How Iceland’s Fermented Shark Is Made: The Ancient Process Behind Hákarl
Hákarl, Iceland’s notorious fermented shark, ranks among the world’s most polarizing foods. To outsiders it smells like industrial cleaner mixed with overripe cheese. To Icelanders it is a national dish rooted in survival, tradition, and a deep respect for the sea. The finished product is not simply “rotted” shark. It is the result of a carefully controlled, multi-month process that transforms poisonous flesh into something safe, shelf-stable, and culturally significant.
The story begins with the Greenland shark (Somniosus microcephalus), a slow-moving, deep-water species that can live for centuries and grow longer than a small car. Unlike most fish, this shark retains high levels of urea and trimethylamine oxide (TMAO) in its tissues. These compounds help the animal regulate osmotic pressure in the cold, high-pressure depths of the North Atlantic. For humans, however, the same chemicals are toxic. Fresh Greenland shark meat can cause symptoms resembling severe drunkenness or poisoning. Icelanders discovered centuries ago that a combination of pressure, drainage, time, and cold air could neutralize the toxins and create a durable food source in a land where winters were long and preservation options limited.
Catching and Preparing the Shark
Historically, Greenland sharks were caught near shore during winter months when they moved into shallower waters. Today many are still taken as by-catch or through targeted fishing by a small number of specialized operators. Once landed, the shark is immediately beheaded and gutted. The head, organs, and much of the cartilage are discarded. The remaining body is cut into large sections or slabs, often with the skin left on. The skin helps hold the soft flesh together during the long curing process.
In the traditional method, the pieces are placed in a shallow pit dug in gravelly or sandy ground, frequently close to the shoreline. The cleaned body cavity is sometimes positioned over a small mound of sand so fluids can drain more effectively. Heavy stones are piled on top to press liquid from the flesh. In some older accounts the pits were positioned so that seawater could wash over them at high tide, adding another natural rinsing effect. The shark remains buried under this weight for six to twelve weeks, depending on the season and ambient temperature. Cool conditions are essential; warmer weather risks uncontrolled putrefaction rather than the desired controlled breakdown.
The Fermentation Stage
What happens underground is not classic lactic-acid fermentation of the kind used for sauerkraut or kimchi. It is better described as a combination of enzymatic autolysis and selective microbial activity. The shark’s own enzymes begin breaking down proteins. At the same time, naturally present bacteria convert the toxic TMAO into trimethylamine and related compounds while urea is broken down into ammonia. The continuous pressure and drainage allow these fluids to escape, carrying much of the toxicity with them. Over the weeks the flesh softens, the pH rises from roughly neutral toward alkaline levels near 9, and the characteristic ammonia aroma develops.
Modern producers have refined the process for greater control and hygiene. Instead of open gravel pits, large pieces of shark are stacked in perforated wooden or plastic containers. These boxes sit in cool storage rooms so liquid can drain freely through holes in the bottom. The fermentation period is typically six to nine weeks. Some operators freeze the meat briefly before beginning the process; studies have shown that frozen and fresh meat follow slightly different microbial pathways early on but converge toward similar end results. No salt, starter cultures, or urine (a persistent tourist myth) are added. The transformation relies entirely on time, temperature, pressure, and the bacteria already present on the fish and in the environment.
Air-Drying: The Final Transformation
Once the fermentation stage is complete, the meat is removed, sometimes rinsed, and cut into manageable strips or chunks. These pieces are then hung in traditional wooden drying sheds known as hjallar. The sheds are open-sided or loosely boarded so cold, dry Icelandic wind can circulate freely while still offering protection from direct sun and rain. The drying phase lasts anywhere from two to six months, again depending on weather and the thickness of the pieces.
During drying a dark brown crust forms on the exterior. Inside, the flesh firms up, residual moisture evaporates, and remaining volatile compounds slowly dissipate. The ammonia smell becomes less aggressive, though it never disappears entirely. Fats oxidize slightly, contributing to the complex flavor profile. By the end of the process the meat is firm enough to cut into small cubes yet still retains a distinctive texture that ranges from soft and almost cheese-like in some cuts to denser and chewier in others.
Two classic styles emerge. Skyrhákarl, taken from the thicker body meat, is pale and relatively mild. Glerhákarl, from the belly, is darker, more translucent, and stronger in both flavor and texture. Producers remove the outer crust before packaging the finished cubes.
Science, Safety, and Sensory Reality
Scientific examination of the process confirms that TMAO levels drop below detectable limits within the first five weeks of fermentation. Ammonia and related amines rise, then stabilize. Bacterial communities shift over time, moving from early colonizers such as Photobacterium or Pseudoalteromonas toward later groups better adapted to the increasingly alkaline, ammonia-rich environment. The end product is microbiologically stable and safe when properly prepared. It can be stored for months without refrigeration once fully dried.
Taste remains intensely polarizing. The first impression is usually ammonia, followed by a fishy, slightly sweet, umami-rich flavor with notes that some compare to very strong blue cheese or cured fish. The aftertaste lingers. Many first-time tasters reach immediately for a shot of brennivín, the caraway-flavored Icelandic spirit traditionally served alongside hákarl. The spirit’s sharpness cuts the richness and helps the experience feel more complete. Anthony Bourdain famously called it the single worst thing he had ever eaten. Plenty of Icelanders still enjoy it, especially during the midwinter Þorrablót festival when þorramatur—a selection of traditional preserved foods—is served.
Cultural Continuity
Hákarl was once a practical necessity. In a country with limited arable land and long, dark winters, any reliable source of protein and fat was valuable. Shark liver oil had earlier commercial importance for lighting, but the flesh itself found its place on the table through this labor-intensive curing method. Today production is small-scale. A handful of families and companies, notably those connected to the Bjarnarhöfn Shark Museum on the Snæfellsnes peninsula, continue the craft. Visitors can tour the drying sheds, smell the ammonia in the air, and sample the finished product on site.
The process has changed remarkably little in its essentials. Whether the shark spends its first weeks under stones in a gravel pit or in a perforated box in a cool warehouse, the goal remains the same: convert a toxic deep-sea fish into a durable food that carries the memory of older Icelandic self-reliance. The long timeline—often half a year or more from catch to table—forces patience. There are no shortcuts that preserve both safety and the traditional character of the dish.
Hákarl will never become a mainstream delicacy outside Iceland, and that is part of its identity. It stands as a reminder that some foods exist not to please every palate but to express a particular relationship between people, climate, and the resources the ocean provides. Understanding how it is made does not make the first bite easier, but it does reveal a sophisticated, time-tested technology of preservation that turned a poisonous shark into one of Iceland’s most enduring cultural foods.
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