Effect of marine protected areas (MPAs) on consumer diet: MPA fish feed higher in the food chain

Abstract

Marine Protected Areas (MPAs) are often established to mitigate the effects of overfishing and other human disturbances. In Fiji these are locally managed and, where enforced, have significantly higher coral cover, higher fish biomass, and lower seaweed cover than in the adjacent, unprotected reefs (non-MPAs). We investigated how the isotopic signatures of a common, mid-level consumer, Epinephelus merra, differed among three small (0.5- 0.8km²) MPAs versus adjacent, unprotected reefs. Isotopic ratios suggested that the fish in the MPAs fed higher in the food chain than those in the adjacent non-MPAs, despite being slightly smaller in size. Calculations using a brown alga as representative of the basal level of the food chain estimate this difference to be about half a trophic level. Thus, the isotopic ratio of a mid-level consumer can be noticeably altered over scales of only a few hundred meters. This may result from more complete food webs and hence greater prey choice and availability in the MPAs and implies that MPAs affect not only species' abundance and diversity, but also diet composition and trophic biology of member individuals. Our findings suggest E. merra exhibits considerable site fidelity in its feeding biology and thus provides a localized isotopic signal of its reef of residence. If the isotopic signal of this mid-level carnivore is reflective of the composition of the food web beneath it, the signal might provide an easily obtained indication of reef conditions in that area.

Keywords: Nitrogen; Phaeophyte; Turbinaria conoides; carbon; grouper; isotope; trophic position.

Figure 1

Map of sampling locations on Fiji’s Coral Coast. MPAs are in grey, the fished area is in white and distances between the protected and fished collection sites are shown for each village. The pairs of numbers are the sample size for each site: the number of algal samples is below the number of fish samples.

Figure 2

Isotopic cross plot showing the relationship between algal δ¹⁵N and δ¹³C (mean± 1SE) by village and protection status. Villages are abbreviated as: Votua (VOT), Vatu-o-lailai (VLL) and Namada (NAM) with protected area (MPA) and fished area (NON) in each village. T. conoides was absent from Namada’s protected site (NAMMPA). N for each location is indicated in parentheses in the legend. Analysis by Blocked ANOVA found no significant difference in δ¹⁵N (p=0.067), while algae from the MPA were significantly enriched in ¹³C (p<0.001).

Figure 3

Isotopic cross plot showing the relationship between fish δ¹⁵N and δ¹³C (mean± 1SE) by village and protection status. Symbols, analyses and site abbreviations as in Figure 2; analysis by Blocked ANOVA found fish from the MPA were significantly enriched in ¹³C (p<0.001) and ¹⁵N (p<0.001).

Figure 4

Mean (± 1SE) fish total length and mean δ¹⁵N in each site; N is shown in parentheses at the base of each bar. The dashed columns are the protected sites in each village; site abbreviations as in Figure 2. Analysis by blocked ANOVA found individuals from the MPA were significantly smaller (p=0.036) and significantly enriched in ¹⁵N (p<0.001).