Ayu fishing is documented in the Man'yoshu (8th century CE) and was a designated tribute fish (mitsugi) sent to the imperial court from mountain provinces; the Nagara River (Gifu) and Kuma River (Kumamoto) are the most celebrated ayu rivers; tomozuri (friend fishing) was developed specifically for ayu because their territorial nature makes conventional net fishing difficult; the Gifu cormorant fishing (ukai) tradition uses trained cormorants to catch ayu and has been practised for 1300+ years
Ayu (鮎 — sweetfish, Plecoglossus altivelis) is Japan's most celebrated freshwater fish and the quintessential summer luxury — a small (15–20cm), silver-sided fish that migrates up cold mountain rivers from May to August and feeds almost exclusively on river algae (diatoms), giving its flesh a distinctive watermelon and cucumber fragrance that has no equivalent in any other fish. Ayu is salt-grilled (shio-yaki) in its entirety — unscaled, ungutted, with the intestines (wata) contributing a slight pleasant bitterness that is inseparable from the eating experience; removing the intestines would destroy the characteristic flavour. The skewering technique is highly specific: the ayu is threaded onto a long metal or bamboo skewer with the S-curve of a swimming fish — not straight — which allows all surfaces to be exposed to the binchotan charcoal heat uniformly. The roe and milt in season (July–August) add additional flavour dimensions. Taimatsu (torchlight) fishing is the traditional harvest method — ayu are attracted to light and move upstream; they are territorial fish that are also caught by the 'friend fishing' (tomozuri) method using a live decoy ayu to attract territorial responses from wild fish.
The watermelon-cucumber fragrance of ayu is produced by the breakdown of algal carotenoids (primarily beta-carotene and zeaxanthin) in the ayu's digestive system — the fish's algae diet is literally expressed as aromatic compounds in the flesh; farmed ayu fed commercial pellets lacks these algal carotenoids entirely, producing a fish that resembles ayu structurally but not aromatically; this is the most complete case in Japanese cuisine of flavour being entirely determined by diet and habitat
Shio-yaki (salt grilling) only — no marinades or sauces; the intestines are eaten and contribute essential bitter umami complexity; the S-curve skewering exposes all surfaces to heat; the river variety is superior to farmed (farm-raised ayu lacks the algae diet and therefore the characteristic fragrance); serve immediately — ayu loses its delicate fragrance within 20 minutes of cooking.
Shio-yaki technique: salt the ayu immediately before grilling (not in advance — advance salting draws moisture); apply heavier salt to the tail fin which dries fastest (prevents burning); grill over binchotan at medium distance for 12 minutes total, 8 minutes first side; the proper test: the fin should not burn but the skin should blister lightly; ayu shiokara (intestine paste) is a byproduct of the season — intensely bitter-umami paste made from fermented ayu innards, used as a condiment in Kyoto cuisine.
Removing the intestines before grilling (eliminates the characteristic flavour complexity); straight skewering produces uneven cooking; using strong sauces (masks the delicate watermelon-fragrance); over-salting before grilling (the salt's function is to create a crust, not season deeply — ayu's flavour is intrinsic); eating cold ayu (the fragrance is temperature-sensitive, cold ayu is not the same experience).
Tsuji, Shizuo — Japanese Cooking: A Simple Art; Murata, Yoshihiro — Kaiseki