Climate change is a hot-topic in the news media in recent years and rightly so! But, whilst we often hear about dramatic and disastrous impacts climate change may have on a global scale (such as sea level rise, desertification and extreme weather events), it can be easy to overlook how climate change may affect species on an individual scale... So does climate change actually affect individual sharks? Could it really have an impact on a particular species? And if so, how!?
Brave New World
In the past, we used to use the term "global warming" to describe how air temperatures around the world were rising as a result of greenhouse gas emissions. However, as we now know that this warming is also associated with a whole suite of other changes - ocean deoxygenation, rising sea levels, ocean acidification and more extreme weather events, to name but a few - we now use the term "climate change" to encapsulate all the effects that human activity is having on the natural conditions of our planet.
As we are only just beginning to understand how climate change might affect the world around us, there is still a lot we do not know about how it may impact sharks and their relatives. Yet, even with only a few studies having been conducted so far, there is already evidence that climate change will be damaging for sharks (Rodda, 2000; Izzo & Gillanders, 2020; Pistevos et al, 2015).
It's Getting Hot in Here!
Scientists monitoring Port Jackson sharks (Heterodontus portusjacksoni) in Australia, have discovered that their growth is already being affected by climate change (Izzo & Gillanders, 2020).
Over a 14 year period (between 1996 - 2010), scientists noticed that the sharks in the Gulf of St Vincent in South Australia, were growing noticeably more slowly than they should. Sadly, they learned that this decline was directly related to the rising sea surface temperatures in the region (Izzo & Gillanders, 2020).
These findings are consistent with studies of Port Jackson sharks in captivity, which have been kept in conditions that mirror those predicted to arise because of climate change. But now we know that these impacts are happening right now out in the wild (Rodda, 2000; Pistevos et al, 2015).
This pattern has been recorded in other species of fish and it is thought it happens because growth is reduced as the oceans become warmer (Izzo & Gillanders, 2020).
A few species of sharks are able to keep their body temperature higher than that of the surrounding water (to learn more you can check out Sharks Packing Heat). However, like most sharks, Port Jackson sharks are "poikilothermic" and so their body temperature is equal to that of their surroundings. If Port Jackson sharks find themselves in an area where the temperature is suboptimal - too hot or too cold - they will move into waters that are a more comfortable temperature. This is known as "behavioural thermoregulation" (Izzo & Gillanders, 2020).
However, Port Jackson sharks also have a very strong drive to return to a very specific location to breed. This is known as "philopatry". The researchers in South Australia hypothesised that the Port Jackson sharks were returning to their breeding grounds even though they were now too warm to be optimal for them. As a result, as they were no longer actively regulating their external environment and persisted in suboptimal temperatures, their growth was impaired (Izzo & Gillanders, 2020).
What this means is that, as temperatures continue to rise, Port Jackson sharks will continue to grow more slowly and therefore it will take longer to reach sexual maturity; delaying how much they can breed. Over time, it is very likely that this will cause their population sizes to decrease (Izzo & Gillanders, 2020).
I Just Want to Go Home
Port Jackson sharks are so named because their limited range in temperate waters of southern Australia, means they are often sighted in waters surrounding the town of Port Jackson. The problem is that, because these sharks live in this small area, there is concern that their habitats will constrict as a result of climate change - they basically won't have anywhere to live (Izzo & Gillanders, 2020).
Comparatively, species of shark which are migratory and/or "pelagic" (aka live in the open ocean) will be more freely able to move to find more favourable conditions in response to climate change (Izzo & Gillanders, 2020).
This highlights how the impacts that climate change may have on one species cannot be carried over to predict how they may affect another species. As extrapolation is impossible, it is vital that scientists continue to investigate how different species of sharks may be affected by climate change on an individual level.
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. Access online.
Rodda K (2000). Development in the Port Jackson Shark Embryo. Adelaide, SA: University of Adelaide. Access online.