A Quantitative Assessment of Ra'ui (a Traditional Approach to Marine Protected Areas) on the Fishes and Invertebrates of Rarotonga, Cook Islands
Authors:   , Miller Sonja Lee 
Published: November 2021

<p>Ra'ui (traditional marine managed areas) in Rarotonga, Cook Islands, are a form of marine protected area re-implemented in the late 1990s when communities became concerned about declining fish and invertebrate stocks in the lagoon. In this thesis I quantified the effects of Ra'ui on fish and invertebrates. First, for a single focal species of abundant reef fish (Ctenochaetus striatus), I developed a novel framework to incorporate environmental heterogeneity into a Control-Impact assessment of Ra'ui effectiveness using an index of habitat selectivity (Manly's alpha) to 'adjust' the density of a reef fish by 'preferred' substrates. My results empirically demonstrated that substrate heterogeneity impinged upon the interpretation of MPA effects. This suggests that habitat heterogeneity should be quantitatively incorporated into analyses of MPA effects to provide a more robust and defensible set of inferences. The novel framework was then used to assess the effects of Ra'ui on densities of other common reef fish and invertebrates, as well as fish functional groups. Averaged across all Ra'ui, a higher percentage of the invertebrate species (20%) had greater abundances inside Ra'ui relative to corresponding Control sites than fish species (15%). This may suggest spatial management is more effective for sessile organisms such as the invertebrates in this study. Even when environmental heterogeneity was 'controlled' in my analyses, responses to Ra'ui were variable between species and sites, illustrating that factors other than environmental heterogeneity may be driving varying patterns of species abundances. However, for the broad-scale functional groups, there were indications of inverse relationships between predator and prey functional groups at 3 of the Ra'ui. I applied meta-analytical techniques to my data to assess whether there was an island-wide effect of Ra'ui on the densities of fish and invertebrate species, and functional groups. There was no evidence in the metaanalyses for an island-wide Ra'ui effect for any species or functional groups. However, analysis of covariance suggested there was an island-wide Ra'ui effect for a number of fish and invertebrate species. The body size of fish and invertebrates is another metric of MPA effectiveness that is expected to increase with protection. However, in determining an island-wide effect of Ra'ui on size, only two fish species, Mulloidichthys flavolineatus and Stegastes nigricans, had greater mean size in Ra'ui relative to Control sites, and Siganus argenteus was the only reef fish to exhibit greater maximum size in the Ra'ui. Two of the lagoon invertebrates (Echinometra mathaei and Tripneustes gratilla) and three of the reefcrest invertebrates (Echinothrix diadema, Echinometra oblonga, and Trochus niloticus) exhibited an island-wide positive response to Ra'ui for mean size. At individual Ra'ui sites, a number of invertebrates had greater mean and maximum size. Further, at individual sites, M. flavolineatus had greater mean size across the most Ra'ui (4 sites), again indicating that the site fidelity of a species may influence its response to protection. Ontogenetic shifts in habitat use are known to occur in fish and invertebrates. For five fish species, juveniles used habitat differently to adults of the same species. Further, of the five species, only densities of both juvenile and adult Acanthurus triostegus exhibited a positive relationship with the proportion of their preferred habitat (juveniles: r2 = 0.44, p = 0.011; adults: r2 = 0.46, p = 0.009). Densities of adult Ctenochaetus striatus also increased linearly with the proportion of preferred substrate cover (r2 = 0.43, p = 0.035). These results suggest that habitat within an MPA may be limiting for different benthic stages of an organism's ontogeny. Some knowledge of the characteristics that underpin MPA effectiveness can aid in their design. Using commonly occurring fish and invertebrate species, I explored the effectiveness of individual Ra'ui using three separate metrics of effectiveness (the proportion of fish and invertebrates exhibiting 1) greater densities, 2) greater mean size, and 3) greater maximum size in Ra'ui relative to Control sites. Ra'ui area and total fish density in each Ra'ui were characteristics that best described the effectiveness of Ra'ui in enhancing mean fish and invertebrate size. The level of compliance with Ra'ui may also have some impact on Ra'ui effectiveness. This study highlights the importance of incorporating environmental heterogeneity into assessments of MPA effects. Further, temporary prohibitions such as Ra'ui may be more effective for some species if certain conditions are met e.g., compliance is good, and the species Ra'ui are protecting / enhancing have high site-fidelity and high growth rates. These findings provide important information for fisheries and conservation managers (e.g., traditional leaders, governmental agencies, non-governmental agencies, communities) that will aid in better design of monitoring programmes and facilitate improved MPA design, not only in the Pacific region, but worldwide.</p>