The goblin fish thieves can be found on the opposite bank of the lake, directly at the northern bridge from Liannon.
Tyrgar, a fisherman from Liannon, owes a shipment of fish to Sunder. To his annoyance, the goblins have caught all the fish from the lake. The goblins must be dealt with and the stolen fish must be delivered to Tyrgar.
Keep in mind that although all the goblins are weak they are great in numbers.
Best fight them with another hero at your side or summonable companions.
Once the fish thief is dealt with, the fish needs to be brought back to Tyrgar. He then asks the Rune Warrior to bring the fish to Sunder directly who can be found in Liannon village.
Sunder accepts the fish rather gruntly but then gifts the Rune Warrior a Bone Shield as compensation for his troubles.