This July marks the 30th anniversary of Shark Week— Discovery Channel's wildly popular television event featuring eight straight days of all things shark. Millions of viewers tune in to watch. And it's no wonder: sharks are fascinating. They've been around for millions of years and there are more than 500 species of all sizes and shapes—from great whites rocketing out of the water chasing prey to camouflaged wobbegongs lurking on the ocean floor. Sharks play important roles in ocean food chains and help keep ocean ecosystems in balance. One thing you might not know about sharks is that, like humans, they need oxygen to stay alive. And, like humans, what they breathe is under threat from the same, growing concern.
How Sharks Breathe
Sharks don't have lungs, but they do have to breathe oxygen to survive. Instead of breathing air, though, sharks get oxygen from the water that surrounds them. The concentration of oxygen in water is much lower than in air, so animals like sharks have developed ways to harvest as much oxygen as they can. The breathing process for sharks begins and ends with their gills, which they use to both extract oxygen from water and rid their bodies of carbon dioxide.
As water passes over the gills, small capillaries allow oxygen to enter the bloodstream.
The oxygenated blood is then pumped throughout the shark’s body.
The blood then enters the heart and is pumped to the gills, where carbon dioxide is released and the process begins again.
Most sharks get water to flow over their gills by swimming and moving through water, while some sharks will hold water in their cheeks and pump it over their gills—allowing them to breathe while resting on the ocean bottom.
The High and Low
Oxygen levels vary depending on depth: higher at the surface of the ocean and lower in deeper water.
Some sharks, especially bigger and more active sharks, require more oxygen than others, and some sharks are actually able to adapt to low oxygen conditions. "While all sharks (and their relatives) require a certain amount of oxygen to survive—and higher levels are better—some species can tolerate low levels of oxygen for prolonged periods of time," says Rachel Skubel, Ph.D., student at the Abess Center for Ecosystem Science and Policy and researcher at the Shark Research & Conservation Program at University of Miami. For example, the epaulette shark is regularly exposed to low or no oxygen in its shallow tidal habitat and tolerates it by lowering its energy (and thus oxygen, which produces energy) demand. There are instances of other sharks, such as the shortfin mako, that make deep dives into low-oxygen areas, likely in search of food, says Skubel. But sharks that are able to use these tactics (i.e., energy production in the absence of oxygen) have to recover from these efforts—just like humans would recover from running fast sprints!
Declining Oxygen in Our Oceans
Still, sharks depend on oxygen-rich water to keep them alive, along with all marine life. Declining oxygen levels in the world's oceans are a threat for sea creatures and the habitats in which they exist. A study published last year shows that oxygen levels have been declining for more than 20 years—faster than anticipated. Areas with low levels of oxygen are expanding, causing fish, shrimp and other organisms to flee or die, and their feeding habits to change.
What does it mean for sharks?
"Low oxygen can reduce abundance of prey species," explains Skubel. "If there is less food available, sharks might move to other areas (if they can)." She also points out that a change in the depth of oxygen minimum zones could impact sharks. "The higher-oxygen surface waters are important for active species of sharks and their prey, so a reduction in 'vertical habitat' could restrict their access not only to food, but also to cooler deeper waters to regulate their body temperature."
Oxygen naturally fluctuates in marine waters, but the decline in oxygen levels is happening faster than predicted. Warmer oceans may be to blame. Scientists point to carbon pollution and climate change as a likely driver of this trend. Carbon pollution—carbon dioxide and other gases emitted from cars, factories, electricity production and agriculture—is one of the biggest contributors to climate change. These gases get trapped near the earth's surface creating warmer temperatures. As carbon pollution warms the earth, oxygen in the oceans drops because warmer water holds less oxygen.
What's more, the oceans naturally absorb carbon dioxide (which actually helps stabilize the earth's climate). But as carbon pollution increases, it increases the acidity of seawater (called ocean acidification). Skubel points out there is potential for acidification to alter a shark's sense of smell and ability to track prey. A 2015 study from the University of Adelaide showed "warmer waters and ocean acidification will have major detrimental effects on sharks' ability to meet their energy demands, with the effects likely to cascade through entire ecosystems."
On Dry Land
Of course, climate change doesn’t just affect the oxygen sharks rely on—it also threatens the air humans breathe. The same warmer temperatures that are concerning for sharks and marine life underwater increase dangers for humans, too, contributing to high-ozone days and wildfires with their pervasive smoke. The added smog and soot increases health risks such as asthma attacks, heart disease and even early death. That's why it's so important to reduce the carbon pollution that causes climate change. Land or water, it impacts all life on this planet.
So, what can you do? Right now, you can join the fight for healthy air. Lifesaving clean air and climate protections are under attack. For example, transportation is the nation's number one source of carbon pollution, but the Trump Administration has signaled that they plan to roll back cleaner cars standards. These standards are strongly supported by the public and they're working to reduce carbon pollution. Other policy issues include a proposal to censor science, riders that give breaks to industrial polluters and threats to the Clean Air Act. You can help us speak up, spread the word and take action for healthy air by signing and sharing our petitions and letters to policymakers.