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Trophic niches determined from fatty acid profiles of sympatric coral reef mesopredators

Stacy L. Bierwagen*, Heidi Pethybridge, Michelle R. Heupel, Andrew Chin, Colin A. Simpfendorfer

*Corresponding author:

ABSTRACT: Previous research has identified similar trophic levels for a wide range of coral reef sharks and large teleost fishes but has been unable to resolve the extent of dietary overlap and resource sharing that lead to interpretation of functional roles and, hence, adequately describe interaction strengths in food webs. We used fatty acid (FA) profiles of muscle and plasma of several mesopredators reflecting diet to better understand the trophic structure and functioning of Great Barrier Reef ecosystems. Multivariate analysis identified significant dietary overlap between 2 shark species (whitetip reef sharks and grey reef sharks), but whitetip reef sharks occupied a smaller niche area than grey reef sharks. We also found clear niche separation between sharks and teleost fishes (red throat emperor, coral trout and grass emperor). All mesopredators sampled had high proportions of polyunsaturated FAs linking them to carnivory, but species differences suggest distinctive dietary sources from trophic biomarker ratios; one based on pelagic diatoms for sharks and flagellates and algae for teleost fishes. However, based on a degree of niche overlap of species sharing common diet, reef-dwelling teleost fishes such as coral trout also derive nutrients from pelagic environments. A faster analytical method to determine FA profiles, proven highly useful in tuna, was tested and deemed viable for muscle tissue of tropical teleost fishes and sharks, while questions remain for use of FA extraction of plasma in ecological studies. These results demonstrate that fine scale inter-specific differences in diet exist between mesopredators in coral reef ecosystems. Our results raise important questions about the ecological influences of these species and energy flow among mesopredators in coral reef ecosystems.