Calahan is a fisherman in Ulgoth's Beard who will approach you after your talk with Mendas. For 20 gp he will tell you that Mendas is missing the truth, but only if you told Mendas, that "knowledge is a worthy pursuit."

In Baldur's Gate II he will be on the island of Brynnlaw. You will need his help for the Island Concerns: Ginia's Tale quest.