The Dolphins of Pern — Anne McCaffrey

When the first humans came to settle the planet Pern, they did not come alone: intelligence-enhanced dolphins also crossed the stars to colonize the oceans of the new planet while their human partners settled the vast continents. But then disaster struck. The deadly silver spores called Thread fell like rain from the sky, and as the human colonists' dreams of a new, idyllic life shattered into a desperate struggle for survival, the dolphins were forgotten...

Now, centuries later, as the dragonriders of Pern were preparing to complete the momentous task of ridding their world of Thread forever, T'lion, a young bronze rider, and his friend Readis, son of the Lord Holder of Paradise River Hold, made contact with the legendary "shipfish." And as the dragonriders grappled with the ending of an era, T'lion, Readis, and the dolphins faced the start of a new one: reviving the bond between land- and ocean-dwellers--and resurrecting the dreams of the first colonists of Pern!