Indirect consequences of Marine Protected Areas (MPAs) as revealed through stable isotope analysis of coral reef fishes

Background/Question/Methods

Marine Protected Areas (MPAs) have been established to combat the unprecedented declines of coral reefs and have been documented to increase the density, biomass, and size of marine fauna. MPA assessments that focus solely on direct effects, such as increased abundances of targeted species, overlook important indirect effects on food web interactions that may change as targeted species, many of which are top level predators, recover within MPAs. Such a rise in predator abundance is likely to impact the abundance, behavior, and foraging strategies of their prey and could potentially change competitive interactions among fishes at lower trophic levels. To determine if there are changes in food web dynamics as the result of protection we measured stable isotopes (δ¹⁵N & δ¹³C) of common reef prey fishes (Sparisoma aurofrenatum, Acanthurus coeruleus, Haemulon flavolineatum and H. sciurus) to quantify their diet breadth and trophic position in sites with different levels of protection (take and no take reefs) in The Florida Keys National Marine Sanctuary (FKNMS). These two isotopic niche dimensions represent each organism’s trophic role in the community. Understanding how MPAs affect these non-target prey fishes is essential to predict ecosystem consequences of MPAs and to guide future conservation efforts.

Results/Conclusions

For reef herbivores (S. aurofrenatum and A. coeruleus), δ¹⁵N values were significantly positively correlated with body size (p<0.001) and were significantly different between take and no take reefs (p<0.05). For the grunts (H. flavolineatum and H. sciurus), δ¹⁵N values were significantly positively correlated with body size (p<0.01), with the exception of H. flav at take reefs. For all species, with the exception of H. sciurus, δ¹³C values were significantly positive correlated with body size for take reefs only (p<0.005). Correlations of δ¹⁵N values with body size suggest that with ontogeny, species shift their diets towards prey of higher trophic levels. Differences in δ¹⁵N values and differences in the relationship of δ¹³C and body size between take and no take reefs suggest the potential for behavioral differences in foraging strategies and/or diet choice for these organisms. The lack of a relationship between δ¹³C and body size at no take reefs for all species with the exception of H. sciurus may indicate that diets are constrained to certain carbon sources as the result of predatory fish populations increasing inside of reserves. These preliminary results suggest that there are species-specific alterations in prey behavior in response to protection.