Before the Alola archipelago was discovered by foreign sailors, maps often described the section of the Pacific now known as the Alolan sea as the "Sea of Bubbles" - and, when the maps were intended for use in navigation, often supplemented with clear warnings of danger. Curiously, "islands of bubbles" is also a proposed etymology for Alola's indigenous name, its modern use as a greeting a later development from the ancient warnings shouted from the coastline. For the beautiful songs of Primarina echo throughout the seas around Alola, a Sparkling Aria which makes its listeners plunge overboard to a watery grave.
Primarina do not sing because they hunt sailors as prey, despite the frequent depictions of such in ancient Alola's artwork and mythology, but to protect the lives of their young. Popplio have notoriously long childhoods and have long been hunted for meat by humans and pokemon alike, and their lopsided gender ratio further slows population growth. While even dragons shy away from confronting a fully grown Primarina, only the haunting and deadly songs of the elders of their species could protect wild Popplio from harm.
Ultimately, the Primarina could not defeat faster ships and more advanced weaponry, and only domestication could save their species from extinction in the wild. But individual Primarina are rumored to live for hundreds of years, and not every domestic pokemon spends their whole life that way. Every now and then, sailors in remote parts of Alola steer away from small rocks or unmarked islands in terror, and report back to shore that they heard Primarina's song. But no one knows what, if anything, the songs of these wild Primarina are intended to protect.