Japan-wide — anago from Tokyo Bay (Edomae tradition); unagi from various freshwater sources
Japanese eel cuisine distinguishes sharply between two different eel species used in fundamentally different ways. Unagi (freshwater eel, Anguilla japonica): the prestige eel, high-fat, rich, deep-flavoured, prepared as kabayaki (split, steamed, grilled with tare) — associated with summer vitality, extremely expensive, critically endangered. Anago (conger eel, Conger myriaster): the sea eel, available from Tokyo Bay and all Japanese coasts, lower in fat than unagi, more delicate in flavour, and traditionally the eel of Edomae sushi. Anago is prized for its melt-in-mouth tenderness and delicate sweetness when steamed and cooled — a top-tier sushi topping (nigiri anago) typically brushed with tsume (reduction of the eel's cooking liquid and soy) rather than wasabi. Anago is also prepared in similar kabayaki style (simmered and grilled) as a more affordable eel alternative. The two are genuinely different flavour experiences — unagi is richer, more assertive; anago is more delicate and sea-forward.
Anago: delicate, sweet, oceanic — tender to the point of melting when properly prepared; unagi: rich, deep, fatty — the more assertive, umami-forward eel; both require sweet-soy tare but express completely different eating experiences
Unagi requires the split-steam-grill kabayaki technique for its fat content and density; anago is typically simmered in dashi-soy until tender then either served at room temperature as sushi or grilled briefly for warmth; anago's tsume (the reduced cooking liquid) is the sushi chef's signature — the concentration level and balance of sweet-soy in the tsume defines the restaurant's anago character; season for anago: summer (June–August) when sea eels are plump.
Anago sushi benchmark: Fukamachi (Nihonbashi, Tokyo) serves possibly Japan's finest anago nigiri — the eel barely set above the rice, almost falling apart, brushed with concentrated tsume; home anago preparation: purchase fresh anago, simmer in dashi+soy+mirin+sake for 15 minutes until just tender, remove, brush with reduced cooking liquid glaze, serve over rice or as sushi; anago no hako-mushi (anago in a lidded steamed dish over rice) is a Seto Inland Sea specialty from Hiroshima — gentle steaming preserves the eel's delicate texture.
Treating anago and unagi as interchangeable (the fat content, texture, and preparation method are all different — applying unagi kabayaki technique to anago produces overcooked, dry eel); using tsume for anago that is too sweet (tsume should be sweet-savoury balanced, not candy-sweet); serving anago cold directly from refrigeration (anago sushi should be at room temperature or slightly warm).
Japanese Cooking: A Simple Art — Shizuo Tsuji