The ability of a shark to sense its environment through smell is its most highly developed sense, playing a central part in its survival. Sharks can detect trace amounts of chemicals in the vastness of the ocean, allowing them to locate prey from great distances. While this acute sense is typically used for tracking food, scientists have leveraged this sensitivity to identify specific chemical cues that trigger an immediate and powerful avoidance response. These aversive compounds offer a non-lethal method for deterring sharks.
How Sharks Detect Scent
A shark’s nose, unlike a human’s, is used exclusively for smelling, not breathing. Water flows into two small openings, or nares, located beneath the snout as the shark swims. Inside each nare is a highly folded structure called the olfactory rosette, which is lined with sensory cells.
The water passes over these cells, which detect dissolved chemical particles. Signals are then sent to the olfactory bulb, which in some species can make up two-thirds of the brain’s weight, reflecting the importance of this sense. This system allows some species, such as blacktip sharks, to detect fish extracts at concentrations as low as one part per 10 billion parts of seawater.
Sharks also use “stereo olfaction” to navigate toward a scent source. By detecting a slight delay in the time an odor reaches one nostril versus the other, the shark determines the direction of the chemical plume. This mechanism enables them to pinpoint a distant chemical source with remarkable speed and precision.
Scientifically Verified Aversions
The most potent and naturally occurring chemical deterrents are necromones, compounds released by decaying shark tissue. These function as an alarm signal or “smell of death” that instinctively causes other sharks to disperse. The repellent effect comes from a complex mixture of substances, including specific amino acids, organic acids, and long-chain unsaturated fatty acids.
One of the most effective necromone components is oleic acid, an unsaturated fatty acid. When released, these semiochemicals signal that a conspecific (another shark) has died nearby, indicating danger. This natural aversion response is powerful enough to cause sharks to rapidly cease feeding and flee the area, even when highly stimulated by bait.
Another class of effective chemical repellents is surfactants, like sodium dodecyl sulfate (SDS), an anionic detergent. The Moses sole fish (Pardachirus marmoratus) naturally secretes pardaxin, a toxin with surfactant qualities that repels sharks. SDS mimics this effect, acting as an irritant rather than an odor.
SDS works by attacking the cell membranes in the delicate gill filaments of the shark. This causes a massive influx of sodium ions from the surrounding seawater, which is highly distressing to the animal. Studies show that SDS is effective at concentrations as low as 36 parts per million, causing sharks to reject food and depart quickly.
Commercial and Experimental Repellents
The understanding of these aversion responses has led to the development of practical chemical repellents, focusing on necromone-based semiochemicals. Modern commercial repellents utilize synthetic or extracted versions of the compounds found in decaying shark tissue. These formulations are being developed for applications such as personal defense and the reduction of shark bycatch in commercial fishing.
When deployed, these agents create a concentrated plume that mimics the “smell of death,” causing sharks to avoid the area. The application of these semiochemicals has been shown to reduce shark bycatch by as much as 71% in some trials, indicating a high level of efficacy in a real-world setting.
While surfactants like SDS are potent, their practical use is often limited to directional application because the concentration required for a broad deterrent cloud is prohibitively high. Chemical signals are considered advantageous over physical or electrical signals because a chemical plume can travel much further in the marine environment. Ongoing research focuses on creating concentrated, stable, and environmentally safe delivery systems for these necromones to maximize their range and duration.
Limitations and Common Misconceptions
Despite decades of research, no chemical shark repellent is considered 100% effective in all situations. Effectiveness varies significantly depending on the shark species, water temperature, current, and the concentration of the chemical agent. For instance, a compound that repels a reef shark may have a diminished effect on a large pelagic species, like a great white shark.
A widespread misconception stems from the historical “Shark Chaser,” a product issued by the U.S. Navy during World War II. This repellent was a cake of copper acetate and a black dye, based on the theory that copper ions and acetic acid from decaying shark flesh were the active deterrents. However, controlled studies later determined that the copper acetate had little value in repelling aggressive shark behavior. Any minor effectiveness was likely due to the black dye providing a brief visual barrier. Many other household chemicals or simple oils similarly lack scientific validation as effective shark deterrents.