Relationships between pair formation, site fidelity and sex in a coral reef cardinalfish

Coral reef fishes are characterised by extreme site fidelity and are often socially monogamous, forming pair bonds within larger social groups. Despite this, the strong link between reproductive behaviour and site fidelity in such social species is poorly understood. We examine these relationships in the cardinalfish Ostorhinchus cyanosoma on the central Great Barrier Reef. We tagged and followed over 100 individuals for 5 weeks to investigate pair fidelity, and behavioural differences between pairs and singles and between sexes, and we experimentally tested the strength of site and mate fidelity. Tagged pairs were typically highly site attached, and lasted throughout the study period. Sex had very little effect on pairing behaviour or habitat use. Paired individuals showed three times higher site fidelity than single ones, with singles frequently relocating. There was a two-fold increase in the movement of individuals that had their partners experimentally removed. Paired individuals exhibited greater homing success, and homed regardless of whether their mate had been displaced with them or was left on the home site. These results suggest that individuals of this species form at least seasonally stable monogamous pair bonds within larger groups, and that pair formation is closely associated with site fidelity.