor them.