Updated: May 14
21 - 27th September
We all know that climate change is looming... We have been warned that in the near future, we will see melting ice caps, rising seas, worsening droughts and more extreme storms, as global temperatures rise. Amongst all these dramatic predictions, we sometimes become so wrapped up in imagining disaster-movie-style catastrophes, that we often forget that climate change will not only affect human beings. The climate emergency will affect every single species living on this planet, including every species of shark! Whatsmore, these effects might not be dramatic and sudden, but insidious... in fact, research is showing that these changes are already having an effect right now!
As atmospheric "greenhouse gases" (we often discuss carbon dioxide (CO2), but this also includes methane, nitrous oxide and fluorinated gases) are released into the air through burning fossil fuels and agriculture, they accumulate in the atmosphere in higher concentrations than normal. As a result, heat from the sun which would normally bounce back into space, is trapped within the atmosphere more than it would normally. This causes a global rise in temperature, which can melt polar ice, raise sea surface temperatures and cause more extreme weather. Furthermore, as this excess CO2 dissolves from the air into the ocean, this also causes the water to become more acidic (known as "ocean acidification").
It is predicted that by the year 2100, atmospheric CO2 will pass 1020 ppm. Consequently, average sea surface temperatures will rise by as much as 0.5 - 0.6°C globally and the water will drop into acidic levels by as much as 0.14 - 0.5 pH units (Rosa et al, 2014).
Several studies have been conducted to assess how these changing conditions are affecting sharks and how these effects might advance under the predicted future conditions.
For example, a research group studied how rising temperatures and ocean acidification altered the development of bamboo shark (Chiloscyllium punctatum) embryos (Image A). They found that, under future conditions, the growth rate of the embryos was significantly altered, which lead to poor body condition when they were born (Image B). As the neonate sharks grew towards adulthood (Image C), they suffered a 50% mortality rate (Rosa et al, 2014).
Subsequently, this group sought to understand how these conditions affected the digestion of this species. They reported that under predicted future conditions, brown banded bamboo sharks had much higher levels of certain enzymes in their gut, which seriously altered their metabolism (Rosa et al, 2016).
They concluded that future conditions will push this species beyond their limits and the scientists expressed concern that if the same is true for other species, there could be serious consequences for sharks which are already endangered. (If you would like to learn about these studies in more depth you can check out my article Climate Change... What a Pain in the Gut!)
Another research group have also studied how future conditions will affect physiology by looking at structure of the skin. They reported that, when keeping puffadder shysharks (Haploblepharus edwardsii) in warm, acidic water, the microscopic structure of their skin was affectively dissolved. This changed the hydrodynamic properties of the skin, which could affect their ability to swim and hunt, but could also mean they are more susceptible to parasites and bacterial infections (Dziergwa et al, 2019).
But it it not only shark physiology which could be changed in response to climate change, scientists have also reported that sharks behave differently when kept under future conditions! It has been shown that lesser spotted catsharks (Scyliorhinus canicula) alter their natural swimming patterns (Green & Jutfelt, 2014) and both smooth dogfish (Mustelus canis) (Dixson et al, 2014) and Port Jackson sharks (Heterodontus portusjacksoni) hunt less efficiently in acidic, warm water (Pistevos et al, 2015).
Whatsmore, there are also many indirect effects which could have an impact on sharks. Indirect effects are those which have a direct effect upon the habitat, which then has a subsequent impact on the animals that live there. For example, rising air temperatures, leading to more extreme weather events could cause irreparable damage to reef ecosystems. This would indirectly affect sharks, as it would degrade their habitats, and impact their food supply or reproductive strategies. Similarly, increasing global temperatures melting ice caps, could cause a change to the global circulation of cool and warm water (known as the "ocean conveyor belt"), which could affect shark distributions and shift their migration patterns. Also, the rising seas resulting from ice melting could hinder shark reproduction, as higher waters would flood shallow, coastal areas, like mangroves, which are critical habitats for juvenile sharks.
The problem is that we are only just beginning to understand how climate change may affect sharks and there are so many more scenarios that scientists simply have not had the time (or the funding!) to study. This means that we are going into the future somewhat blind. We can only have some idea how to protect sharks from the damaging impacts of climate change, if we understand what the impacts might be. Therefore, it has never been more critical to invest in our wonderful scientists who are studying climate change and ecology, and place our trust in their vital work! Hopefully, they can advise us how to protect our sharks, so they can survive anthropogenic climate change and be around for many, many years to come!
Dixson DL, Jennings AR, Atema J. & Munday PL (2014). Odor tracking in sharks is reduced under future ocean acidification conditions. Global Change Biology, 21, 1–9.
Dziergwa J, Singh S, Bridges CR, Kerwath SE, Enax J & Auerswald L (2019). Acid-base adjustments and first evidence of denticle corrosion caused by ocean acidification conditions in a demersal shark species. Scientific Reports, 9:1, DOI: 10.1038/s41598-019-54795-7. Access online.
Green L & Jutfelt F (2014). Elevated carbon dioxide alters the plasma composition and behaviour of a shark. Biology Letters, 10:9, doi: 10.1098/rsbl.2014.0538.
Pistevos JCA, Nagelkerken I, Rossi T, Olmos M & Connell SD (2015). Ocean acidification and global warming impair shark hunting behaviour and growth. Scientific Reports, 5:1, DOI: 10.1038/srep16293. Access online.
Rosa R, Baptista M, Lopes vm, Pegado MR, Paula JR, Tru¨benbach K, Leal MC, Calado R & Repolho T. (2014). Early-life exposure to climate change impairs tropical shark survival. Proceedings of the Royal Society B, 281:1793, DOI: 10.1098/rspb.2014.1738. Access online.
Rosa R, Pimentel M, Galan JG, Baptista M, Lopes VM, Couto A, Guerreiro M, Sampaio E, Castro J, Santos C, Calado R & Repolho T (2016). Deficit in digestive capabilities of bamboo shark early stages under climate change. Marine Biology, 163:3, DOI: 10.1007/s00227-016-2840-z. Access online.
Kempster R, Hart Nathan & Collin S (2009). Survival of the stillest: predator avoidance in shark embryos. PloS One, 8, e52551.