• Harriet

Sharks and the Climate Crisis

What is the role of our ocean in climate change?

The ocean plays an invaluable role in solving the climate crisis, capturing between five to twelve billion tons of carbon dioxide (CO2) from the atmosphere annually. It is fueled in part by phytoplankton at the ocean surface which consume CO2 through photosynthesis. When phytoplankton die or are eaten by larger organisms, the carbon-rich fragments and faecal matter sink to the seafloor and are sequestered into the sediments. Locking carbon into ocean sediments is a key function of the oceanic carbon cycle and necessary in reducing atmospheric carbon.

But what has carbon got to do with rising sea temperature anyway? This video provides the perfect explanation.

How exactly is climate change affecting our oceans?

The ocean and its inhabitants are under attack due to overexploitation and increased levels of destructive fishing methods, but now they have a new worry. The climate crisis poses a plethora of threats including, but not limited to, rising water temperatures, ocean acidification, reduced availability of oxygen, intense marine heat waves, coral bleaching events, and the continent-wide shifting of marine life.

It was previously argued that sharks would likely be uneffected by rising CO2 levels, as modern sharks evolved in the Devonian era, when atmospheric CO2 levels were many times greater than the current day. As more and more studies take place, we are unearthing that this may not be the case.

As most Chondrichthyan's (sharks, rays, skates, and chimaera) are ectotherms, water temperature directly regulates biological and physiological processes that are crucial for survival. A rise in water temperature makes sharks more vulnerable due to their already globally threatened status, poorly characterised basic life history, and an inability to evolve fast enough to survive drastic changes due to slow generation times and low reproductive output. Scientists have already reported that in 28 species of shark, skate and chimaera as water temperature rises, incubation time decreases (more on why this appears to be bad news for sharks below).

This may seem overwhelming, but together we can stop this.

We can change these grim sounding prospects but first we need to understand them, so let’s dive in!

How do sharks affect atmospheric carbon?

Coastal predators such as sharks protect and enhance blue carbon (carbon stored in coastal and marine ecosystems) by limiting grazing on seagrass meadows and kelp forests. These ecosystems are precious not only to the plethora of species they support but also in the role they play in the absorption of carbon due to photosynthesis. For example, tiger sharks frequent shallow seagrass meadows. They prevent overgrazing in concentrated areas from turtles and other prey species, resulting in increased and more evenly distributed seagrass. The increase in meadow mass directly helps rising carbon levels, as seagrass captures carbon from the atmosphere 35 times faster than tropical rainforests.

Large fish such as shark, tuna, mackerel and swordfish are comprised of ~10 to 15% carbon. When they die naturally, their bodies act as direct carbon sinks through 'carcass deadfall'. The carbon held in their bodies is sequestered into the sediment. Put simply, if a carcass is removed from the sea its corresponding carbon is released into the atmosphere but if left to die at sea that carbon is buried. Luckily, these animals are also useful to the climate crisis when they stay alive!

Whaling and Sharks?

The global biomass of whales is less than 25% of pre-whaling times. A study by Pershing et al. shows the implications of removing large marine species on the oceans ability to store carbon. If efforts were increased to restore populations, whales could remove 8,700,000 tonnes of carbon from the environment. That's the equivalent of 110,000 hectares of forest, an area the same size as the Rocky Mountain National Park. If blue whale populations alone were restored they could sequester 3,600,000 tonnes of carbon. The same amount achieved by 43,000 hectares of forest, an area the same size as Los Angeles. But this isn’t the end of the story...

Vicki James, policy manager at Whale and Dolphin Conservation (WDC) told the BBC...

"We need to think of whaling as being a tragedy that has removed a huge organic carbon pump from the ocean that would have been having a much larger multiplying effect on phytoplankton productivity and the ocean's ability to absorb carbon."

The organic pump mentioned above refers to the whales natural ability to provide iron fertilisation to phytoplankton through its faeces. This iron helps phytoplankton blooms thrive and in doing so they have enormous influence, capturing