A Study of Red Snapper (Lutjanus Campechanus) Ecology in the Northern Gulf of Mexico and the Effect of Variable River Outflow Using Stable Isotope Analysis of the Food Web and Eye Lenses

In the Mississippi Bight and surrounding waters, river outflow impacts the basal resources of the Red Snapper food web, altering carbon sources and impacting prey and predator isotopes. In this study, the impact of riverine outflow on nutrients, particulate organic matter (POM), and physical water parameters on Red Snapper and their food web was analyzed using stable isotope and stomach content analysis over 5 years. The Mississippi, Pearl, Pascagoula, and Mobile rivers were included in the analysis of river impact. The Mississippi and Mobile rivers were found to significantly impact nutrients and POM in the region. River outflow was also broken out by high, medium, and low outflow regimes. Trends found in POM and zooplankton isotopes related to river outflow reflected the isotope values of Red Snapper and their prey, and Red Snapper body condition improved under moderate to low outflow regimes. Under these conditions diet changed enough to reflect in the isotopes of Red Snapper muscle tissue, though different diet analysis techniques did not agree on how diet changed. With increased river outflow due to climate change and more frequent and longer openings of river diversions, managers must understand how Red Snapper habitat use and dietary shifts change. Red Snapper eye lenses were compared to otoliths to determine if they could be used as an aging tool, and individual lamina of the eye lenses were analyzed for stable isotopes. Though eye lenses were not particularly useful as a primary aging tool, the successive layering of lens lamina over time was consistent enough to determine a general age. Stable isotope analysis of eye lens lamina captured ontogenetic shifts prevalent in the literature. The use of eye lenses provides the opportunity to use a single fish instead of many from different cohorts to study changes in diet and habitat use at ecologically and ontogenetically important time periods, under different outflow regimes