Top predators can structure communities and ecosystems through direct predation and behavioral changes induced in prey. One factor that is critical in determining a predator’s role in an ecosystem is its position in a food web. Body size is one important determinant of trophic interactions. For raptorial predators, body size usually is positively correlated with trophic level both within and among species. This relationship, however, may not be linear within large-bodied species where the largest individuals are able to consume large-bodied herbivores. For example, tiger sharks (Galeocerdo cuvier), are predicted to increase the proportion of energy they are getting from herbivorous dugongs (Dugong dugon) and green turtles (Chelonia mydas) that they consume as they grow past 3.0 m in total length.. Here, I used stable isotope analysis to investigate the relationship between body size and trophic level of tiger sharks (Galeocerdo cuvier) in Shark Bay, Western Australia. Stable isotopes provide a way to interpret community interactions using long-term trophic and diet information. Specifically, I tested whether δ15N values, which increase with increasing trophic level, were higher in smaller size classes than larger size classes. Data from fin clips taken from 138 individuals showed a somewhat weak (r2 = 0.05) but significant negative linear relationship between δ15N values and body size. These results are consistent with larger sharks feeding at lower trophic levels than smaller sharks, likely due to increasing predation or scavenging of large-bodied herbivores. There were no significant differences in δ15N values of tiger shark fin clips taken before (n=111) and after (n=27) a heat wave (2011) that resulted in catastrophic seagrass declines (up to 70% coverage loss in some sites). It is possible, however, there was not enough time between mortality event and sampling for tiger shark tissues to reflect trophic changes due to the decline. Further insights into trophic shifts of tiger sharks could be gained from compound-specific stable isotope analyses. This analysis could provide us with more precise trophic information that will give us a better understanding of the role body size plays on the trophic interactions of tiger sharks in Shark Bay.
