Trophic ecology of red snapper Lutjanus campechanus on natural and artificial reefs: interactions between annual variability, habitat, and ontogeny

In the Gulf of Mexico (GOM), oil and gas platforms have created an expansive network of artificial reefs. Generally, policies mandate removal of these structures post-production; however, many enter 'Rigs-to-Reefs' (RTR) programs that convert the rig materials into artificial reefs ('reefing'). Despite the growth of RTR programs worldwide, the functionality of the resulting habitats remains uncertain, particularly due to the lack of comparative studies with natural systems. To address this data gap as it relates to trophic ecology, we compared annual, ontogenetic, and habitat-specific diet and stable isotope signatures (delta C-13 and delta N-1(5)) of adult red snapper Lutjanus campechanus (n = 1585) from relic coralgal natural reefs to those of fish from standing and reefed platforms located in similar depth strata of the northwestern GOM. Stomach content analyses revealed significant effects of year, habitat, and total length on prey composition. Subsequent analyses of stable isotope values by size class identified a non-linear relationship with ontogeny. delta C-13 and delta N-15 values at reefed platforms and natural reefs decreased in the medium size class (401-600 mm total length), whereas fish from standing platforms exhibited more consistent feeding patterns across ontogeny. Annual variability was also observed in delta C-13 and delta N-15 values, with 2013 and 2014 significantly different from 2015. These findings suggest that the trophic impacts of habitat type on reef fishes are more complex than previously considered in the GOM and that reefed platforms provide foraging opportunities more similar to natural reefs than standing platforms.