They may have existed for hundreds of millions of years – long before trees – but today sharks and rays are some of the most endangered animals in the world, largely due to overfishing and habitat loss.
Climate change adds another major stress to the mix. So how do sharks cope as the ocean warms?
Our new study looked at Port acksexon sharks to find out. We have discovered individual sharks that adapt in different ways, depending on where they come from.
Port Jackson sharks from the colder waters of the Gulf of Australia struggled to cope with rising temperatures than those living in the warmer waters of Erverwiss Bay in New South Wales.
This is important because it contradicts 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 in one species respond to climate change in the same way, as this may lead to their overestimation or underestimation.
But before we explore this further, let’s look at what exactly sharks will be exposed to in the coming years.
In Australia, the grim reality of climate change is already upon us: we are witnessing intense sea heat waves and coral bleaching events, the disappearance of entire algae forests, mangrove forests, and the movement of marine life across the continent.
Southeast Australia is a hotbed of global change, with water temperatures rising three to four times above the global average. As the water temperature rises, the oceans become more acidic and the amount of oxygen decreases.
Either of these factors is a concern, but all three can work together.
One could argue that sharks have existed for millions of years and have survived multiple climate catastrophes, including several global events of mass extinction.
For this, we 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 continue because they tend to be long-lived with little reproductive output (they do not have many cubs). The time between generations is too long to respond through natural selection.
Coping with rising temperatures
When it comes to dealing with rising water temperatures, sharks have two options: they can change their physiology to adapt or move from pole to colder water.
Moving to colder waters is one of the obvious responses to climate change, while the subtle effects on physiology, as we have studied, have been largely ignored to this day. However, they can have major impacts on the individual, and ultimately on species, distribution, and survival.
We collected Port acksexon sharks from cold water around Adelaide and hot water in Yervis Bay. After raising the temperatures by 3, we studied their thermal limits (how much heat sharks can take in before they lose their balance), swimming activity, and resting metabolic rate.
While all populations were able to adjust their thermal limits, their metabolic rate and swimming activity depended on where the sharks were originally collected.
With the water temperature rising by just 3, the survival energy required is more than double the current daily temperature for the Port acksexon sharks in Adelaide.
The massive change in energy demand we have seen in Adelaide sharks means that they need to prioritize survival (coping mechanisms) over other processes, such as growth and reproduction. This is in line with several other species of sharks that grow more slowly when exposed to warmer waters, including epochal sharks and shark-headed sharks.
On the other hand, the 3 зголем temperature rise did not affect the energy requirements of the Port acksexon sharks of Jervis Bay at all.
Threatening the entire ecosystem
Finding out what triggers heat responses is important in identifying broader patterns. For example, the reduced sensitivity of sharks in Jervis Bay is likely to reflect the region’s thermal history.
Australia’s southeast coast is warmed by the East Australian Current, which varies in strength both year-round and year-on-year. With each generation exposed to these naturally changing conditions, populations along this coast have likely become more heat tolerant.
In contrast, populations in the Greater Australia do not experience such variability, which may make them more susceptible to climate change.
So why is this important? When sharks change behavior, it affects the entire ecosystem.
Implications range from fish stock shifts to conservation management, such as where marine reserves are allocated.
Sharks and rays generally rank at the top or middle of the food chain and
have critical ecosystem functions.
Port acksexons, for example, are hedgehog predators, and hedgehogs feed on algae forests – a rich habitat for hundreds of marine species. If the number of sharks in a region decreases and the number of hedgehogs increases, then this can lead to the loss of algae forests.
There is little research dedicated to understanding how individuals from 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 the most at-risk populations. We have seen this in action at coral bleaching events in various parts of Australia, for example.
We also need a better approach to how a wide range of species will respond to the changing climate. This will help us understand how communities and ecosystems can disintegrate, as each component of the ecosystem responds to warming in different ways and at different speeds.
Steps need to be taken to address these gaps in our knowledge base if we are to prepare for the next one.
Kulum Brown, Professor, Macquarie University and Conor vaerway, Conor vaerway
This article was published by Creative Commons Talk. Read the original article.