Low functional redundancy and high variability in Sargassum browsing fish populations in a subtropical reef system

Establishing levels of functional redundancy in browsing fish populations among sampling periods and across spatial gradients is important in understanding coral reef functioning. We used baited video techniques to determine functional redundancy and variability in browsing herbivores within no-take marine protected areas (MPAs) and reference fished sites across two sampling periods and four reef types (scaling from high to low coral cover) in Moreton Bay, Australia (similar to 27 degrees S, 153 degrees E). We hypothesised higher herbivore abundance and browsing rates in MPAs due to protection from fishing, but lower functional redundancy in Moreton Bay generally than in tropical reefs. The function of Sargassum browsing in Moreton Bay is conducted by a single species (rabbitfish Siganus fuscescens), which is unlikely to browse at ecologically significant rates. Siganus fuscescens abundance was variable between reef types, sampling periods and individual sites, such that their abundance and browsing rates were not higher within MPAs. Similar spatial and temporal variability was found for fish community structure, indicating that other functional roles might not be influenced by MPA protection in Moreton Bay. We highlight the importance of accounting for variability in fish communities and ecosystem processes across spatial and temporal periods in considering the influence of no-take MPAs.