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They may have existed for hundreds of millions of years – long before trees – but sharks and rays are among the most endangered animals in the world today, largely due to overfishing and habitat loss.
Climate change adds another overall stressor to the mix. So how will sharks cope with ocean warming?
Our new research looked at Port Jackson sharks to find out. We found that individual sharks adapt in different ways, depending on where they come from.
Connor Gervais, Author provided
Port Jackson sharks from the cooler waters of the Great Australian Bight have found it harder to cope with rising temperatures than those living in the warmer waters of Jervis Bay in New South Wales.
This is important because it goes against the general assumption that species in warmer tropical waters are at greater risk of climate change. It also illustrates that we should not assume that all populations of a species respond to climate change in the same way, as this can lead to overestimating or underestimating their sensitivity.
But before we explore this further, let’s take a look at what exactly sharks will be exposed to in the coming years.
An existential threat
In Australia, the grim reality of climate change is already upon us: we are witnessing intense marine heat waves and coral bleaching events, the disappearance of entire kelp forests, the dieback of mangrove forests and the displacement of marine life. across the continent.
Southeast Australia is a hotspot of global change, with water temperatures rising three to four times the global average. In addition to rising water temperatures, the oceans are becoming more acidic and the amount of oxygen is decreasing.
Each of these factors is cause for concern, but all three could also act together.
It could be argued that sharks have been around for millions of years and have survived multiple climate catastrophes, including several global mass extinction events.
For this, let’s say that life in the anthropocene is characterized by changes in temperature and carbon dioxide levels on a scale not seen for more than three million years.
Rapid climate change poses an existential threat to all life on Earth and sharks cannot evolve fast enough to keep up because they tend to be long-lived with low reproductive production (they don’t have many pups). The time between generations is too long to respond by natural selection.
Coping with rising temperatures
When it comes to coping with rising water temperatures, sharks have two options: They can change their physiology to adapt or move towards the poles towards colder waters.
The move to colder waters is one of the most obvious responses to climate change, while the subtle impacts on physiology, as we have studied, have largely been ignored to date. However, they can have a great impact on the individual and, ultimately, on species, distribution and survival.
We collected Port Jackson sharks from cold water around Adelaide and warm water in Jervis Bay. After raising the temperatures by 3 ℃, we studied their thermal limits (how much heat the sharks could withstand before losing balance), swimming activity and their resting metabolism.
While all populations could adjust their thermal limits, their metabolic rate and swimming activity depended on where the sharks were originally collected.
With a water temperature rise of only 3 ℃, the energy needed to survive is more than double that of current daytime temperatures for Port Jackson sharks in Adelaide.
The massive shift in energy demand we observed in Adelaide sharks means they must prioritize survival (coping mechanisms) over other processes, such as growth and reproduction. This is consistent with many other shark species that are slower growing when exposed to warmer waters, including shoulder epaulette and bonnhead sharks.
On the other hand, a 3 ℃ temperature rise barely affected the energy needs of Port Jackson sharks from Jervis Bay.
Threatening the entire ecosystem
Finding out what drives heat responses is important in identifying larger patterns. For example, the reduced sensitivity of Jervis Bay sharks likely reflects the region’s spa history.
The southeastern coast of Australia is warmed by the East Australian Current, which varies in intensity throughout the year and from year to year. With each generation exposed to these naturally varying conditions, populations along this coast have likely become more heat tolerant.
Populations in the Great Australian Bight, on the other hand, do not experience such variability, which could make them more susceptible to climate change.
So why is it important? When sharks change their behavior, they affect the entire ecosystem.
The implications range from changes in fish stocks to conservation management, such as the allocation of marine reserves.
Sharks and rays generally rank at the top or center of the food chain e
they have critical ecosystem functions.
Port Jackson sharks, for example, are hedgehog predators, and hedgehogs feed on kelp forests, a rich habitat for hundreds of marine species. If the number of sharks decreases in a region and the number of urchins increases, it could lead to the loss of kelp forests.
What’s next?
There is little research devoted to understanding how individuals of different populations within species respond to climate change.
We need more of this type of research, because it can help identify hidden resilience within species and also highlight populations at greatest risk. We have seen it in action at coral bleaching events in different parts of Australia, for example.
We also need better management of how a wide range of species will respond to a changing climate. This will help us understand how communities and ecosystems could fragment, as each component of the ecosystem responds to warming in different ways and at different rates.
If we are to prepare for what follows, steps must be taken to address these gaps in our knowledge base.
Culum Brown, professor, Macquarie University and Connor Gervais, Connor Gervais
This article was republished by The Conversation under a Creative Commons license. Read the original article.
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