So? If you beat them you will gain 4% of the quest... Press Q and click on the mainline quest, and you will see: <<Zendar asked you to elimenate river pirates.....>> and under it <<quest complete 0%>>. When you make it 100%, return to constable Hareck and you will be awarded.
At 40% you will receive a message from Suck, Zendars' Tavern Keeper. Go speak to him and complete his quest. When you finish, return to Sucker for the reward (he will give you a Medal if I recall correct), finish the Pirate quest and (after the reward) your mainline quest will change...