Cost-benefit of different methods for monitoring invasive corals on tropical rocky reefs in the southwest Atlantic

Biological invasions need to be efficiently monitored in order to detect change in invader abundance and the modification of receptor communities so that management options can be effectively applied. We compared four methods, visual census (VC), photo-quadrats of 50 × 50 cm (PHOT50), mosaic of photo-quadrats of 25 × 25 cm (PHOTS25) and video-quadrats (VIDEO), to determine 1) the percent cover of the most abundant taxa in reef communities undergoing invasion by the corals Tubastraea coccinea and Tubastraea tagusensis and 2) direct counts of the invasive species for density estimates per unit area. The study was carried out on eight islands in the Tamoios Ecological Station Marine Protected Area, Ilha Grande Bay, Brazil. The digital methods did not differentiate some T. coccinea from T. tagusensis and both Tubastraea densities were higher in the VC method, followed by PHOTS25, PHOT50 and VIDEO. An ANOSIM indicated differences among sampled communities but not between the methodologies. The richness, diversity and evenness indices did not differ significantly between the methods for the different benthic communities investigated. In the field, the VC was slower and PHOT50 was faster; however, in the laboratory VC was faster and PHOTS25 was slower. Overall the VC method was quickest, followed by PHOT50, VIDEO and PHOTS25. The overall cost was highest in PHOTS25 method, followed by the VC, VIDEO and PHOT50. VC had the best cost-to-benefit ratio and digital methods were not reliable for estimating the densities of corals.