Aji Sushi
A Comprehensive Overview of Jack Mackerel or Saurel in Japanese Sushi Cuisine

Aji アジ is the Japanese generic term for a group of fish from the spiny mackerel subfamily, which includes many species. In a gastronomic context, the term aji is usually associated with maaji, the Japanese horse mackerel (Trachurus japonicus). Although the term aji can be applied to any member of the Carangidae family that has the ending aji, this is not a common convention in general usage or in gastronomy. In gastronomy, aji typically refers to maaji. Historically, the term aji dates back to the Edo period. To distinguish maaji from other types such as maruaji or muroaji, modifiers such as ma 真 (genuine) or hon 本 (original) were added. This nomenclature emphasizes the maaji as an essential member of the aji family and points to its importance and frequent occurrence.^[1]

Aji is one of the most important representatives of sushi ingredients prepared with the shiny silver skin (hikarimono). The meat of aji is pink and firm. It is also juicy, rich in fat and has a strong characteristic taste. Like other silver-skinned fish, aji is particularly tasty after salting and marinating in vinegar (sujime). Marinating not only preserves and absorbs strong flavors, but also serves to enrich aromas and reduce the “fishy” taste. Contrary to popular belief, soaking in a salt or vinegar solution does not sufficiently kill any parasites that may be present.

In high scale sushi restaurants, such as the famous Sukiyabashi Jiro in Tokyo by Jiro Ono, untreated raw aji is also preferred. According to Mr. Ono's statement, untreated raw aji is not only more difficult to process, but also makes higher demands on quality and freshness than if aji was marinated. It should be noted that the meat of aji can spoil very quickly due to its high enzyme content.^[2]

Usually aji nigiri sushi is garnished with freshly grated ginger (oroshi shōga) and finely chopped spring onion (negi), together this creates an amazingly successful combination of flavors and is more suitable than Japanese horseradish (wasabi). This combination blends wonderfully with vinegared sushi rice. Soy sauce should only be used sparingly for glazing. The meat of aji is rich in omega-3 fatty acids and vitamins B2, B3, B6 and B12.

The optimal season for catching horse mackerel, known as aji in Japanese cuisine, varies significantly between the Pacific and Atlantic oceans. In the Northeast Atlantic, Trachurus trachurus generally reaches peak fat content during the summer months, from August to October, with some evidence of a secondary peak in January along the Portuguese coast.

Mediterranean populations show more variation, with the highest fat in late winter to spring (peaking around March–April). Fat content fluctuates more dramatically in the Mediterranean species, which spawn in summer. For example, one study found muscle fat ranged from near-zero in late summer up to a few percent in spring.

For Pacific species, Trachurus japonicus in the Northwest Pacific tends to have higher fat content in summer and fall, while the limited data on Trachurus murphyi in the Southeast Pacific suggests potential peaks in approximately August–October. Japanese fishermen also recognize summer to early fall as the season of “oily” aji ideal for sushi. By late fall, the fish are still fairly rich, before winter scarcity sets in.

The scientific genus name Trachurus comes from the Greek and means “rough tail” (from trachys for rough and oura for tail). This refers to the rough skin near the caudal fin peduncle of these fish.

Aji is known in English as Atlantic horse mackerel, a name derived from the old Dutch word Horsmakreel. The term refers to mackerel that spawn in shallow sea regions or near the coast, known as hors. The term was adapted by the English as “horse mackerel”, and in Norway the fish is known as heste makrel, heste being the Norwegian word for horse. In the past two decades, the name Paardenmakreel has appeared on the Dutch fish market, a retranslation of the English “horse” into the Dutch paard. It is still the same species of fish. A similar linguistic change can also be found in Portuguese. Another historical Dutch name for the horse mackerel is Maasbanker, which describes spawning in shallow areas of the North Sea, particularly near the River Maas. This name changed to Maasbangu in the Caribbean, but remained in Africa, especially in South Africa, and in Asia as Maasbanker. This name development reflects the influential role of the Netherlands in fishing and trade in the 16th and 17th centuries.^[3]

In Japan, aji is an important ingredient in many dishes. Besides being used for sushi or sashimi, horse mackerel are often grilled, simmered, or fried. The fact that aji is highly appreciated as an ingredient in Japan is reflected by the fact that the Japanese kanji 味 (あじ) for “taste” or “flavor” is also read as aji in Japanese. 

In Japan, Japanese horse mackerel are called seki aji if their fishing area is in the Strait of Bungo 豊後水道. Only specimens caught off the coasts of Saganoseki City are allowed to be called seki aji. The meat is characterized by its balanced amount of fat and firm flesh. They are particularly popular in upscale sushi restaurants and are traded at correspondingly high prices.

Many species categorized by the Japanese as belonging to the group of aji are native to tropical and temperate waters of the world. The most relevant species of this group for the preparation of sushi is the Japanese horse mackerel (maaji), whose range extends from Hokkaido to the South China Sea. In contrast, the most economically important species is the Chilean jack mackerel (chiri maaji). This is also used in Japan, along with related species from the North Atlantic and South Pacific, as a substitute (daiyōgyo) for the preparation of aji sushi. The closely related amberstripe scad (muroaji) is found primarily in tropical and subtropical waters and is also considered an important seafood in Japan.

In commercial fisheries, historical catch data provides insights into the health of marine ecosystems and the sustainability of fishing practices. Data from the Food and Agriculture Organization of the United Nations (FAO) provides insight into global catches of aji, a collective name for various mackerel species of the genus Trachurus, over a period of nearly seven decades.

From 1950 to 2018, the stacked bar chart shows a significant increase in catches, starting at a low level and increasing dramatically in the 1970s and 1980s. The spikes in catches in these decades indicate a period of intense fishing activity, possibly driven by the growing demand for aji in the food industry, and the chart classifies catches by species: Mediterranean horse mackerel, Cunene horse mackerel, Japanese horse mackerel, Cape horse mackerel and Chilean horse mackerel. Each species contributes to the total catch with its own color, which makes it easier to distinguish between species and identify patterns. It is noticeable that certain species, such as the Japanese horse mackerel and the Cape horse mackerel, play a dominant role in the fishery.

After the peaks of the 1990s, the data show a downward trend, with fluctuations that could indicate changes in fish stocks or the introduction of quotas and other fisheries management measures. The significant decline from 2000 onwards raises questions about the causes: Overfishing, changes in marine habitats or possibly stricter regulations and increased awareness of sustainable practices could be factors.