What Is Unagi? Japan’s Summer Eel Tradition Explained
Today I heard several fascinating facts about unagi, and they made me want to understand what is really behind one of Japan’s most recognizable dishes. The essential story is true—but the life of the Japanese eel is even more remarkable than the familiar image of glazed eel over rice suggests.
What is unagi?
Unagi is the Japanese word commonly used for freshwater eel. In Japanese cuisine, it usually refers to the Japanese eel, Anguilla japonica. It is best known as kabayaki: eel opened into fillets, skewered, grilled, and repeatedly brushed with a sweet-savory sauce made from soy sauce, mirin, and sugar. It may be served in a bowl as unadon or in a lacquered box as unajū.
Does unagi swim from the sea into Japanese rivers?
Yes, although the full journey is more precise than simply saying that eel comes from “somewhere at sea.” Japanese eels are catadromous fish: they grow in rivers, lakes, estuaries, and coastal waters, but reproduce in the ocean. Research has located their spawning area in the western North Pacific near the Mariana Ridge. Their transparent, leaf-shaped larvae drift on ocean currents toward East Asia. After transforming into glass eels, many enter estuaries and rivers in Japan and neighboring countries, where they grow for years. Mature eels eventually migrate back toward the open ocean to spawn.
This extraordinary life cycle also explains why eel farming has been so difficult. Most farmed unagi has historically begun with wild-caught glass eels rather than eggs produced at a farm. Japan achieved complete-cycle artificial production in research, but producing artificial glass eels reliably, affordably, and at commercial scale remains challenging. The Fisheries Agency of Japan describes both the breakthrough and the continuing need to reduce dependence on wild seed.
Kanto and Kansai: two ways to grill unagi
The regional difference you heard is broadly correct. In the Kanto style associated with Tokyo and eastern Japan, the fillet is commonly grilled once without sauce, steamed, and then grilled again with sauce. Steaming renders some fat and produces a soft, delicate texture.
In Kansai and much of western Japan, cooks generally grill the eel directly without the intermediate steaming step. This creates a firmer bite, crisp edges, and a more pronounced roasted character. There are also differences in how the eel is opened: traditionally from the back in Kanto and from the belly in Kansai. Individual restaurants have their own methods, so these are regional traditions rather than absolute rules. Japan’s Ministry of Agriculture, Forestry and Fisheries introduces kabayaki as part of Tokyo’s regional food culture.
Why do people eat unagi in summer?
Unagi can be enjoyed throughout the year, but it has a particularly strong connection with summer through Doyō no Ushi no Hi, the Day of the Ox during the midsummer Doyō period. Since the Edo period, people have eaten nutritious eel in the hope of maintaining their strength through Japan’s heat and humidity. The custom is often linked to the scholar and inventor Hiraga Gennai and an advertising idea used to help an eel shop attract summer customers, although the exact origin is not certain.
There is an interesting twist: the culinary custom and the natural season are not the same thing. Wild eel is often considered richest in the colder months as it stores fat before migration. Summer is the famous cultural season for eating unagi, while farmed eel makes the dish available all year. The summer tradition is documented by MAFF’s guide to Japanese seasonal foods.
Unagi versus anago
Unagi and anago look similar on the plate, but they are different fish with different lives. Unagi moves between fresh or brackish water and the ocean. Anago generally refers to marine conger eel, especially the whitespotted conger used in Japanese cooking, and it remains a sea fish rather than migrating into rivers like unagi.
Anago is usually leaner, lighter, and more delicate. It is often simmered for sushi or grilled for dishes such as Hiroshima’s anago meshi. Unagi is richer and oilier, which suits the assertive kabayaki glaze. MAFF’s guides to anago sushi and anago meshi describe its place in Tokyo Bay and Seto Inland Sea food culture.
A dish worth eating thoughtfully
The Japanese eel’s long migration and dependence on wild glass eels also make sustainability important. Resource levels have declined, and Japan regulates eel aquaculture and glass-eel stocking. When possible, choose restaurants and sellers that can explain their sourcing and avoid treating unagi as an everyday, unlimited ingredient.
For me, understanding the journey adds another dimension to the meal. A piece of softly steamed Kanto-style unagi or crisply grilled Kansai-style eel is not only comfort food. It carries a story connecting distant ocean currents, Japanese rivers, regional cooking techniques, and a summer custom that has lasted for generations.
