Joint estimation of crown of thorns (Acanthaster planci) densities on the Great Barrier Reef.

M Aaron Macneil,Jessica Hoey,Hugh Sweatman,Camille Mellin,Kenneth R.N Anthony,Zara L Cowan,Ian Miller,Chris J Fonnesbeck,Sascha Taylor,Steven Moon,Alistair J Cheal,Morgan S Pratchett

doi:10.7717/peerj.2310

Abstract

Crown-of-thorns starfish (CoTS; Acanthaster spp.) are an outbreaking pest among many Indo-Pacific coral reefs that cause substantial ecological and economic damage. Despite ongoing CoTS research, there remain critical gaps in observing CoTS populations and accurately estimating their numbers, greatly limiting understanding of the causes and sources of CoTS outbreaks. Here we address two of these gaps by (1) estimating the detectability of adult CoTS on typical underwater visual count (UVC) surveys using covariates and (2) inter-calibrating multiple data sources to estimate CoTS densities within the Cairns sector of the Great Barrier Reef (GBR). We find that, on average, CoTS detectability is high at 0.82 [0.77, 0.87] (median highest posterior density (HPD) and [95% uncertainty intervals]), with CoTS disc width having the greatest influence on detection. Integrating this information with coincident surveys from alternative sampling programs, we estimate CoTS densities in the Cairns sector of the GBR averaged 44 [41, 48] adults per hectare in 2014.

Highlights

Outbreaking pests generate extensive environmental degradation and billions of dollars in ecosystem damage annually for resources such as crops (Oerke, 2006), forests (Aukema et al, 2011), and fisheries (Knowler & Barbier, 2000)
To account for the influence of habitat factors on detection we developed a hierarchical model using available covariates for detection including site-scale hard coral cover (HC), observer team (OT), the disc-width (DW) of individual crown-of-thorns starfish (CoTS), presence of a tag (PT), and a dummy variable for surveys conducted at night (NI), with corresponding individual, sampling-occasion, and transect-scale parameter estimates (γ1,a1,2,3,4)
Joint zero-inflated survey model Because the Field Management Program (FMP) conducts a two-part monitoring program whereby entire reefs are coarsely surveyed to detect the presence of outbreaks using manta-tows and counts of CoTS outbreaks are made by smaller-scale underwater visual count (UVC) surveys, we developed a two-part mixture model that included explicit outbreak and count components for each manta-tow section surveyed on each reef (Fig. 1D)

Summary

Introduction

Outbreaking pests generate extensive environmental degradation and billions of dollars in ecosystem damage annually for resources such as crops (Oerke, 2006), forests (Aukema et al, 2011), and fisheries (Knowler & Barbier, 2000). Among the most ecologically and economically costly are outbreaks of native crown-of-thorns starfish (CoTS; Acanthaster spp; Carter, Vanclay & Hundloe, 1988), a common coral-eating starfish that outbreaks on many Indo-Pacific coral reefs (Pratchett et al, 2014). CoTS are considered a pest due to their destructive impacts on coral populations when outbreaks occur. Of all the major disturbances to coral reefs (e.g., storms, coral bleaching, fishing, pollution, nutrients, and disease) only CoTS outbreaks have the potential to be actively locally managed without negatively impacting other resource users. There is insufficient information available about many aspects of CoTS population dynamics and life history (Pratchett et al, 2014) to accurately predict the success of management intervention

Methods

Results

Conclusion