Girwyllan Fish
From DQWiki
In the Fastness of Girwyllan a fish returns to the river from which it came from each year and dies. Eating the one magical fish can increase your abilities. The more often you do it, the more you gain, and some say even above normal for your race. Others say that the Harpies will eat you first - as they are fanatical over it - seeing it as a source of life for them to expand their group, by allowing them to make normal Harpies smarter.