How stable carbon isotope analysis can help to protect marine species

By Evangeline Wilby

Stable carbon isotope analysis (CIA) is a chemical approach used to uncover the past of marine organisms, which promises to be invaluable for predicting and protecting their future (Haywood et al., 2019). The Natural History Museum has used this approach to determine the movements of a 4.5 tonne blue whale, nicknamed ‘Hope’, that was sold to the museum after the whale washed up on a sandbar in Ireland. Scientists uncovered the whale’s movements in the last 7 years of her life by analysing the ratio of stable carbon isotopes in her baleen plates, determining the pathway that led her to her unfortunate, premature death (Trueman et al., 2019). 

This method uses intrinsic markers within species to analyse their geographic range and migration routes. It is based upon the fact that carbon exists in heavy and light isotopes that have varying distributions across the ocean, represented by icescape maps. Researchers can extract keratin structures from marine organisms such as baleen from a whale, teeth from sharks or dolphins or even shell from a turtle. The carbon in the food that the organism has eaten will be incorporated into this keratin, which can then be analysed to determine which part of the ocean this individual was feeding in and when, since keratin is built up across an organism’s lifetime in layers. It acts almost like a filing cabinet, storing location at time stamps throughout the organism’s life (Haywood et al., 2019). 

So why do we care about where these individuals have been in the past? How can this help inform conservation, management strategy and policy, the crucial questions in conservation ecology and environmental science? Knowing the location of organisms highlights the important areas that need to be under protection and can provide the crucial evidence needed to propose the boundaries of marine protected areas (MPAs). This is essential to provide these species with protection from industries such as fishing, poaching and tourism (Daly et al., 2018). It is even more important to know the location of species currently, as climate change is causing many species to shift their ranges, particularly into deeper or more polar regions that are less affected by global warming in comparison to water at the surface and equator (Pinsky et al., 2020). Therefore, researchers need to know if these species are moving, where they are moving to and why in order to protect these areas and also predict other areas that the species may move into. 

Examples of studies using this approach to synthesise this evidence include the work by Haywood, to uncover spatial, foraging and reproductive ecology in 6 different species of sea turtles (Haywood et al., 2019). This research emphasises the flexibility and complexity in the life histories of these species, which could mask patterns, making conservation decisions difficult. It may therefore be useful to use multiple technologies with integrative approaches, such as CIA and satellite telemetry, as seen in studies by (Vander Zanden et al., 2015). Satellite telemetry uses a transmitter attached to an organism that will emit signals detected by a satellite every time the organism breaks the surface, collecting data on latitude, longitude and elevation (Hays et al., 2014). Due to the fact each technology has different limitations, using them in conjunction can help to provide a more accurate overview of location. Satellite telemetry is often costly and there are concerns over tags being too large, invasive and harmful to the organism as well as the fact they are dependent on battery life. Whilst CIA can provide data on a vast spatial and temporal scale, samples can become contaminated relatively easily. This could potentially be reduced by standardised protocols for tissue collection (Haywood et al., 2019). This integrative approach to research into geographic range of sea turtles is interesting because it helps to provide evidence that CIA can be used to gather the same results as satellite telemetry and in this particular study elucidated new foraging areas of female loggerhead turtles, a prime example of how these technologies can be used to find areas worth protecting (Vander Zanden et al., 2015). 

Other studies that have used CIA include locating open ocean feeding grounds in salmon. This research highlighted issues surrounding capture-based methods such as satellite telemetry, since it is biased by capture location (MacKenzie et al., 2011). Additionally, research into Hammerhead shark populations in Eastern Australia has used CIA, discussing its advantages over satellite telemetry, as this is only able to provide data for a small number of focal individuals and also take years to produce results. This research illustrated the importance of tracking movement of highly mobile migratory species with faster approaches able to collect more data to be able to inform management strategy on a faster timescale, provided the areas have predictable geographic gradients in isotope values (Ravoult et al,. 2020). 

CIA is also relevant for looking at how species location is affected by other anthropogenic pressures, such as overfishing. CIA of muscle tissue in Indo-Pacific humpback dolphins in China has shown a habitat and dietary shift likely due to increased tourism and fishing here, that has reduced the amount of higher-trophic level prey (Zhang et al., 2019). CIA can therefore be used to provide evidence for negative effects on marine populations to help inform fishing policies in relation to dolphin conservation. 

These studies both show the wide range of marine species that CIA can be used for and also the range of data it can provide evidence on for informing new government policies and conservation strategies. A lot of current research is comparing this approach to more traditional satellite telemetry methods and some are even bringing the two ideas together in unison for optimal effect. Taking this research out into new areas, particularly in developing countries will be critical in promoting sustainable use of the oceans. With institutions such as the Natural History Museum bringing this information into the public eye, it increases public knowledge and awareness, which is crucial in environmental science. Therefore, perhaps a whale that died 129 years ago really can provide ‘Hope’ for the whales and other marine species of the future, that are under constant threat from anthropogenic pressures. 

References: 

Daly, R., Smale, M. J., Singh, S., Anders, D., Shivji, M., Daly, C. A.,K., Lea, J. S. E., Sousa, L. L., Wetherbee, B. M., Fitzpatrick, R., Clarke, C. R., Sheaves, M., Barnett, A. & Wintle, B. (2018) Refuges and risks: Evaluating the benefits of an expanded MPA network for mobile apex predators. Diversity & Distributions. 24 (9), 1217-1230. Available from: doi: 10.1111/ddi.12758. 

Zhang, X., Yu, R.-Q., Lin, W., Gui, D., Sun, X., Yu, X., Guo, L., Cheng, Y., Ren, H. & Wu, Y. (2019). Stable isotope analyses reveal anthropogenically driven spatial and trophic changes to Indo-Pacific humpback dolphins in the Pearl River Estuary, China. The Science of the total environment, 651, 1029- 1037. 

Hays, G.C., Mortimer, J. A., Ierodiaconou, D. & Esteban, N. (2014) Use of Long-Distance Migration Patterns of an Endangered Species to Inform Conservation Planning for the World’s Largest Marine Protected Area. Conservation Biology; Conservation Biology. 28 (6), 1636-1644. Available 

from: doi: 10.1111/cobi.12325. 

MacKenzie, K. M., Palmer, M. R., Moore, A., Ibbotson, A. T., Beaumont, W. R. C., Poulter, D. J. S. & Trueman, C. N. (2011) Locations of marine animals revealed by carbon isotopes. Scientific Reports; Sci Rep. 1 (1), 21. Available from: doi: 10.1038/srep00021. 

Pinsky, M. L., Selden, R. L. & Kitchel, Z. J. (2020) Climate-Driven Shifts in Marine Species Ranges: Scaling from Organisms to Communities. Annual Review of Marine Science; Ann Rev Mar Sci. 12 (1), 153-179. Available from: doi: 10.1146/annurev-marine-010419-010916. 

Ravoult, V., Trueman, C. N, Kingsbury, K. M, Gillanders B. M., Broadhurst, M. K., Williamson, J.E. & Nagelkerken, I. (2020). Predicting Geographic Ranges of Marine Animal Populations Using Stable Isotopes: A Case Study of Great Hammerhead Sharks in Eastern Australia. Frontiers in Marine Science, 7. 

Haywood, J. C., Fuller, W. J., Godley, B. J., Shutler, J.D., Widdicombe, S. & Broderick, A. C. 2019. Global review and inventory: how stable isotopes are helping us understand ecology and inform conservation of marine turtles. Marine Ecology Progress Series, 613, 217-245. 

Trueman, C. N., Jackson, A. L., Chadwick, K. S., Coombs, E. J., Feyrer, L. J., Magozzi, S., Sabin, R. C. & Cooper, N. (2019) Combining simulation modelling and stable isotope analyses to reconstruct the 

last known movements of one of Nature’s giants. PeerJ (San Francisco, CA). 7 e7912. Available from: doi: 10.7717/peerj.7912. Vander Zanden, H.,B., Tucker, A. D., Hart, K. M., Lamont, M. M., Fujisaki, I., Addison, D. S., Mansfield, K. L., Phillips, K. F., Wunder, M. B., Bowen, G. J., Pajuelo, M., Bolten, A. B. & Bjorndal, K. A. (2015) Determining origin in a migratory marine vertebrate: a novel method to integrate stable isotopes and satellite tracking. Ecological Applications; Ecol Appl. 25 (2), 320-335. Available from: doi: 10.1890/14-0581.1.

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