The idea that a shark dies if it stops swimming is a common myth in marine biology. While all organisms eventually die, the necessity of constant movement applies only to a fraction of the over 500 known shark species. This widespread confusion stems from their unique physiology, often conflating the needs of fast-moving oceanic species with their bottom-dwelling relatives. Understanding the different ways these ancient cartilaginous fish breathe reveals why some must remain in constant motion while others can rest on the seafloor.
The Biological Necessity of Movement
The necessity of movement for survival is determined by a shark’s method of oxygen intake, which varies significantly across species. All sharks must have water flowing over their gills to extract oxygen, achieved through two primary mechanisms: ram ventilation and buccal pumping.
Ram ventilation is a passive method where the shark swims forward with its mouth slightly open, forcing water over the gills. This technique is highly efficient for fast-swimming, active species like the Great White, Mako, and Whale sharks, known as obligate ram ventilators. These obligate species lack the muscle structure necessary to actively pump water, meaning if they stop moving, the flow of oxygenated water ceases, leading to suffocation. The inability to stop is a direct consequence of their high metabolic demand for oxygen, which requires the constant, high-volume flow provided by ram ventilation.
Many other species, including the Nurse shark and various bottom-dwellers, use a method called buccal pumping. This active process involves using muscles in the mouth and pharynx to rhythmically open and close the mouth, drawing water in and pushing it over the gills. Buccal pumpers can remain motionless while breathing, often resting on the ocean floor or inside caves.
Some species, such as the Caribbean Reef shark or the Lemon shark, are facultative ventilators, meaning they can switch between buccal pumping when resting and ram ventilation when swimming. Small openings behind the eyes, called spiracles, in some species further aids in drawing water over the gills, even when the mouth is buried in sand. Therefore, the constant swimming requirement applies only to a minority of species.
Natural Limits to Longevity
While many sharks can stop swimming without dying, they are still subject to the natural limits of life, including old age, disease, and predation. Shark lifespans vary dramatically, ranging from a few decades for smaller species to over a century for others. The Greenland shark, for instance, is one of the longest-living vertebrates on Earth, with an estimated lifespan that can exceed 250 years.
Natural mortality occurs through predation, most often affecting younger or smaller individuals. Larger sharks may prey on smaller species, and intraspecific predation, or cannibalism, occurs between members of the same species. Marine mammals, such as Orcas, are known to hunt and kill even large Great White sharks.
Sharks generally grow slowly, mature late, and produce relatively few offspring compared to bony fish. This reproductive strategy makes their populations inherently vulnerable to any sustained increase in mortality. Disease and natural wear contribute to the death of older sharks, representing the baseline mortality that balances populations in a healthy ecosystem.
The Impact of Human Activity on Mortality
Human activity is responsible for the greatest threat to global shark populations, vastly exceeding natural mortality rates. Fishing is the primary cause of unnatural death, occurring either through targeted commercial fishing or as unintentional bycatch. Estimates suggest that tens of millions of sharks are caught annually, with a significant number dying as bycatch in fisheries targeting other species.
The demand for shark fins, primarily for use in shark fin soup, drives the wasteful practice of finning. Fins are removed and the shark’s body is discarded, often while the animal is still alive. This practice, along with the demand for shark meat, liver oil, and cartilage, places intense pressure on species worldwide.
Beyond fishing, habitat destruction and pollution also contribute to shark mortality. Coastal development can degrade or destroy important nursery grounds and pupping areas, reducing the survival rate of young sharks. Pollution, including plastic ingestion and the bioaccumulation of heavy metals and toxins from industrial runoff, weakens immune systems and affects reproductive capabilities. This combination of direct fishing pressure and indirect environmental degradation makes human activity the leading factor driving the decline of many shark species.