Fishing can be started at level 7 in Loa or Braiken castle.
The fish you catch can be used like potions, increasing health or mana.
Since action 6, Mana Shock has been implemented, using fish can counteract the rather natsy effect of being unable to use skills in pvp.
You can Fish in Both Loa and Braken Castle with fishing skill Level 1. You can Fish in Draco with fishing skill Level 3.