After reading comments here and at wowhead and thottbot i decided to aim for Howling Fyord schools.
My results there are:
2647 total casts from the first cast going for the achievement (99% of those are in schools)
1401 fangtooth herrings
0 dark herrings
That was in a period of around 10 days aiming for 100 fangtooth herrings at least per day.
My average time spent per day fishing was 1-2 hours.
I was starting to get really frustrated especially after reading posts that people had caught dark herring in a few casts.
Also i got tired moving from school to school and thought to try mulgore and stonebull lake.
My results there:
332 brilliant smallfish
216 long jaw mud snappers
5 10 pound mud snappers
4 minor mana potions
2 minor healing potions
2 12 pound mud snappers
1 warrior bracers
1 15 pound mud snappers
total casts 563
All those casts at 5 October
So basically if you are getting tired with not getting the dark herring is not bad idea to try mulgore.
I know i was lucky getting it that fast but still 2.5k+ casts in howling fyord make up for that :).
Also Stonebull lake is good for people that can do other things same time they are fishing like watching movies etc. It maybe take a little longer than dark herring ( according to stats that is) but is a lot more easier just camping one place and fishing 1-2 hours than moving all the time from school to school.
