Twenty-five Dolphins made the journey to Pern with their human companions. They were specially enhanced dolphins, with the ability to communicate with humans. They had been partnered with humans, in a relationship similar to - but far more equal and voluntary - to that of Dragons and humans in later Pern.
During the First Pass, humans lost contact with the dolphins, and eventually forgot about the old relationship. The dolphins came to be referred to as "ship fish". The dolphins themselves never forgot, however, and continued to aid humans in various ways, such as directing fishermen to rich fishing areas. Humans rewarded them for this by giving them a portion of their catch, and being careful not to snare dolphins with their catch.