Sharks have undeservedly earned a bad reputation, striking fear in the hearts of many, who often describe oceans as "infested" with the creatures. But it's not technically possible for them to infest their own home. Indeed, sharks have roamed the oceans for 450 million years, producing giants like the Megalodon, the Otodus and the Ptychodus. For many ages of world history, sharks could be regarded as the unchallenged masters of the ocean.
With such robust evolutionary versatility, these cartilaginous fishes have survived not one, not two, but five mass extinctions in Earth's history. On geological timescales, nature regularly goes on killing sprees, wiping out countless species forever. But sharks have surprising resilience and have weathered the Late Devonian Extinction, which ended the "golden age" of sharks; the Permian-Triassic mass extinction that wiped out 90 percent of marine species; and even the Cretaceous-Paleogene Extinction, which famously wiped out the non-avian dinosaurs about 66 million years ago.
Despite how resilient sharks may seem, though, they may not be able to dodge the fate of the dodo or the passenger pigeon. Human activity, which seems to be driving a sixth mass extinction, is making it harder and harder for them to survive. It begs the question: Will sharks be able to outlive us?
Sharks have been getting a raw deal from humans for a long time. Almost sixty years before the classic 1975 creature feature "Jaws" scared moviegoers away from the beaches, President Woodrow Wilson infamously declared a "war on sharks" to retaliate against the species for a series of New Jersey shore attacks. Countless innocent sharks were slaughtered by hunters eager to claim state rewards.
As humans pump greenhouse gases into the atmosphere by burning fossil fuels, out-of-control heat waves are causing "weird" weather while killing thousands of humans. Sharks are also heavily impacted by these rising temperatures, as they inhabit an ocean that traps increasing quantities of heat causing sea levels to continuously rise and millennia-old ocean current patterns to shift.
"Temperature is the most pervasive climate change stressors on sharks, affecting their metabolic rates, distribution patterns and overall survival, as it influences every aspect of their physiology and behavior, from growth and reproduction to prey availability and habitat selection," Dr. Jodie L. Rummer, a James Cook University marine biology professor, told Salon. "Elevated water temperatures increase metabolic rates in sharks, necessitating higher food intake while reducing prey availability. For instance, a 3° C increase can significantly impact energy levels and recovery times of reef shark pups, making them vulnerable to predation and exhaustion."
The sharks aren't just tired from this global heating, they are also shrinking, less resembling the Hollywood "monsters" from "Jaws" and "The Meg."
"Warmer temperatures are causing sharks to grow to smaller sizes, which can limit their ability to swim long distances and migrate to new habitats as their current ones become unsuitable," Stockholm University zoology assistant professor Dr. Valentina Di Santo said. "Ocean acidification, driven by higher levels of carbon dioxide, affects the mineralization of shark skeletons. This makes their bones denser and heavier, reducing their buoyancy and increasing the energy they need to swim."
As if being punier is not bad enough, the sharks also become inferior hunters.
"Acidification also interferes with their ability to detect prey," Di Santo said. "These combined stressors not only affect individual sharks but also have broader implications for shark populations and the health of marine ecosystems."
Sharks are even vulnerable to global heating before they have been born.
"Early life stages, such as newborn and juvenile sharks, are particularly vulnerable due to their narrow temperature tolerance ranges and the dynamic conditions of their coastal nursery habitats," Rummer said. "For example, epaulette 'walking' sharks, which develop in an egg (i.e., kind of like a chicken), show a reduced incubation time (i.e., from 4 to 3 months) under warming conditions, and those neonates hatch smaller, with less yolk reserves (energy), but with higher metabolic costs than their current day temperature counterparts."
Rummer added, "Finally, being K-selected species characterized by slow growth, late maturity and low reproductive rates, makes sharks more vulnerable to these changes, as they cannot adapt quickly enough (i.e., over generations) to keep pace with the rapidly shifting environmental conditions around them."
In addition to greenhouse gas emissions, sharks are endangered by human activities such as overfishing, pollution, habitat destruction and as the unintended victims of commercial fishing, or "bycatch," in which the become tangled in commercial fishing gear, leading to injury or death.
"Sharks are heavily targeted for their fins, meat and liver oil," Rummer said. "The global shark fin trade is particularly devastating, with millions of sharks killed annually."
The animals also rely on habitats like coral reefs and mangroves that humans are destroying, all of which makes "it harder for sharks to find suitable breeding, nursery and feeding grounds," Rummer said. Finally, "chemical pollutants and plastics accumulate in sharks, causing health issues and impacting reproductive success."
Both Rummer and Di Santo agree that sharks are worth protecting because of their intrinsic value. Rummer works for The Physioshark program, which works to establish and protect marine protected areas and shark sanctuaries. Predominantly doing field work in French Polynesia, Rummer works in the world's largest shark sanctuary which "provides an ideal environment for studying and protecting shark populations without fishing pressures."
"My team’s work with newborn sharks in Mo'orea involves capturing, tagging, and studying these animals in their natural habitats," Rummer said. "These experiences deepen our understanding and commitment to shark conservation, highlighting the importance of protecting these vulnerable stages of shark life. Since we have been doing this for over a decade now, we are starting to see the babies of our first babies, which is really remarkable."
She recalled several special moments: "Capturing a newborn shark that was probably born within the hour, learning to tell how old a newborn shark is by how much their umbilical scar (i.e., their belly button) has healed, using an ultrasound on a female blacktip reef shark to confirm she’s pregnant, and of course learning from Polynesians about the rich culture surrounding sharks."
Di Santo, by contrast, has a formative shark memory with which millions can immediately identify.
"When I was about three years old, my older brother thought it would be hilarious to watch 'Jaws' with me," Di Santo said. "I was absolutely terrified— For weeks, I was convinced there was a shark lurking in our bathtub. Over the years, I’ve been lucky enough to study many different shark species. While Hollywood loves to spotlight the great whites, I’ve gravitated towards the less glamorous, but equally fascinating benthic sharks (and stingrays and skates.) These bottom-dwellers might not get the same attention, but their unique locomotor adaptations (some can walk and swim!) are endlessly intriguing to a biomechanist like me."
