They may have been around for hundreds of millions of years – long before trees – but today sharks are among the most threatened animals in the world, largely because of overfishing and habitat loss. Climate change adds another overarching stressor to the mix. So how will sharks cope as the ocean heats them?
A new research looked at Port sharks to find out that individual sharks adapt in different ways, depending where they came from. Port sharks from cooler waters in the Great Australian Bight found it harder to cope with rising temperatures than those living in the warmer water from New South Wales.
This is important because it goes against the general assumption that species in warmer, tropical waters are at the greatest risk of climate change. It also illustrates that we shouldn't assume all populations in one species respond to climate change in the same way, as it can lead to over- or underestimating their sensitivity.
In Australia, the impacts of climate change are visual: marine heat waves, coral bleaching events and disappearance of mangrove forests. The southeast of Australia is global change hotspot, with water temperatures rising at three to four times the global average. In addition to rising water temperatures, oceans are becoming more acidic and the amount of oxygen is declining.
When it comes to dealing with rising water temperature, sharks have two options: they can change their bodies to adapt, or move towards the poles to cooler waters.
The researchers collected Port sharks from cold water around Adelaide and warm water in Jervis Bay. After increasing temperatures by 3oC, they studied their thermal limits (how much heat the sharks could take) and concluded that with a rise in water temperature of just 3oC, the energy required to survive is more than twice that of current day temperatures for the Port sharks. The massive shift in energy demand means they have to prioritize survival over growth and reproduction. This means sharks in Adelaide will have slower growth when exposed to warmer waters, including Bonnethead sharks and Epaulette sharks.
On the other hand, a 3oC temperature rise hardly affected the energy demands of the Port sharks from Jervis Bay at all. The explanation can only be that Australia's southeastern coastline is warmed by the East Australian Current. With each generation exposed to these naturally variable conditions, populations along this coastline have likely become more tolerant to heat.