The observation of a stable pair bond in nature tends to lead to an initial presumption that the partnership is for mating. However, pairs can also form as cooperative alliances for acquiring access to resources or commodities, such as the coalitions that form between male cheetahs or greylag geese. In cases where species do not exhibit obvious sexual dimorphism and where the act of reproduction is difficult to observe, establishing whether the apparently simple phenomenon of pairing is based on mating mutualism or cooperation for other reasons can be difficult. This is especially true for aquatic organisms, where the observational challenges are extreme. In the tropical marine fishes known as rabbitfish (Siganidae), approximately half of the extant species live in schools while the other half form socially monogamous pairs. Pair formation has been assumed to be for mating, yet almost nothing is known about the reproductive ecology of the pairing species. We used remote acoustic telemetry to track the movements of one such species (Siganus doliatus) over the course of a reproductive season and, in doing so, provide the first evidence that adult paired fishes undertake highly synchronised migrations with conspecifics on a monthly cycle. All tagged individuals migrated along the same route in three consecutive months (Oct-Dec). These 2-3 day long migrations always occurred shortly after the new moon. The timing and directionality of the migrations suggests that Siganus doliatus breeds in mass aggregations, throwing into question the long-held assumption that pairing in this family of fishes is based on the benefits of monogamous mating. The use of acoustic tracking to test the evolutionary basis of pairing in organisms highlights the extraordinary diversity of behavioural and evolutionary hypotheses now testable using tracking technology.