• Amy Webber

Our Earth’s climate is changing, but what does this mean for sharks?

Climate change is a term many are now familiar with as the Earth’s surface temperature continues to rise at a rate that has doubled since the 1980s, resulting in a 1 degree Celcius increase in the global average temperature in comparison to the pre-industrial era. The major causes of this rising temperature are the anthropogenic activities that release greenhouse gases into our atmosphere, and the largest consequence is a long term shift in the planet’s weather patterns.

The Earth’s global average surface temperature has risen by nearly 1oC since the 20th century during the pre-industrial era. Graph from NOAA Climate.

The 26th UN Climate Change Conference of the Parties, commonly known as COP26, has just been hosted in Glasgow. Here, world leaders, representatives and businesses discussed climate goals to tackle the climate emergency and reduce anthropogenic impacts on the planet’s ecosystems and resources. It is widely accepted that failure to meet these climate goals will have devastating effects on our natural environment. Mass bleaching events of coral reefs, melting of ice caps and increasingly unpredictable patterns of droughts and flooding are some of the impacts that have become well recognised as consequences of a warming climate.

When we consider the oceans specifically, they are experiencing some of the greatest effects of climate change. Altered oceanic conditions have the ability to affect organisms individually, and ecosystems as a whole. This latter phenomenon has been extremely prevalent in coral reef environments. The rise in oceanic water temperatures has caused mass events of coral bleaching, a process in which coral polyps expel the live photosynthetic algae, called zooxanthellae, leaving behind a white, bare skeleton, which can eventually result in the death of the coral. A reduction in the number of live corals means less food for fish species that rely on live coral as a food source. This in turn means less food for larger predators, such as reef sharks. Coral reefs are places of delicate balance, where all inhabitants rely on each other, and so the death of coral means the death of an ecosystem.

Coral bleaching occurs as a result of higher temperatures caused by global warming. The zooxanthellae are expelled, turning a once vibrant and colourful ecosystem to a barren, white environment. Image from Coral Watch.

When we consider the direct effects of climate change on sharks and rays, there can be unintended consequences on where they choose to live. Even as far back as 2003, a study by Dr Baum revealed evidence to suggest that great white sharks were beginning to migrate polewards in response to rising oceanic temperatures in tropical and equatorial regions. Top predatory sharks are considered to be keystone species, having large impacts across the ecosystems they live in. Therefore, the change in distribution and presence of these animals can cause ecosystem imbalances, having negative consequences on a range of other species.

Another important facet to consider is that not only can sharks and rays alter where they live in response to changing temperatures, but so can their prey species. The east coast of South Africa is well known for the annual occurrence of the sardine run where countless sardines migrate to more suitable water temperatures during the winter. The Save Our Seas Foundation has been documenting this event for many years; they have seen that the sardine run often occurs later in the year or it can even fail to occur at all. Sardines are an important prey source to many shark species, for example bronze whaler sharks, so a lack, or delay, of the sardine migration may have negative impacts on them. This once again highlights the importance of understanding the sensitivity of multiple aquatic species to a warming environment.

Bronze whaler sharks feeding on a bait ball of sardines on the east coast of South Africa. Image from Wildest Animal.

The effects of warming oceans through climate change, however, are not simply limited to changes in predator-prey relations and distribution. The way in which sharks' bodies are functioning is changing too. These animals are ectotherms, otherwise known as cold-blooded, meaning their internal body temperature is regulated by the water within which they live. In a study by Dr Hight (2007), it was identified that an increase in the temperature of surface waters directly caused the core body temperature of sharks to increase. Higher core body temperatures affect the rate at which biological functions occur, most notably metabolism. Warmer waters caused a faster rate of metabolism and therefore digestion, increasing the amount of food an individual needs. The need for more food combined with the reduction of habitat and change in prey species distribution has the potential to seriously negatively affect sharks’ ability to access the necessary amount of food to survive.

There is no question as to whether the sharks residing within our ocean are threatened by climate change. The negative effects of warming surface temperature of the ocean have shown to alter the way shark species behave and their ability to survive, from migration to habitat loss and reduced food resources. Ignoring these effects will have catastrophic consequences for our oceans, an environment where balance and delicate food web relationships are fundamental to the success and survival of an ecosystem. Taking action and mitigating the increasingly harmful effects of climate change will not only save our sharks, but save our oceans.

Reference and further reading resources:

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