Trophic Niche Partitioning of Five Sympatric Shark Species in the Tropical Eastern Pacific Ocean Revealed by Multi-Tissue Fatty Acid Analysis

Abstract

Comparative studies investigating how species differ in their utilization of dietary resources, that is, the trophic niche differentiation, is one of the prerequisites for species coexistence. Fatty acid (FA) analysis of consumer tissues has recently shown utility in drawing further inferences about trophic niche dynamics of marine predators such as sharks. Intraspecific variability in FA profiles would be expected to reflect ontogenetic dietary changes and sexual differences related to physiological needs. In this study, we examined liver, plasma, and muscle FAs in five sympatric or coexisting species of oceanic pelagic sharks (blue ( Prionace glauca) , silky ( Carcharhinus falciformis) , bigeye thresher ( Alopias superciliosus) , pelagic thresher ( Alopias pelagicus) , and scalloped hammerhead ( Sphyrna zygaena )), inhabiting the eastern Pacific Ocean. Results showed complex inter- and intra-individual and tissue variation among the 5 shark species. Based on multivariate analysis and mixing models of the muscle FAs, P. glauca and C. falciformis were shown to occupy the largest FA niche width, indicating diverse feeding habits or habitat isolation, whereas A. pelagicus and A.superciliosus have increased trophic specialization. Percentages of fatty acid niche overlap indicated strong resource competition between C. falciformis and S.zygaena and a limited degree of dietary isolation between P. glauca and other sharks. Trophic conclusions differed based on the analysis of plasma FAs, which can be attributed to higher dietary FA turnover rates. However, high and unexplained intra-specific variance in liver FAs, made the analysis of trophic niche metrics unreliable. Overall, our multi-targeted approach reveals the magnitude of potential competitive interactions among coexisting tropical shark species and indicates that ontogenetic niche differentiation function as a major factor determining the coexistence of tropical open-ocean sharks. It also expands our understanding of inter-tissue variability and best practices when using fatty acid analysis to estimate trophic niche metrics.

Keywords: pelagic shark, fatty acid, Biomarkers, feeding ecology, coexistence mechanism, multiple tissues