CURRENT RESEARCH

I am especially interested in feeding and foraging (food-finding) behavior in marine mammals, especially cetaceans (whales, dolphins, and porpoises) and how it varies with demographic differences (e.g., age, sex, relatedness between animals), prey behavior and abundance, geographic location, environmental conditions, and short- and long-term disturbances (e.g. harmful algal blooms, human development, changes in climate and water conditions). I aim to conduct research with real-world applications, and communicate those research findings effectively to both technical and public audiences. 

Marine mammals increasingly face diminished fitness and increased mortality due to natural and anthropogenic (human-caused) disturbances that disrupt their access to prey. Understanding 1) how what they eat (their "diet") varies naturally under undisturbed conditions, and 2) how disturbances impact that diet is key to evaluating cetacean vulnerability and implementing responses and policies to mitigate disturbance effects. 

But, how do we figure out what they eat? Most marine mammals feed underwater, away from human view, so directly observing feeding is difficult and rarely enough to fully estimate prey consumption. Instead, multiple indirect methods exist, including analyzing hard parts (e.g., whole prey, fish ear bones, squid beaks) found in stomach contents, fecal samples, or regurgitate, using DNA metabarcoding to identify prey species in those same samples, or comparing the quantities of biochemical tracers like stable isotopes or fatty acids that transfer from prey to predators. 

My PhD research focuses on validating and applying quantitative fatty acid signature analysis (QFASA), one model-based method for estimating diet. QFASA's application in marine mammals has so far been limited to several seal and sea lion (pinniped) species, polar bears, and three cetacean species. 

What are fatty acids, and how can we use them to estimate diet? Essentially, fatty acids are chains of carbon, hydrogen, and oxygen atoms that are the building blocks of fat in an animal’s body (including in humans!).  When a marine mammal consumes prey, fatty acids from the prey are deposited into the thick layer of fat under a marine mammal's skin (their "blubber") with little structural change and remain there for weeks to months. We can then use mathematical models to compare the proportions of different fatty acids found in a marine mammal and their potential prey items and estimate the animal's average diet over that extended week-to-month time period.

My study species is the bottlenose dolphin, Tursiops truncatus, and much of my work involves a community of free-ranging dolphins that lives year-round in Sarasota Bay, Florida and has been studied since the 1970s by the Sarasota Dolphin Research Program.

I start by collecting small minimally invasive dolphin blubber samples and a representative sample of potential dolphin prey. Then, I extract the lipids (fats) from the samples, turn the fatty acids in those lipids into fatty acid methyl esters (FAMEs), and run those through a gas chromatographer with flame ionization detection. This yields a fatty acid signature, the relative proportions of different fatty acids, for each sample. Finally, I create model diets from the "library" of prey fatty acid signatures and compare them to the actual fatty acid signatures of each predator's blubber; the model diet with the signature that is most similar to the predator's represents a close approximation of that predator's diet. 

Once I have an estimate of a predator's diet from the previous several months, I can investigate all sorts of interesting research topics. For instance, how does diet vary between different sexes or ages of dolphins? Do moms and calves feed on the same prey, and does that go along with any kind of learned foraging behaviors? How does diet change following a harmful algal bloom that decimates prey fish populations, or when recreational or commercial fishing changes the kinds of prey fish available?  These are the kinds of questions that diet estimates, determined using QFASA, can help us answer. 

More broadly, with QFASA’s detailed, long-term diet information, researchers can determine diet and disturbance vulnerability in cetaceans worldwide, including endangered species that rarely strand or cannot be temporarily captured. Knowing key prey species and how disturbances affect diet also helps wildlife managers and government organization create targeted and justified responses, such as limiting certain fisheries or nutrient run-offs, with a greater chance of compliance and success.

References: 

Bowen, W.D. and Iverson, S.J. (2013). Methods of estimating marine mammal diets: A review of validation experiments and sources of bias and uncertainty. Marine Mammal Science 29(4), 719–754. https://doi.org/10.1111/j.1748-7692.2012.00604.x

Budge, S.M., Iverson, S.J., Koopman, H.N. (2006). Studying trophic ecology in marine ecosystems using fatty acids: a primer on analysis and interpretation. Marine Mammal Science 22(4), 759–801. https://doi.org/10.1111/j.1748-7692.2006.00079.x

Goetsch, C., Connors, M.G., Budge, S.M., Mitani, Y., Walker, W.A., Bromaghin, J., et al. (2018). Energy-rich mesopelagic fishes revealed as a critical prey resource for a deep-diving predator using quantitative fatty acid signature analysis. Frontiers in Marine Science 19(5). https://doi.org/10.3389/fmars.2018.00430

Iverson, S.J., Field C., Bowen, D., Blanchard, W. (2004). Quantitative fatty acid signature analysis: a new method of estimating predator diets. Ecological Monographs 74(2), 211–235. https://doi.org/10.1890/02-4105

Newsome, S.D., Clementz, M.T., Koch, P.L. (2010). Using stable isotope biogeochemistry to study marine mammal ecology. Marine Mammal Science 26(3), 509–572. https://doi.org/10.1111/j.1748-7692.2009.00354.x

Nielsen, J.M., Clare, E.L., Hayden, B., Brett, M.T., Kratina, P. (2018). Diet tracing in ecology: Method comparison and selection. Methods in Ecology and Evolution 9(2), 278–291.  https://doi.org/10.1111/2041-210X.12869

Ning, X., Gui, D., He, X., Wu, Y. (2020). Diet Shifts Explain Temporal Trends of Pollutant Levels in Indo-Pacific Humpback Dolphins (Sousa chinensis) from the Pearl River Estuary, China. Environmental Science and Technology 54(20), 13110–13120. https://doi.org/10.1021/acs.est.0c02299

Knox, T.C., Callahan, D.L., Kernaléguen, L., Baylis, A.M.M., Arnould, J.P.Y. (2019). Blubber fatty acids reveal variation in the diet of male Australian fur seals. Marine Biology 166, 117. https://doi.org/10.1007/s00227-019-3552-y

Nordstrom, C., Wilson, L.J., Iverson, S.J., Tollit, D.J. (2008). Evaluating quantitative fatty acid signature analysis (QFASA) using harbour seals Phoca vitulina richardsi in captive feeding studies. Marine Ecology Progress Series 360, 245–263. https://doi.org/10.3354/meps07378

Meynier, L., Morel, P.C.H., Chilvers, L.B., Mackenzie, D.D.S., Duignan, P.J. (2010). Quantitative fatty acid signature analysis on New Zealand sea lions: model sensitivity and diet estimates. Journal of Mammalogy 91(6), 1484–1495. https://doi.org/10.1644/09-MAMM-A-299.1

Pierce, G.J. and Boyle, P.R. (1991). A review of methods for diet analysis in piscivorous marine mammals. Oceanography and Marine Biology: An Annual Review 29, 409–486.

Remili, A., Dietz, R., Sonne, C., Iverson, S.J., Roy, D., Rosing-Asvid, A., et al. (2022). Validation of quantitative fatty acid signature analysis for estimating the diet composition of free-ranging killer whales. Scientific Reports 12, 7938. https://doi.org/10.1038/s41598-022-11660-4

Thiemann, G.W., Rode, K.D., Erlenbach, J.A., Budge, S.M., Robbins, C.T. (2022). Fatty acid profiles of feeding and fasting bears: estimating calibration coefficients, the timeframe of diet estimates, and selective mobilization during hibernation. Journal of Comparative Physiology B 192(2), 379–395. https://doi.org/10.1007/s00360-021-01414-5

Trites, A.W. and Spitz, J. (2017). “Diet,” in The Encyclopedia for Marine Mammalogy 3rd ed., eds. B. Wursiig, J.G.M. Thewissen, K.M. Kovacs. (London: Academic Press), 255–259. https://doi.org/10.1016/B978-0-12-804327-1.00007-8

Wells, R.S., McHugh, K.A., Douglas, D.C., Shippee, S., Berens McCabe, E.J., Barros, N.B., et al. (2013). Evaluation of potential protective factors against metabolic syndrome in bottlenose dolphins: feeding and activity patterns of dolphins in Sarasota Bay, Florida. Frontiers in Endocrinology 4, 139. https://doi.org/10.3389/fendo.2013.00139

Wells, R.S., 2014. Social Structure and Life History of Bottlenose Dolphins Near Sarasota Bay, Florida: Insights from Four Decades and Five Generations, in: Yamagiwa, J., Karczmarski, L. (Eds.), Primates and Cetaceans, Primatology Monographs. Springer Japan, Tokyo, pp. 149–172. https://doi.org/10.1007/978-4-431-54523-1_8 

Xie, Q., Ning, X., He, X., Deng, L., Wu, Z., Huang, B., et al. (2022). First evaluation of quantitative fatty acid signature analysis (QFASA) in dolphins. Regional Studies in Marine Science 50, 102141. https://doi.org/10.1016/j.rsma.2021.102141