The video above is the The Witcher 3 Wild Hunt Black Pearl Secondary Quest Walkthrough and shows how to complete Black Pearl, the quest featured in The Witcher III: Wild Hunt.
The video walkthrough covers the following objectives:
Meet Nidas near the collapsed bridge to the east of Arinbjorn.
Follow Nidas to the pearl bed.
Find a black pearl.
Return to Nidas.
Meet Nidas in the Golden Sturgeon to collect your reward.