Interactions between herbivorous fishes and macro-algae on a temperate rocky reef

Abstract

In a survey of the herbivorous fishes at a number of rocky reef locations in Victoria, Australia, the densities, of the damselfish Parma victoriae (Gunther) and a guild of herbivorous leatherjackets (Monacanthidae) were negatively correlated with the cover of large brown macro-algae. Within sites, these fishes occurred predominantly below a continuous zone of the kelp Ecklonia radiata (J.Ag.), where they fed on a variety of red and green turfing algae. Some damselfish were found on small patches of turf where there were gaps in the kelp canopy. The only herbivorous fishes associated with the kelp were Odax cyanomelas (Richardson), which feeds directly on Ecklonia radiata, and the leatherjacket Penicipelta vittiger (Castelnau), which consumes epiphytic red algae. Direct observations of quadrats at various depths indicated that by far the greatest amount of herbivorous fish feeding occurred below the kelp zone. Experimental kelp clearances in shallow water resulted in a major increase in the feeding by leatherjackets and damselfish, the latter establishing territories on cleared patches. This corresponded with an increase in the abundance of turfing algae following canopy removal. A fine-scale examination of the feeding activity of Parma victoriae and leatherjackets below the kelp line suggested that the damselfish controlled the local use of space by the more wide-ranging leatherjackets. Experimental removal of P. victoriae resulted in a substantial increase in leatherjacket feeding. There was no difference between P. victoriae territories and adjacent areas in terms of the distribution and abundance of algae, suggesting that neither group has a major impact on the benthic flora. The results suggest that large brown macro-algae are key organisms affecting the distribution of herbivorous fishes at a number of spatial scales. Aggressive behaviour by the damselfish also affects local patterns in the dispersion of non-territorial species.