Trophic ecology of the common thresher (Alopias vulpinus) and shortfin mako (Isurus oxyrinchus) from the Northeast Pacific (funded by NOAA Fisheries Service)       

The common thresher, Alopias vulpinus, and the shortfin mako, Isurus oxyrinchus, are top predators in marine pelagic food webs.  What is currently known about the diet of these two large sharks comes primarily from anecdotal observations of their stomach contents. Stomach contents provide a snapshot view of an individual’s most recent diet providing high resolution, but large year-round standardized sampling is necessary to capture temporal differences, and the large volume of unidentifiable digested matter confounds interpretations.

Stable isotopes are naturally occurring forms of an element that differ only in their number of neutrons. Stable isotope analysis can be used to infer the feeding ecology of organisms because the isotopic signature of an organism is derived from its diet and environment. Thus, stable isotopes can be used to complement and validate stomach content-based diet studies.  

In this study, Sean investigated the trophic ecology of the common thresher and shortfin mako from the Northeast Pacific by using stable isotope analysis of δ¹³C and δ¹⁵N from white muscle, liver, and vertebral centra. He compared the trophic position between the two sharks where they co-occurred, and investigated both intraspecific and interspecific ontogenetic differences in their isotopic signatures, including suspected shifts in diet. By utilizing multiple tissues with different rates of isotopic turnover, Sean also investigated localized and cumulative feeding patterns of these species.