Hákarl (kæstur hákarl) is Greenland shark that has been fermented for several weeks and then hung to dry, until it can be eaten in small cubes. It’s Iceland’s most notorious traditional food: pungent, ammonia-scented and usually served with a shot of brennivín. It isn’t something to make at home – fresh Greenland shark is poisonous, and the process depends on specialist producers – but here is how it’s made, what the science says happens to the shark, and how to try it. Greenland’s other well-known ferment, kiviaq, carries a botulism risk instead.
Why Fresh Greenland Shark Is Poisonous
The flesh of the Greenland shark (Somniosus microcephalus) is toxic to people and dogs, especially when fresh. When Danish researchers analyzed it, the only suspect compound present in large amounts was trimethylamine oxide (TMAO), which deep-sea sharks accumulate in their tissues. Their conclusion was that the poisoning is most likely acute trimethylamine (TMA) poisoning, caused when gut bacteria convert the TMAO to TMA after the meat is eaten (Anthoni et al., 1991).
How Hákarl Is Made
1. Fermenting (several weeks)
The shark is gutted and cut into large pieces. Traditionally these were buried in gravel or pressed under stones near the shore so liquid could drain away; today producers typically use draining containers. Over the following weeks, bacteria break down the flesh.
In a 2023 Icelandic study that followed the process, the shark was fermented for 7–8 weeks. TMAO fell below detectable levels within five weeks, while TMA and dimethylamine rose sharply and the pH climbed from about 6 to 9 (Jensen et al., 2023). This is an alkaline fermentation, closer to controlled decomposition than to the lactic fermentation behind sauerkraut or yogurt.
2. Drying (weeks to months)
The fermented pieces are hung in an airy drying shed, where they develop a brown crust. In the 2023 study, drying lasted five weeks, and bacterial counts fell during it. Traditional producers often dry for several months. The crust is trimmed off and the pale inside is cut into cubes.
3. What’s in the finished product
Analyses of ready-to-eat hákarl found an alkaline product (pH about 8–8.8) whose bacteria were dominated by Tissierella, Atopostipes and Pseudomonas, not lactic acid bacteria (Osimani et al., 2019; Cardinali et al., 2026). No Shiga toxin-producing E. coli was found in the samples tested. The powerful ammonia-like smell comes mainly from trimethylamine, with phenol and sulfur compounds adding to it (Cardinali et al., 2026).
Researchers suggest Pseudomonas may help break down TMAO and TMA during fermentation, but exactly how the process makes the meat safe to eat isn’t fully worked out. What producers have is centuries of practice.
Why You Shouldn’t Make It at Home
- Fresh Greenland shark is poisonous, and there’s no simple home test to show when it has become safe.
- It’s an uncontrolled, alkaline, protein-breakdown fermentation – none of the protections of a salty, acidic vegetable ferment apply.
- The process is specific to Greenland shark. Don’t apply it to other shark species or fish.
- Greenland sharks grow extremely slowly and mature very late (see below), which is a conservation concern.
If you want to try hákarl, buy it from an established Icelandic producer.
The Greenland Shark: Centuries Old
The animal is remarkable in its own right. Radiocarbon dating of the eye lenses of 28 female Greenland sharks showed they live at least 272 years; the largest, at 5 m long, was estimated at 392 ± 120 years old, making it the longest-lived vertebrate known. The same study estimated that females reach sexual maturity at no less than 156 ± 22 years, and the researchers raised concerns about the species’ conservation (Nielsen et al., 2016).
How Hákarl Is Eaten
Hákarl is served in small cubes on toothpicks, usually with a shot of brennivín, Iceland’s caraway-flavored spirit. It’s most closely tied to Þorrablót, the midwinter festival in January and February when Icelanders eat traditional preserved foods such as dried fish, soured meats and singed sheep’s head. For most Icelanders it’s an occasional festival food rather than an everyday one.
In Iceland, hákarl is sold in supermarkets and by producers. The farm at Bjarnarhöfn on the Snæfellsnes peninsula produces it and runs a shark museum where visitors can see the drying shed and taste it (Bjarnarhöfn).
What Hákarl Tastes Like
The smell arrives first: sharp and ammoniac, which is why many people eat it outdoors or breathe out before taking a bite. The flavor is milder than the smell – fishy, salty-savory and somewhat cheese-like – with a chewy texture. A small first piece and a chaser is the usual advice.
Hákarl belongs to a family of northern fermented fish that includes Sweden’s surströmming and Norway’s rakfisk. For fermented fish you can make yourself, see our fermented fish sauce guide, and for more of the world’s ferments, the 25 best fermented foods in the world.