Previous work on the trophic levels of sharks has primarily used stomach content analysis to assess where sharks sit in the food web. Assessing trophic position by examining stable isotope ratios is a less invasive method than stomach content analysis.
Sharks are apex predators and play a pivotel role in marine ecosystems. Detailed understand of where apex predators sit in the food web, in relation to each other, gives us a better understanding of the top down effect they may have on an ecosystem.
The five species included in this study were the blue shark (Prionace glauca), bigeye thresher (Alopias superciliosus), silky shark (Carcharhinus falciformis), scalloped hammerhead (Sphyrna lewini) and oceanic whitetip (Carcharhinus longimanus). These five species are all caught by the Chinese longline fleet in the mid-east Pacific.
Stable isotope analysis (SIA) can be used to assess a number of mechanisms in the food chain. By examining δ15N isotopes in organisms, researchers can make an estimate of the position that organisms inhabits in the food web. δ15N isotopes move through the food chain as organisms consume each other. Therefore organisms residing in a higher position in the food web will have higher δ15N values than those further down.
The researchers also examined δ13C isotopes; these isotopes originate from the primary producer and remain relatively unchanged as they make their way through the food web; they can be used to give an indication of the source of primary production supporting the entire food web.
From these analyses, the team discovered that A. superciliosus occupies the highest trophic position in the food web; possibly owing to the fact that this species consumes more cephalopods than the others - cephalopods have higher δ15N and δ13C values than fish prey. The team also discovered that A. superciliosus had the largest variation of δ15N - possibly owing to its highly migratory lifestyle.
C. longimanus occupied the lowest trophic position in the food web.
Li, Yunkai , Gong, Yi , Chen, Xinjun , Dai, Xiaojie and Zhu, Jiangfeng. Trophic ecology of sharks in the mid-east Pacific ocean inferred from stable isotopes. Journal of Ocean University of China 13(2):278-282
