I've found the easiest place to catch the bear acuda is Helgrind Warren in Nidavellir. You would need to fight the first battle and then the doors open. Then fight the storm dudes in the chamber with the water (and the yardbird). Catch the fire fish in there, I've always found a bear acuda hiding in there
If you do not have access to Nidavellir yet you can port into it. If you need someone to take you there and clear the enemies for you to go fishing just let me know your usual availability (include time zone) and I'll help you
Bear Acuda are found mainly in Grizzleheim. Catch Fire fish in Savarstaad Pass, Mirkholm Keep, or Vigrid Roughland, and you'll eventually get one. If you have Winnow Fish Rank 2 or Reveal Fish Rank 1, a Bear Acuda will either be the only Fire fish left, or the one that isn't highlighted. (As a side note, if you see an Ice fish in the waterfall in Vigrid Roughland, catch it! It's an even rarer Polar Bear Acuda!)
