Shark stranding, the event of a shark leaving the water and becoming stuck on land, is a rare and tragic phenomenon. For these marine predators, leaving their aquatic environment is an act of desperation, not a natural behavior. The causes are rarely simple, resulting from a complex combination of factors that incapacitate the animal. Understanding these strandings requires investigating the underlying biological and environmental stressors.
Internal Health and Biological Distress
A significant number of shark strandings result from a natural decline due to poor health. Sharks suffering from chronic illness or severe parasitic infestations become weakened and disoriented, losing the strength and coordination needed to navigate the ocean. Infections, such as bacterial meningitis, have been identified in necropsies, suggesting a neurological cause for disorientation. This inflammation can severely impair the shark’s ability to swim or sense its surroundings. High parasitic loads can drain energy reserves or cause tissue damage, making the shark susceptible to strong currents or shallow waters.
Environmental Factors and Natural Toxins
External factors, particularly natural toxins, can directly incapacitate a shark and force it ashore. The most widely documented cause involves Harmful Algal Blooms (HABs), which produce potent neurotoxins. Brevetoxins bioaccumulate through the food web and affect a shark’s central nervous system. Exposure to these toxins can cause hyper-excitability, seizures, and paralysis. A shark experiencing such neurological distress loses motor control and may be unable to avoid the shoreline.
Rapid changes in local water temperature, known as cold shock, can also lead to stranding, especially for species that migrate to avoid cold, such as thresher sharks. These animals can become lethargic and physiologically compromised if they encounter unexpectedly frigid currents, leaving them vulnerable to washing ashore.
The natural behavior of sharks when pursuing prey can also lead to accidental stranding as tides recede. Certain coastal shark species, including tiger sharks and juvenile salmon sharks, are known to follow prey into extremely shallow water or tidal pools. If a shark becomes trapped in a rapidly receding low-tide pocket, it loses the ability to move water over its gills, leading to suffocation. The physical exertion and stress of being stranded often prove fatal.
Indirect Effects of Human Interaction
Human activities introduce stress and injury to shark populations, often leading to stranding as an indirect consequence. The unintentional capture of non-target species, known as bycatch, is a major contributor to physiological stress. Sharks released alive after being caught frequently suffer from severe exhaustion and internal injury, leading to high post-release mortality rates. Species like scalloped hammerheads and dusky sharks are particularly vulnerable, with survival rates sometimes dropping below 40% following capture. This acute stress response impairs their ability to swim, navigate, and maintain homeostasis, making them prone to drifting into coastal areas.
Intense underwater noise pollution can disorient sharks by affecting their highly sensitive sensory systems. High-powered sources, such as naval sonar and seismic surveys, introduce powerful acoustic signals that can interfere with a shark’s electroreception and hearing. This acoustic trauma can cause disorientation, navigational errors, and a stress response that pushes the animal toward the shore. Chronic exposure to pollutants or the ingestion of marine debris also causes chronic illness and blockages, weakening the shark until it is unable to survive in its environment.
Post-Stranding: The Immediate Scientific Response
When a shark is discovered stranded or freshly deceased, a swift scientific response is initiated to determine the cause. Marine biologists prioritize collecting perishable data and tissue samples before decomposition obscures evidence. The primary investigative tool is the necropsy, which involves a detailed internal examination of the sharkâs organs. Scientists look for signs of infectious disease, parasites, injuries, and stomach contents to identify potential sources of poisoning or trauma. Samples are sent for laboratory analysis to detect specific agents, such as viral pathogens or neurotoxins.