Decades of tagging white sharks in the eastern Pacific have taught us that large white sharks are more of an oceanic pelagic
The stable isotope composition of white sharks and their potential prey from the eastern Pacific. The stable isotope composition of white sharks (black star) is between California prey (green circle) - such as pinnipeds, small cetaceans - and offshore prey from Café-like habitats (blue square) and Hawaii (red triangle), which includes species such as tunas, squid, and whales. The relative importance of different prey can be generally estimated by how close white sharks are to the different prey (i.e. if white sharks only ate California prey they would overlie their California prey). Figure modified from Carlisle et al. 2012
Carlisle, A.B., Kim, S.L., Semmens, B.X., Madigan, D.J., Jorgensen, S.J., Perle, C.R., Anderson, S.D., Chapple, T.K., Kanive, P.E. and Block, B.A., 2012. Using stable isotope analysis to understand the migration and trophic ecology of northeastern Pacific white sharks (Carcharodon carcharias). PloS ONE, 7(2), p.e30492.[/caption]
Dr. Aaron Carlisle is marine biologist interested in the spatial, trophic, and physiological ecology of marine fishes. His work has largely focused on pelagic fishes and elasmobranchs (sharks, skates, and rays), and he employs a variety of research techniques, including chemical tracers (e.g. stable isotope analysis), electronic tagging, and modeling approaches.[/caption]
Clues In the Tissue
The stable isotope composition (the ratio of heavy to light isotopes of carbon and nitrogen) of an animal’s tissues can be used as an intrinsic marker to understand its diet. After a predator consumes and digests its prey, it assimilates the protein from the prey into its own tissues, transferring the stable isotope signature of that prey to the predator. Furthermore, if an animal migrates between regions where the stable isotope composition of their prey changes, the stable isotope composition of the migratory animal will shift over time to reflect its diet in its new habitat.
These stable isotope dynamics, combined with the the migratory pattern of white sharks, means that we can collect tissue samples from sharks in California and understand what they might be eating offshore. Prior research from Barbara Block’s lab, led by myself, used this approach and demonstrated that white sharks feed enough while offshore to incorporate a clear isotopic signature of prey from the White Shark Café and other oceanic habitats. However, our results also suggested that they fed at a much lower rate than when in coastal habitats. While this provided valuable initial insight into what white sharks were doing offshore, additional research is required to fully answer these questions, and there remains much that is unknown about how white sharks use oceanic ecosystems such as the White Shark Café.
Our Goal
One of the goals of this research cruise is to characterize the stable isotope composition of the different parts of the pelagic and mesopelagic food webs that support white sharks in the Café in order to better understand their structure and function, as well as to provide insight into how these food webs can support a large apex predator such as the white shark. We will accomplish this by collecting tissue samples from animals that we collect during our trawling, fishing, and tagging efforts, and then analyzing those samples once we are back home. This stable isotope dataset - in combination with the tissue samples that have continued to be collected from white sharks in California - will allow us to better understand how white sharks fit into these oceanic ecosystems. Sometimes being able to answer a seemingly simple question such as “What does a white shark eat in the Café?” or “How much does a white shark eat in the Café?” can provide unique and important insight into why the sharks may be out here in the first place. Luckily, because white sharks “are what they eat,” we may finally be able to answer these basic but fundamentally important questions.
