UESP says
Bugs
- During http://www.uesp.net/wiki/Skyrim:Supply_and_Demand the Jarl in Riften asks you to clear this cave. Clearing it and/or killing some of the inhabitants or the bandit outside the cave may cause it to be unable to be completed.
If this happens, you may need to visit the cavern up to 3 times for this quest to complete. On Xbox, the game seems to look for 7 Cragslane Cavern humans to be killed after the quest is assigned. If you have cleared the cavern before this quest is assigned, only 3 humans will spawn around the cavern -- one outside and two inside. To get the required 7 kills, kill the bandit outside, kill the 2 bandits inside, and then wait in Solitude for at least a month in order to allow the cavern to reset. Travel back to the cavern and get 3 more kills. Finally, wait once more for a month in Solitude to allow the cavern to reset again, and then the quest should clear the next time you kill the bandit outside.
Edit, Oh, and welcome. A http://www.uesp.net/wiki/General:Fishy_Stick for you.