Maaji Sushi
A Comprehensive Overview of Japanese Horse Mackerel in Japanese Sushi Cuisine

The Japanese jack mackerel (Trachurus japonicus), also known as the Japanese horse mackerel or Japanese scad, is called maaji (真鯵, まあじ) in Japanese. Under that name, it is the default "aji" of Japanese gastronomy. When markets, menus, and sushi counters use the unqualified term aji, they mean this species. Within the broader aji group, which encompasses dozens of species, maaji holds the central position: it is the most commercially important, the most widely consumed, and the species against which all other aji are measured.^1, 2

As a sushi ingredient, maaji belongs to the hikarimono (光り物, silver-skinned fish) category alongside kohada, saba, iwashi, sanma, and sayori. Hikarimono preparation is considered a test of the sushi chef's skill because these fish spoil rapidly and demand precise handling. Maaji is distinctive within the group for its traditional garnish of grated ginger and sliced scallion rather than wasabi, a pairing that addresses the fish's oily richness more effectively than wasabi alone.^3, 4

The flesh is pink, firm, and slightly elastic, with moderate fat content (a national average of 4.5%), though premium branded specimens from specific populations can reach 15% or more. The flavor is driven by robust umami and a characteristic seafood sweetness. A distinctive aroma, produced by a volatile compound that maaji generates at an unusually high rate among red-fleshed fish, sets it apart from all other hikarimono.^5, 6, 7

Maaji occupies an unusual position in Japanese food culture: simultaneously an affordable everyday fish and a vehicle for luxury branding. Standard specimens wholesale at the Toyosu market for approximately ¥600–750/kg, while premium branded variants (Seki Aji from the Bungo Strait, Donchicchi Aji from Hamada, Gon Aji from Nagasaki) command prices five to fifteen times higher. This price spectrum exists within a single species because the key quality variable is not genetics but ecology: resident populations known as ki-aji (黄アジ, yellow aji), which feed in nutrient-rich coastal environments, develop markedly higher fat content than migratory populations known as kuro-aji (黒アジ, black aji).^8, 9, 10

Maaji is unusual among sushi fish. Its muscle is classified as red-fleshed by hemoglobin content, but its protein composition is closer to white-fleshed fish than to mackerel or tuna, giving it neither the heaviness of maguro nor the delicacy of hirame. This makes it one of the most versatile sushi fish: raw, sujime-cured, kobujime, aburi, tataki, namero.¹¹

The distinctive maaji taste derives from several interacting components. The primary umami component is inosinic acid (IMP), which peaks approximately ten hours after death, a timeline relevant to aging decisions. Salt inhibits the enzyme that degrades IMP, which is one reason why salting during sujime helps preserve umami. Glutamic acid and aspartic acid provide additional umami depth, particularly in fish that have been rested for more than a day. Glycine and alanine contribute the characteristic seafood sweetness.^6, 12

Among red-fleshed species, maaji generates hexanal at an unusually high rate. Hexanal is a product of fat oxidation and constitutes the distinctive volatile signature of fresh maaji, the compound primarily responsible for the aroma that distinguishes it from other hikarimono.⁷

In early Edomae sushi, maaji was always served vinegar-cured (sujime, 酢締め) with the silver skin intact, the only safe method before refrigeration. The transition to optional raw service, enabled by post-WWII cold-chain development, represents one of sushi's most significant twentieth-century shifts. Today both approaches coexist, and the chef's preference between them reflects their approach to the fish.^3, 13

Jiro Ono of Sukiyabashi Jiro reportedly prefers raw, untreated maaji, stating that raw service demands both superior fish quality and greater technical skill than vinegar-cured preparation. He judges salt and vinegar timing "in an instant upon opening the body," a calibration that accounts for the individual fish's fat content, freshness, and size.¹³

Professional sujime protocols vary from chef to chef, but the core sequence is consistent: salt generously, rest (duration adjusted to fish condition: 15 minutes for small or lean specimens, 40–60 minutes for larger or fattier fish), rinse, immerse briefly in rice vinegar. A professional cook with approximately 30 years of experience notes that the vinegar bath must be kept ice-cold, with the bowl set over ice water. If vinegar is applied at room temperature, the ginpi (銀皮, silver skin) loses its lustrous color during peeling, a visual defect that undermines hikarimono presentation.¹⁴

Takai Hidekatsu, an NHK cooking program contributor, provides additional detail: fish pieces must not overlap in the vinegar bath because overlapping causes scale transfer and flesh damage. Pin bone removal and skin peeling are performed after sujime, not before; the acid-firmed flesh handles more cleanly and bone extraction is easier in the set texture.¹⁵

Resting the fish overnight after curing allows salt and vinegar to fully harmonize. One practitioner describes this as transforming raw aji as ingredient into aji as finished work (shigoto, 仕事). This distinction between raw material and completed preparation is a core Edomae concept.¹⁴

The processing sequence unique to maaji begins with removing the zeigo (ぜいご, lateral line scutes), bony, keeled plates running along the lateral line that are absent in most other sushi fish. This step is among the first knife skills taught to Japanese cooks. After zeigo removal: scale the remaining body, remove the head, gut and wash, then three-piece fillet (sanmai oroshi, 三枚おろし).^3, 8

To prepare each piece of fish for the rice (neta), belly bones are scraped with the knife, pin bones pulled with wet tweezers toward the head end. For raw service, the skin is peeled by hand from head to tail in one motion, then the flesh is scored with multiple shallow parallel cuts (kazari-bōchō, 飾り包丁), improving appearance, helping release flavor, and preventing curling on rice. For sujime service, the silver skin is typically retained for hikarimono visual effect, and the fish is brushed with nikiri (煮切り, reduced soy glaze) rather than served with separate dipping soy.^3, 4

The traditional condiment for aji nigiri (握り) is freshly grated ginger (oroshi shōga) and finely sliced scallion (negi), placed between rice and fish. This pairing is specific to aji and some other hikarimono, replacing wasabi. Ginger's sharpness cuts the fish's oily richness more effectively than wasabi's heat, while providing complementary antibacterial properties.^3, 13

Atari-negi (あたりねぎ) has become a widely adopted alternative: pounded asatsuki chives with ginger, producing an aroma reminiscent of garlic. It was invented by Okada Shūzō, former master of Kozasa Sushi (小笹寿し) in Shimokitazawa, Tokyo, and has become the standard condiment at many sushi shops for aji. The name follows the euphemistic pattern of replacing suri (to grind) with atari (to win), the same convention seen in atarime for surume.^16, 17

In high-end sushi restaurants, condiment restraint matters. The natural aroma of the horse mackerel should not be overwhelmed by excessive yakumi (薬味, aromatic garnish). Some contemporary chefs use all three (wasabi, ginger, and scallion), applied as the individual fish's condition dictates.¹⁶

Kobujime (昆布締め, kelp curing) is applied to maaji, though less commonly than for white-fleshed fish. Most often it is combined with vinegar as a hybrid sujime-kobujime: kombu added to the vinegar bath, or placed atop fillets during resting for 30–60 minutes. The result layers the fish's own umami with kombu's glutamic acid.¹⁸

Maaji spoils rapidly due to high enzyme activity and relatively high histidine levels. Immediate refrigeration below 4°C is essential, and at counter temperature, degradation is perceptible within hours. Brief resting of one to two days can develop umami through inosinic acid accumulation as energy compounds in the muscle break down, but extended aging as practiced with white-fleshed fish or tuna is risky; freshness degrades faster in red-fleshed species.^8, 12

In omakase progression, maaji as hikarimono is positioned mid-course: after lighter white-fleshed fish, often alongside or just before fattier items. At Sushi Aoki in Ginza, aji appears as approximately the seventh item in the neta box, after lighter items such as kohada and kisu kobujime.^19, 20

Raw nigiri is the modern standard at high-end counters: skin removed, flesh scored, garnished with ginger and scallion. In traditional Edomae style, the fillet is sujime-treated with the silver skin retained and brushed with nikiri. Aburi (炙り, torch-searing) is occasionally applied but is not a traditional preparation; it melts fat beneath the skin and crisps the silver skin, producing a smoky aroma. It is best suited for fattier specimens.

Beyond nigiri, maaji appears in several other forms. As sashimi, the fillets are served without rice, often with the same ginger-scallion garnish. Tataki (たたき) is lightly pounded and dressed with ginger, scallion, and miso. Namero (なめろう), a Bōsō Peninsula specialty, takes the mincing further: the flesh is chopped with miso, ginger, scallion, and shiso (perilla) until sticky and viscous. Nanbanzuke (南蛮漬け) follows a different logic entirely: the fish is lightly fried, then marinated in sweet vinegar, a preparation that traces back to Portuguese escabèche introduced through the nanban trade.²¹

Ko-aji no oshizushi (小鯵の押し寿司, pressed small-aji sushi) is a regional pressed-sushi format from the Sagami Bay area on the Pacific coast west of Tokyo, where maaji were once landed in great abundance. Small fish are cured with salt and vinegar, then half-fillets are placed on shari and pressed in a wooden box. The format combines the sujime work of the Edomae tradition with the pressing technique of Kansai oshizushi: Ōfunaken, the Ōfuna railway-station vendor that introduced its version in 1913, describes the style as "shaped in the Kantō manner, pressed in the Kansai manner." It became famous as ekiben (駅弁, station box-meals) along the Tōkaidō line. Tōkaken in Odawara has sold its ko-aji oshizushi continuously since 1903, and several other stations along the Tōkaidō and Izu coasts maintain their own variants. The format remains one of the few in which maaji is the defining ingredient rather than one option among many.^{22, 23, 24}

Historically, aji belonged to the hikarimono category for which Edomae cooks developed their salt-and-vinegar techniques in the first place. Kohada, saba, iwashi, sayori, and aji were the fish that nineteenth-century Edo treated with sujime both to extend shelf life and to temper their pronounced, slightly oily flavors. This hikarimono lineage is classically paired with shari (シャリ, sushi rice) made from akazu (赤酢, red vinegar): an aged sake-lees vinegar that carries roughly ten times the free amino acids of komezu (米酢, rice vinegar) and enough umami depth to stand up to a vinegar-cured fish. Its fermented character pairs naturally with the fermented note that sujime preparation brings out. In a strict Edomae reading, aji is therefore an akazu candidate, though less unambiguously than kohada.²⁵

Modern practice has shifted, and the reason lies in how aji is now most often served. Unlike kohada, which still arrives at the counter almost always in some salt- or vinegar-cured form, aji today is overwhelmingly served fresh: briefly trimmed, garnished with finely chopped scallion and grated ginger directly on the neta, sometimes finished with a touch of nikiri. Even at strictly traditional shops, the trend is toward minimal intervention. At Tokyo's Kizushi (㐂寿司), one of the city's most traditionally-minded Edomae houses, the fourth-generation owner Yui Kazuhiro describes salting the fish for only a few minutes and "passing it through" the vinegar bath rather than marinating it. In this fresh presentation, aji's sensory profile sits closer to a clean, mildly fatty white fish than to a classic cured hikarimono.²⁴

This shift has consequences for the shari. At high-end Edomae shops that calibrate their rice to the fish, the documented principle is to match heavier, fattier, or more heavily-worked neta with akazu, and lighter, cleaner, more delicate neta with komezu. Takahashi Shingo of Aoyama's Takumi Shingo blends his rice seasonally, using an akazu-dominated mix in winter when fish are fattier and a komezu-dominated mix in summer when they are lighter. Sushi Akazu Kanayama in Shinjuku, despite the akazu in its name, also divides its rice by ingredient on this principle, reserving red vinegar for fatty and richly flavored items and white vinegar for clean and delicate ones. Under this logic, fresh aji served with scallion and ginger, the form most counters use today, sits on the komezu side. When a traditional shop chooses to serve aji with even a brief sujime treatment, the calculation shifts and akazu, or an akazu-leaning blend, becomes the more coherent partner.^25, 26

Maaji is a single-species ingredient: the authentic aji for sushi is Trachurus japonicus exclusively. No other species is intentionally marketed as a sushi-grade substitute in Japan.¹

The most consequential distinction for sushi quality is not between species but between ecotypes of the same species. Resident ki-aji populations settle on reef structures in nutrient-rich coastal waters, developing taller, rounder bodies, golden-yellow coloring, and high fat within the flesh from diets rich in crustaceans. Migratory kuro-aji travel open-ocean routes, maintaining slender, dark-backed profiles with lower fat from the energy expenditure of constant movement. These are not subspecies; they are behavioral variants of a single species, with some intermediate individuals.^2, 10

Ki-aji is generally superior for sushi: higher fat produces richer mouthfeel, and flesh firmed by fast-current dwelling develops distinctive texture. The Seki Aji cooperative describes their resident fish as "well-fattened from abundant food, flesh thoroughly firmed by fast currents." Color alone is unreliable for identification: stale fish also turn yellowish, and body depth ratio is a more reliable indicator.¹⁰

The primary source of retail confusion is maruaji (マルアジ, Decapterus maruadsi), sometimes sold under the bare label "aji" or "maaji" without qualification. The fastest diagnostic: check for a small detached finlet behind the dorsal and anal fins: present in maruaji, absent in maaji. Additionally, maaji's zeigo scutes extend the entire lateral line (69–73 scutes), while maruaji's scutes cover only the rear section. Body cross-section differs as well: maaji is flattened sideways, maruaji more cylindrical (hence maru, "round"). Sensorially, maruaji is leaner, less sweet, and slightly fishier.^1, 2

Two other Trachurus species reach Japan through imports, neither for raw consumption. Nishi-maaji (ニシマアジ, T. trachurus), the Atlantic horse mackerel, is imported from the Netherlands, Norway, and Ireland. It is slightly drier and less sweet than domestic maaji, used primarily for himono (干物, dried fish). Chiri-maaji (チリマアジ, T. murphyi), the Chilean jack mackerel, is larger with coarser texture, used for processed and fried products.²⁷

Outside Japan, what arrives as aji depends on geography. T. japonicus is native to the Northwest Pacific (Japan, Korea, and the East China Sea) and does not occur in the Eastern Pacific, the Atlantic, or European waters. In the United States, high-end restaurants may import T. japonicus through Japanese specialty distributors, but the locally available Eastern Pacific species T. symmetricus (Pacific jack mackerel, found from Alaska to Baja California) and imported T. trachurus are also plausible sources at other price levels. In Europe, aji on a sushi menu is almost certainly local T. trachurus (Atlantic horse mackerel): same genus, same zeigo, same basic preparation. In either case, the fish on the plate belongs to the same genus as maaji and handles similarly in the kitchen, but it is not the same species.

The nationally cited shun (旬, peak season) for maaji (May through August) reflects a Kantō-centric perspective. Spawning stretches from January through November across all of Japan, progressing northward: Kyushu in January–May, Shikoku in April–July, central Honshū in April–July, Tōhoku in July, Hokkaido in August. Because the best eating condition occurs during the pre-spawn fattening period, the optimal window shifts accordingly by region.^{28, 29, 30}

Fat content varies significantly: Japan's national food composition database reports an annual average of 4.5%, while monthly measurements in T. japonicus populations range from 3.94% to 5.75%, with a strong inverse correlation to water content. This relationship is directly perceptible: fattier fish contain less water, producing firmer, richer mouthfeel. Branded ki-aji populations vastly exceed these averages: Donchicchi Aji must certify ≥10% fat, with peak specimens reaching 15–20%.^{5, 31, 32}

The ecotype distinction matters more than calendar date for quality. Resident ki-aji populations show less seasonal variation because stable, nutrient-rich nearshore environments keep them consistently well-fed. Migratory kuro-aji follow a more pronounced seasonal fat cycle. Maaji is best raw at peak fat; lean-season fish benefits from sujime, which adds acid brightness to compensate for reduced richness. Post-spawn maaji is lean and depleted, more commonly diverted to frying or dried products.²

No formal government grading system exists for maaji. Market differentiation operates through branding and the ki-aji/kuro-aji distinction. Quality indicators: eyes plump, convex, and clear; gills bright crimson; body firm and springing back when pressed; fins erect and moist; belly firm with clean iridescent silver; scales intact. Fresh maaji has a clean, sea-like aroma; any "fishy" or ammoniac smell indicates spoilage. Experienced sushi chefs additionally assess fat content by the translucency of the fillet against light: fattier flesh is more opaque.^6, 8

Toyosu size categories: mame-aji (under 15 cm), zengō/ko-aji (15–25 cm), chū-aji (25–30 cm), ō-aji (30 cm and above). For sushi, fresh whole fish delivered same-day or ikejime-killed (活け締め, brain-spiked) with ice slurry is the standard. Live maaji traded in some wholesale markets is almost entirely farm-raised; national aquaculture production totals only 553 tonnes.^9, 33

Maaji was among the original Edomae sushi ingredients, prepared with sujime as hikarimono. No primary source pinpoints the exact date of its inclusion; the earliest systematic Edomae descriptions postdate the tradition's emergence in the 1820s–1830s by decades, but multiple converging sources confirm aji as part of the early repertoire. The transition to optional raw preparation is more precisely dateable: circa 1965, when a Shinjuku ryōtei chef adapted a raw aji dish he had encountered in Izu.^3, 8, 13

Namero (なめろう) originated on fishing boats off the Bōsō Peninsula (房総半島, Chiba Prefecture). Raw maaji is finely minced, combined with miso, scallion, ginger, and shiso, then pounded until sticky and viscous. The name derives from either the expression "so delicious you'd lick the plate" (皿をなめるほど旨い) or the clinging texture itself. Daitokuya (大徳家) in Chikura, Minami-Bōsō (founded in 1869) claims to be the first restaurant to serve namero commercially; the current fifth-generation owner chairs the Minami-Bōsō Namero Research Association. In 2023, the Agency for Cultural Affairs designated the region's aji culture as a "100-Year Food."²¹

Seki Aji (関アジ) from the Bungo Strait launched Japan's entire branded fish (burando-gyo, ブランド魚) industry. The name was coined in 1982 after television commentator Fujiwara Hirotasu publicized the fish during a visit. Caught exclusively by line fishing, a multi-hook handline technique locally called tobashi, with approximately 100 lure-baited hooks on a 150-meter main line set at roughly 140 meters depth. These resident ki-aji develop superior fat content from feeding in the nutrient-rich, fast-tidal-current environment of the Hayasui Strait. Fish are never touched by hand, killed by ikejime and shinkeijime (神経締め, spinal cord destruction), and shipped at 5°C.^{10, 34, 35}

The Saganoseki branch of the Oita Prefecture Fishery Cooperative filed a trademark in 1992 and received registration in 1996, the first-ever trademark for a fishery product in Japan. In 2006, Seki Aji additionally obtained regional collective trademark (chiiki dantai shōhyō, 地域団体商標) registration. Each fish bears a branded tail-tag seal; without it, the product cannot legally be sold as Seki Aji. Identical fish caught from the Ehime side of the same strait must be sold as Hana Aji (岬アジ) at lower prices.^36, 37

The vulnerability of this institutional model was exposed in 2012, when the Misaki Fisheries Cooperative in Ehime Prefecture surrendered its brand certification after an investigation revealed that purse-seine-caught aji from an Oita-based company had been shipped as ipponzuri "Hana Aji" from May 2007 to August 2011. In the documented period of April–August 2011 alone, 2,508 kg of fraudulent fish entered the market, exceeding 10% of the total Hana Aji volume for that fiscal year. The cooperative cited declining pole-and-line catches and inability to meet order demand.³⁸

Other branded maaji systems use different quality approaches. Donchicchi Aji from Hamada, Shimane Prefecture, was branded in 2003 as the first use of near-infrared fat measurement technology for fish branding. A portable spectrometer pressed against the fish's posterior dorsal area reads lipid content non-destructively, with an accuracy that closely matches laboratory analysis. Fish must certify ≥10% average fat and ≥50 grams body weight; the high fat is attributed to Calanus copepods (tiny crustaceans), whose body composition exceeds 50% fat, abundant in Shimane's western offshore waters.^32, 39

Gon Aji from Nagasaki's Gotō-nada uses a different approach entirely: after seine-net capture, fish are transferred alive to offshore live wells and held for seven to ten days without feeding. Lactic acid accumulated from capture stress dissipates, and without feed intake, the fish metabolizes stored fat, distributing it evenly throughout the flesh, producing a shimofuri (霜降り, frost-like marbling) effect analogous to premium beef.^40, 41

The branded maaji landscape now includes over ten named products across Japan. These differ fundamentally in institutional structure: Seki Aji, Donchicchi, and Gon Aji are cooperative-managed systems with defined catch criteria, quality thresholds, and trademark protection. Ōgon Aji (黄金アジ) from Kanaya, Chiba (perhaps the most media-visible name) is by contrast a general term (ippan meishō) that cannot be registered as a trademark; individual restaurants trademark their specific dish names but the fish designation itself is unprotected.⁴²

Japan's domestic catch reached 99,295 tonnes of maaji in 2022, representing a 27% decline from 2012. Nagasaki Prefecture alone accounts for 47% of national production. Historical catches have fluctuated dramatically: exceeding 500,000 tonnes in the 1980s, collapsing to approximately 50,000 tonnes by the late 1980s, recovering through the 1990s, and fluctuating around 100,000 tonnes in recent years.^8, 43

Two distinct stocks with divergent status are managed separately. The Tsushima Current stock had a biomass of 384,000 tonnes in 2023, with breeding-population biomass of 261,000 tonnes, above the maximum sustainable yield (MSY) target of 254,000 tonnes. The Pacific stock is in worse condition: total biomass of 56,000 tonnes in 2022, below the MSY biomass target of 60,000 tonnes, with breeding-population biomass of only 26,000 tonnes. The IUCN (International Union for Conservation of Nature) rates the species as Near Threatened.^{44, 45, 46}

Toyosu wholesale pricing averages ¥588/kg (April, lowest) to ¥749/kg (July, highest) across the 2021–2025 five-year period. The year 2025 is running notably elevated, with March reaching ¥889/kg versus a five-year average of ¥652, a 36% premium.⁹

Maaji aquaculture is minimal: 553 tonnes nationally, less than 1% of wild catch. Its primary function is supplying live fish for sushi shop tanks. Dominant fishing methods include purse seine for volume harvests from the Tsushima Current stock, set nets for Pacific coast catches, and pole-and-line for premium branded catches.^33, 47

Trachurus japonicus Temminck & Schlegel, 1844, was described in Fauna Japonica. It belongs to the family Carangidae (order Carangiformes), a genus of 14 species worldwide. Adults reach a maximum of 50 cm total length, commonly 35 cm, and live up to 12 years. First maturity occurs at approximately 19–20 cm body length. Spawning occurs in multiple rounds from late winter through early summer, at optimal water temperatures of 16–21°C, progressing northward with the season.^{29, 30, 46, 48}

The distinction between resident ki-aji and migratory kuro-aji is not merely a market category; it reflects a genuine biological divergence within T. japonicus. Some populations settle permanently on coastal reef structures and remain there year-round, while others follow seasonal migration routes through open water. Why certain individuals become resident is not fully understood. The Seki Aji cooperative hypothesizes that stable year-round water temperature and abundant food in the Bungo Strait eliminate the need to migrate. Research from the strait supports this: fat content in resident fish remains nearly constant year-round, versus extreme seasonal variation in migratory fish. The stability is attributed to the strait's unusual temperature regime, approximately 6°C cooler in summer due to upwelling and warmer in winter due to Kuroshio influence.¹⁰

Maaji are omnivorous, feeding on crustaceans, small fish, marine worms, and squid, increasingly feeding on fish as they grow. Diet composition directly affects flavor: the exceptionally high fat content of Donchicchi Aji is attributed to Calanus copepods, the same fat-rich crustaceans described in the branding section above. For ki-aji populations feeding on reef-based crustaceans, the high-protein, high-fat diet translates directly to richer umami and firmer texture.^10, 32

Nutritional data per 100g raw flesh with skin: 112 kcal, 19.7g protein, 4.5g lipid. Notable micronutrients include vitamin B12 at 7.1 μg (approximately 296% of daily value), vitamin D at 8.9 μg, selenium at 46 μg, DHA at 570 mg, and EPA at 300 mg. DHA comprises 25–45% of total fatty acids.^5, 31

The etymology of "aji" and the linguistic history of the aji name complex (including the connection to 味 (aji, "taste/flavor") attributed to Arai Hakuseki's Tōga (東雅, 1717), alternative derivations, the kanji variants 鰺 and 鯵, and the English "horse mackerel" derivation from Dutch horsmakreel) are covered in the aji group article. The species name maaji (真鯵) uses the prefix 真 (ma-, "true/genuine"), marking it as the standard-bearer of the group, the same naming pattern seen in madai (真鯛), maguro (鮪), and masaba (真鯖).^1, 49

Maaji carries the same parasite and bacterial risks as other red-fleshed sushi fish. Approximately 14% of individual specimens test positive for Anisakis larvae. Vinegar curing does not kill them: the Japanese Ministry of Health, Labour and Welfare (MHLW), the Ministry of Agriculture, Forestry and Fisheries (MAFF), and all major food safety authorities explicitly state that vinegar, salt, soy sauce, and wasabi are ineffective. Only freezing or thorough heating reliably kills the parasite. Sujime is a flavor and texture treatment, not a food safety measure.^{50, 51, 52}

Histamine is the second concern. Maaji is classified by the MHLW among histidine-rich fish susceptible to bacterial conversion of histidine to histamine, alongside tuna, mackerel, sardine, and other red-fleshed fish. The largest histamine outbreak in world history involved dried T. japonicus: 2,656 cases in Japan in 1973. Once formed, histamine is heat-stable; no amount of cooking destroys it. Some histamine-producing bacteria remain active even under refrigeration, so an unbroken cold chain from landing to service is the only reliable defense.^{53, 54, 55}

A second parasite, Kudoa trachuri, has been described from T. japonicus specimens collected off Nagasaki and forms visible whitish cysts in the trunk muscle. It should not be confused with K. septempunctata, the species responsible for temporary diarrhea after raw consumption, which is overwhelmingly linked to olive flounder (hirame). Kudoa in maaji is a quality and aesthetic issue, not a gastrointestinal hazard. Mercury is not a concern: the FDA classifies horse mackerel among "Best Choices" for two to three weekly servings.^{56, 57, 58}

Regulatory approaches differ substantially across jurisdictions. Japan has no mandatory freezing requirement for raw fish service; restaurants serve fresh, unfrozen aji sashimi legally, relying on visual inspection and rapid gutting. The European Union and the US FDA both mandate prior freezing for raw-served fish, and the FDA additionally lists Trachurus for both parasite and histamine hazards under its HACCP (hazard analysis and critical control points) framework. This regulatory gap explains why fresh, unfrozen aji sashimi is common at Japanese sushi counters but would require prior freezing in Europe or the United States.^51, 58