After the Great Jagras hunt you’ll be sent off on a mandatory expedition to the Ancient Forest (listed under the page Rebuild the Ancient Forest Camp in the walkthrough). While your objective is - as indicated by the aforementioned page name - to set up a new camp in the Ancient Forest, you’ll also be able to talk to some new characters, including the Piscine Researcher. Ask him about fishing, then indulge in this past-time after he challenges you to catch a Pink Parexus (part of the bounty Research Help: Pink Parexus Capture). Simply catching any first will do, and it’s painfully easy - for most fish, you need merely press [Circle]/[B] when it bites (the bobber goes under the water).