Effects of microhabitat availability on estimates of density of a reef fish: implications for assessments of marine protected areas

Many estimates of ‘marine protected area (MPA) effects’ may be confounded by environmental heterogeneity between MPA and ‘Control’ sites. However, the magnitude and extent of such confounding is generally unknown. Here, the effects of microhabitat availability on estimates of MPA performance were explicitly explored. Abundance of a reef fish species, Ctenochaetus striatus (Quoy & Gaimard, 1825), available microhabitat, and, microhabitat preference for C. striatus within six ‘Ra’ui’ (traditionally managed MPAs) and six paired ‘Control’ sites on the island of Rarotonga, Cook Islands, were estimated. Response ratios accounting for available microhabitat qualitatively modified inferences of Ra’ui effectiveness for two of the six Ra’ui when contrasted with response ratios not accounting for available microhabitat. However, analysis of covariance (ANCOVA) indicated that available microhabitat accounted for significant variation in C. striatus densities between Ra’ui and Control, rather than protection. Our results suggest that adjusting for microhabitat availability may significantly alter our perception of the effects of Ra’ui on C. striatus. Our framework, in concert with our ANCOVA models, provides a stronger assessment of MPA effects. Further, we conclude that metrics of environmental heterogeneity should be incorporated into future assessments of MPA effectiveness, with our work describing one potential framework to accomplish this.