Functional Connectivity of Coral Reef Fishes in a Tropical Seascape Assessed by Compound-Specific Stable Isotope Analyses

Kelton McMahon, Ph.D., 2011
Simon Thorrold, Advisor

The ecological integrity of tropical habitats, including mangroves, seagrass beds and
coral reefs, is coming under increasing pressure from human activities. Many coral reef
fish species are thought to use mangroves and seagrass beds as juvenile nurseries before
migrating to coral reefs as adults. Identifying essential habitats and preserving functional
linkages among these habitats is likely necessary to promote ecosystem health and
sustainable fisheries on coral reefs. This necessitates quantitative assessment of
functional connectivity among essential habitats at the seascape level. This thesis presents
the development and first application of a method for tracking fish migration using amino
acid (AA) δ13C analysis in otoliths. In a controlled feeding experiment with fish reared on
isotopically distinct diets, we showed that essential AAs exhibited minimal trophic
fractionation between consumer and diet, providing a δ13C record of the baseline
isoscape. We explored the potential for geochemical signatures in otoliths of snapper to
act as natural tags of residency in seagrass beds, mangroves and coral reefs in the Red
Sea, Caribbean Sea and Eastern Pacific Ocean. The δ13C values of otolith essential AAs
varied as a function of habitat type and provided a better tracer of residence in juvenile
nursery habitats than conventional bulk stable isotope analyses (SIA). Using our otolith
AA SIA approach, we quantified the relative contribution of coastal wetlands and reef
habitats to Lutjanus ehrenbergii populations on coastal, shelf and oceanic coral reefs in
the Red Sea. L. ehrenbergii made significant ontogenetic migrations, traveling more than
30 km from juvenile nurseries to coral reefs and across deep open water. Coastal
wetlands were important nurseries for L. ehrenbergii; however, there was significant
plasticity in L. ehrenbergii juvenile habitat requirements. Seascape configuration played
an important role in determining the functional connectivity of L. ehrenbergii populations
in the Red Sea. The compound-specific SIA approach presented in this thesis will be
particularly valuable for tracking the movement of species and life-stages not amenable
to conventional tagging techniques. This thesis provides quantitative scientific support
for establishing realistic population connectivity models that can be used to design
effective marine reserve networks.