Relative contribution of five species of herbivorous fish to the consumption of epilithic algal matrix in the southern Great Barrier Reef

Abstract

The importance of herbivorous organisms to the ecology of coral reef systems has been well documented, with most studies concentrating on the visually conspicuous families of Pomacentridae, Scaridae, Acanthuridae and Siganidae. Recent studies have shown that the visually cryptic fish from the tribe Salariini (Blenniidae) may also be important contributors to the maintenance of coral reef ecosystems. Herbivores are thought to play a key role in suppressing the growth of algae and allowing space for new corals to settle and grow, keeping coral reef systems coral-based. The principle algal food source utilised by herbivorous organisms in coral reef systems is the epilithic algal matrix (EAM). It consists of a turf algae mat with associated detritus, meiofauna, phytoplankton, bacteria and sediment baffled among the fronds. To determine the relative contributions of representative herbivorous fishes from the above-mentioned families to the consumption of EAM on Heron Island reef, the abundance, biomass, distribution, morphology, assimilation efficiency, behaviour and grazing contribution were investigated.To address concerns of past studies investigating the importance of herbivorous blermies in coral reef systems, the numerical abundance, biomass and distribution of Salariini blenny assemblages at Heron Island, Lady Elliot and North West reefs were compared. Species composition and biomass, but not density differed significantly among the three reefs. At Heron Island reef, density and biomass of combtoothed blermies exceeded published estimates by 27% -98% for most groups of conspicuous herbivorous fish in equivalent zones indicating that inconspicuous grazers deserve closer attention. At Heron Island reef, biomass densities and mean wet mass of Ward's damselfishPomacentrus wardi and the jewelled blenny Salarias fasciatus were not significantly different, whereas total bites per day, epilithic algal matrix (EAM) per bite and total organic carbon consumed by S. fasciatus significantly exceeded those of P. wardi.Territorial behaviour differed between the two species. The damselfish P. wardi spent most of the time chasing roving grazers fi-om their territories, while largely ignoring blennies. While, S. fasciatus chased more than c. 90% of other blennies from their territories, largely ignoring (or avoiding) roving grazers. Both P. wardi and S.3 fasciatus rarely chased non-grazers. P. wardi possibly tolerates S. fasciatus grazing within its territory as the presence of the latter contributes to territory defence from other blennies.A comparison of the abundance and biomass between inner and outer reef flat zones for five species, each representative of its family, was conducted: Pomacentrus wardi (Pomacentridae), Salarius fasciatus (Blenniidae), Chlorurus sordidus (Scaridae), Siganus Uneatus (Siganidae) and Ctenochaetus striatus (Acanthuridae). The abundance of the species chosen as representatives of the five families were all relatively common, representing between 25% (C sordidus) and 75% (C. striatus) of al...