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Sharky Isotopes! What is a stable isotope? What do they have to do with sharks? - By Alina Hussey

When a shark workup takes place, a team of scientists will gather all sorts of data and samples. This might include the sex of the shark, various body measurements, and samples of blood, muscle tissue, and fin clips. But what is done with this information? How can such small tissue samples help researchers better understand these elusive underwater animals?

[Alina and the team taking samples of a juvenile lemon shark - Photo by Chelle Blais]

Chemical tracers or chemical footprints such as stable isotopes can provide insight into the diet and behaviour of sharks; this information can be derived from just a small fin clip or muscle biopsy. These tissue samples contain ratios of isotopes that change throughout the ocean - varying from the type of food eaten to the habitat where an animal occurs.

But what exactly is a stable isotope?

Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. Isotopes can be stable or unstable - when they are unstable, they will turn into another isotope or element over time. Stable isotopes remain constant, making them preferable for biological studies. Carbon and nitrogen are examples of elements that are commonly used in stable isotope analyses. The ratio of carbon and nitrogen isotopes found within an animal is what gives information on what they have been eating and where they have been eating it.

We’ve all heard of the saying “you are what you eat”, and this is true for sharks! Isotopes are found within all living things, including the fish which sharks feed upon. The bodies of sharks will therefore contain these distinct isotopes; however, isotope concentrations vary in different underwater habitats and environments.

The shark tissue samples collected by scientists contain these isotopes, which, when analyzed, can answer questions about where the shark lives and its movement patterns, what they are eating, their position in the food chain, and so on. After the sample is collected it is frozen and taken back to the laboratory where it is washed, dried, and ground into a fine powder. The processed sample is then analyzed in a machine called a mass spectrometer.

[A tiny fin clip is taken from the trailing edge of the dorsal fin of a juvenile lemon shark - Photo by Chelle Blais]

Carbon isotopes are generally good for determining foraging strategies and feeding locations, whereas nitrogen isotopes can be used to determine the trophic position of a shark (where it lies in the food web).

Why are stable isotopes important?

The waters surrounding Bimini contain a variety of habitats…

Seagrass habitats:

[Lemon shark in seagrass meadow - Photo by Chelle Blais]

Mangrove habitats:

[Lemon sharks in pristine mangrove habitat - Photo by Chelle Blais]

Seagrass and mangrove habitats are isotopically distinct. Analysis of fin clips or muscle tissue can answer questions about which of these habitats an individual shark prefers to use. Isotope data can show how different individuals of the same species may display very different foraging behaviours. Some lemon sharks show preference to feeding in the mangrove habitats, while others show preference to seagrass areas. Stable isotope data can then be used in conjunction with other data such as growth rates. For example, scientists can look at where sharks of different ages primarily reside and forage and whether this changes as they get older. Researchers can also seek to ask behavioural questions – such as if bolder sharks forage out in riskier seagrass environments compared to shy sharks that may prefer to hide in the mangroves.

This information (all from just a tiny sample!) can be used in so many ways to ask and answer a variety of different questions about sharks, further increasing our understanding of this species and the environment they live in.

Watch the video below to see how stable isotope analysis is integral in understanding shark behaviour and personality:

References and Further Reading:

Just what is an isotope? and how can we use it to unlock the secrets of deep-water sharks? Save Our Seas Foundation. we-use-it-to-unlock-the-secrets-of-deep-water-sharks/

Tool talk: Stable isotopes. ACER. isotopes/

Hussey, N. E., DiBattista, J. D., Moore, J. W., Ward, E. J., Fisk, A. T., Kessel, S., Guttridge, T. L., Feldheim, K. A., Franks, B. R., Gruber, S. H., Weideli, O, C., & Chapman, D. D. (2017). Risky business for a juvenile marine predator? Testing the influence of foraging strategies on size and growth rate under natural conditions. Processings of the Royal Society B: Biological Sciences, 284(1852), 20170166. https://doi.or/10.1098/rspb.2017.0166

Reum, J. C. P., Williams, G. D., & Harvery, C. J. (2017). Stable isotope applications for understanding shark ecology in the Northeast Pacific Ocean. Advances in Marine Biology, 149- 178.

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