The effect of fish size, habitat type, and the deepwater horizon oil spill on red snapper, lutjanus campechanus, diet and trophic ecology in the northern gulf of Mexico

Red snapper, Lutjanus campechanus, were sampled with hook and line at natural (n = 33) and artificial (n = 27) reef sites in the northern Gulf of Mexico from 2009-2011. Stomachs (n = 708) were extracted and their contents preserved for gut content analysis, and muscle tissue samples (n = 200) were dissected and frozen for stable isotope ratio-mass spectrometry (delta 13C, delta 15N, and delta 34S) analysis. Forty-eight percent of sampled fish had identifiable prey which were grouped into 7 categories: fish, decapods, cephalopods, stomatopods, gastropods, zooplankton, and other invertebrates. A permutational multivariate analysis of variance (PERMANOVA) was computed to test the effect of fish size, habitat type, and the Deepwater Horizon Oil Spill (DHOS) on red snapper diet. Fish size (PERMANOVA, p = 0.021), and the DHOS (PERMANOVA, p = 0.001) were both significant effects in the model, but interactions between habitat type and the DHOS (PERMANOVA, p = 0.049) was also significant. Significant differences in diet among red snapper size categories were due to low trophic position prey items, such as pelagic zooplankton, being more abundant in the diet of larger (>500mm) red snapper, while red snapper in smaller size classes contained greater amounts of higher trophic level prey such decapods and fish. Although no significant difference existed in red snapper diet between habitat types, they consumed slightly higher amounts of decapods at artificial reefs (21.9% of total diet) than natural (14.8%). The significant DHOS effect was driven by an increase in fish and a decrease in zooplankton in red snapper diet following the spill; however, this effect cannot be interpreted independently given the significant interaction between habitat type and the DHOS effect. Zooplankton prey consumption declined and fish consumption increased for red snapper sampled at both artificial and natural reefs after the DHOS, but fish consumption increased to a greater extent at artificial reef sites following the spill. A significant increase in delta15N after the DHOS (ANOVA, p <0.001) also indicates red snapper sampled fed on higher trophic position prey following the spill, and lower delta34S values indicate an increase in benthic versus pelagic prey consumption following the DHOS (ANOVA, p <0.001). The ontogenetic shift in diet observed via gut content analysis was also apparent in the significant effect of fish size on delta15N (ANOVA, p = 0.015). Overall, results clearly indicate the DHOS affected red snapper diet and trophic position, and likely affected the abundance of prey resources as well. Study results also confirm the utility of stable isotope analysis to infer aspects of reef fish trophic ecology.