Response of herbivorous fishes to crown-of-thorns starfish Acanthaster planci outbreaks. II. Density and biomass of selected species of herbivorous fish and fish-habitat correlations

Abstract

Density, biomass and fish-habitat correlations were estimated for selected species of herbivorous fish on crown-of-thorns starfish (COTS) impacted and control reefs in the central Great Barrier Reef. The objective was to investigate a long-term numerical and/or biomass response to increased food availability (algal turfs) on COTS affected reefs. Out of 14 variates investigated, 2 showed evidence of a numerical response on COTS affected reefs. Importantly, total number of individuals and total biomass showed no response. Instead, a negative effect (i.e. higher abundances on control reefs) was detected for Zebrasoma scopas, although this was attributable to very high abundances on one of the non-impacted reefs (Coil). Density of Siganus corallinus was higher on COTS impacted reefs; however, this was not greater than variability over time. On COTS affected reefs, there was a negative correlation of herbivorous fish with thick turf algal abundance and a positive correlation with live coral cover, whereas on non- affected reefs, there were a similar number of positive and negative correlations with thick turf algae and live coral. Additionally, a distinct cross-shelf trend in length frequency of Acanthurus nigrofuscus was observed. Differences in food availability, wave energy and intraspecific competition were hypothesized to be important factors in this observed distribution.