What will climate change mean for sharks? It’s complicated.

By Rachel Skubel

When someone asks me this question – what will climate change mean for sharks – I feel the need to start off with a caveat. There are more than 500 species of sharks, and elasmobranchs (sharks, skates, and rays) bring that total to more than 1000 species. They inhabit most oceanic environments, and some are even specialized to rivers. To complement (or complicate) this biological and ecological diversity even further– climate change is having, and will continue to have, an enormously varied set of impacts around the world.

Overall, we can expect these physical changes:

• A further 2.6 – 4.8 °C warming by 2100 (IPCC, 2013)
• A further decrease in pH by 0.4 units by 2100 (IPCC, 2013)
• Decreases in dissolved oxygen, and more ‘dead spots’ of very low oxygen (IPCC, 2013)
• Changes in salinity depending on location (increases with warming, but can also decrease owing to other oceanographic factors) (IPCC, 2013)

However, all of these projections should have asterisks because of their highly variable nature – not just on a “now to 2100” scale, but on an hour to hour, day to day, week to week…. and so on, scale. Not to mention, the environment in, for example, a coastal bay, might be very different from an open ocean environment just five kilometers away – the depth is much greater, there is more circulation, and there may be larger currents which are contributing to its physical conditions.

I’ve given you the climate side of the equation, so here is the shark component:

• Most sharks’ body temperature follows that of the environment – unlike mammals, they are ectothermic and cannot generate their own body heat
• Sharks, like other animals, have an ideal range for temperature, above or below which they are not able to perform basic functions (Pörtner & Farrell, 2008) [see image below]

All animals have a certain thermal range in which they can operate aerobically (using oxygen for energy), above and below which their performance declines to the point of mortality (figure from Portner and Farrell 2008).

• We can measure this range in a lab setting very precisely by turning the temperature up and down, and measuring performance until we see drop-offs
• We can approximate this range in a field setting by tracking the animal’s movements, seeing what temperatures it is using, and to an extent its activity in those temperatures [this is what I am doing!]
• Below certain oxygen concentrations, performance declines (basically, ability to swim aerobically, and not transition to the more limited anaerobic energy system – similar when we are sprinting and our muscles being burning) but we have recorded sharks using low-oxygen areas for limited amounts of time.
• Similar to temperature, there is a certain salinity range a shark can survive in without being compromised – after all, in order to breathe, sharks must continually take in water to extract its oxygen, and this process is fine tuned to a certain amount of salt being in the water (Tunnah et al., 2016).
• Decreases in pH can actually trigger a neural response in some sharks, wherein they cannot smell as well, and not locate their prey (Pistevos et al., 2015). Some are notably resilient, like the astounding epaulette shark (Heinrich et al., 2014).

The epaulette shark has been a model of tolerance to environmental variability – owing to the extreme circumstances it has been able to cope with in its natural environment, including regular air exposure at low tide (photo: BBC One)

So, what will climate change mean for sharks? To generalize, we can follow the ‘adapt, move, or die’ narrative from Harbary et al. (2016)

• They could shift their population to follow optimal conditions (Sunday et al., 2012; Jones & Cheung, 2015).
  • If so, we have to ask whether they will find enough food in that area, and whether their habits for mating, giving birth, and growing from a pup to an adult will be able to adapt as well.

• They could be very specific to their environment, and not able to shift, so be subjected to less ideal conditions.
  • If so, they might adapt and persist, or not adapt, and if not, experience more stress. This could mean it’s harder to find enough food (Pistevos et al., 2015), they become more stressed from catch-and-release fishing because they are already stressed out from the environment not being ideal.
• All of this is not to mention, climate is currently on the backburner of shark and elasmobranch threats – overfishing is a more immediate concern (Dulvy et al., 2014). Climate change has the potential to ‘amplify’ this threat by raising the overall stress level.

You can see that we are still in the process of figuring out what climate change will ultimately mean for sharks. For my part, I’m using satellite tags which can be attached to a shark to give their position in the ocean, as well as the depth and temperature of the water they are using. This helps us detect the optimal conditions for those sharks. I’m also looking at how certain signatures in their blood change with temperature and oxygen, which can shed light on whether they make adjustments to cope with a changing environment.

Drawing blood from a nurse shark, and conducting on-board tests of blood samples (photos: Nicole Lin and Gammon Koval, sharktagging.com)

Practically, though, it is important to consider how we can share this information with decision makers, like fishery managers. If we identify a habitat that needs to be protected, or if fishing pressure should be adjusted because sharks are more likely to die in bycatch (unintentional fishery catch, after which the shark is sometimes released), how can we share this information so that it is actually used? This is a story for another post, but we are seeing promising developments such as climate vulnerability assessments (e.g. USA, International), which translate relative sensitivity and exposure of a species in a given fishery to its managers; and dynamic ocean management – using real-time or near real-time predictions of animal location to regulate fishing pressure. Importantly, these conversations are beginning now, so we can work towards productive and proactive science-decision maker-fishery relationships as our oceans continue to change.

References

Dulvy NK, Fowler SL, Musick J a et al. (2014) Extinction risk and conservation of the world’s sharks and rays. eLife, 3, 1–35.

Habary A, Johansen JL, Nay TJ, Steffensen JF, Rummer JL (2016) Adapt, move or die – how will tropical coral reef fishes cope with ocean warming? Global Change Biology, 1–12.

Heinrich DDU, Watson S-A, Rummer JL, Brandl SJ, Simpfendorfer CA, Heupel MR, Munday PL (2014) Foraging behaviour of the epaulette shark Hemiscyllium ocellatum is not affected by elevated CO2. ICES Journal of Marine Science.

IPCC (2013) Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (eds Stocker TF, Qin D, Plattner G-K, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V, Midgley PM). Cambridge, United Kingdom and New York, NY, USA, 1535 pp.

Jones MC, Cheung WWL (2015) Multi-model ensemble projections of climate change effects on global marine biodiversity. ICES Journal of Marine Science, 72, 741–752.

Pistevos JCA, Nagelkerken I, Rossi T, Olmos M, Connell SD (2015) Ocean acidification and global warming impair shark hunting behaviour and growth. Scientific Reports, 5, 16293–16293.

Pörtner H, Farrell A (2008) Physiology and Climate Change. Science, 322, 690–692.

Sunday JM, Bates AE, Dulvy NK (2012) Thermal tolerance and the global redistribution of animals. Nature Climate Change, 2, 686–690.

Tunnah L, MacKellar SRC, Barnett DA et al. (2016) Physiological responses to hypersalinity correspond to nursery ground usage in two inshore shark species (Mustelus antarcticus and Galeorhinus galeus). The Journal of experimental biology, 219, 2028–2038.

Rachel is a 2^nd year Ph.D. student at the University of Miami’s Abess Center for Ecosystem Science and Policy, where she works with the Shark Research and Conservation program lead by Dr. Neil Hammerschlag. Originally from London, ON, Rachel completed her MSc at McMaster University in Hamilton. Her PhD research is focusing on what climate change will mean for shark movements and fishing-induced stress, and how to translate this information for fishers and decision makers so they can respond to environmental change. Her studies are supported by an NSERC PGS-D scholarship. Follow her on twitter @RachelSkubel to stay in touch with ongoing research!