After centuries of slaughter, their sentience was recognised by Humans and they were accepted as legal entities with the passage of the Cetacean Rights Act. Since then, dolphins have helped humans study oceanography and marine biology throughout Earth and Known Space. 
Dolphins are a Handicapped species but, as Terrestrial mammals, their flippers possess unused muscles, as well as hand and arm bones for which Humans designed prosthetic limbs controlled by these muscles. Humans subsequently transported them to colonise and explore oceans on other planets. This included running a fishing industry on Down.