Macroalgal resource use differences across age and size classes in the dominant temperate herbivorous fish Aplodactylus lophodon (Aplodactylidae)

Herbivorous fishes comprise a substantial proportion of temperate fish communities, although there is little understanding of their trophic resource use and whether this changes throughout post-settlement ontogeny. With increasing loss of macroalgal forests, understanding how temperate fishes use macroalgae will be vital in predicting future effects on temperate fish biodiversity. The Australian rock cale (Aplodactylus lophodon) is one of the most abundant herbivorous fish inhabiting shallow temperate south-eastern Australian reefs. We examined gastrointestinal contents throughout ontogeny and demonstrated that this species maintains a herbivorous diet through all life stages. Differences in algal taxa consumed were apparent through ontogeny, with the juvenile diet dominated by filamentous red and green algae and the adult diet dominated by brown and calcareous red algae. Relative gut length increased through ontogeny, potentially facilitating dietary transition to less digestible algae, but no concurrent increase in jaw power was observed. The results highlight the diversity of trophic resource use in a temperate marine herbivore, but the near-complete dominance of dietary algae throughout ontogeny indicates the reliance on primary producers across all life stages. Given the importance of fucoid resources in the adult diet, any loss of macroalgal forests within south-eastern Australia may affect foraging success and persistence.