What is Ayu (Sweetfish)?
Ayu, commonly called “sweet fish”, is a stint-like fish that spawns in autumn and tastes best when it has gained fat over the summer months. Young ayu (waka-ayu, 若鮎), on the other hand, are caught in spring. Ayu is not only a popular fish in Japan, but is also firmly connected to Japanese tradition and culture.
Ayu as Ingredient for Sushi or Sashimi
Since ayu is a freshwater fish, it is not a traditional edomae sushi ingredient. Nowadays, despite its availability from Aqualtutur, it is encountered sometimes as a cooked, simmered or flamed (aburi) ingredient.
For the preparation of Sushi or Sashimi it is strongly recommended to use ayu from aquaculture, because they are not infested by potential parasites. Ayu represents an intermediate host for parasites, especially for the Yokogawa sucker worm (lat. Metagonimus yokogawai). The Japanese Food Safety Commission strongly advises against the consumption of raw ayu [Fscj, 2014].
Don't be fooled by its inconspicuous appearance, ayu appeals by its excellent taste. Ayu is a highly valued seafood in East Asia because of its unmistakable sweetish taste, whose aromas are said to remind of melon and cucumber. It has a pleasant texture and a strong umami taste. The intense taste known from wild caught ayu fish is not present in fish from controlled breeding.
When the ayu fish appear, this is the sweet sign of summer.
- Japanese phrase
Compared to wild catches, farmed fish from aquacultures have a very high fat content. When prepared raw, they are about three times as fat. When they are prepared in whole, the internal organs have more than five times the fat content of wild caught fish. It is not unusual to cook ayu with its innards. The bitterness of the offal is considered an important part of the taste. Ayu from aquaculture is available all year round. You can distinguish a wild caught ayu from an aquaculture bred ayu by its appearance: Wild-caught ayu have a long, pointed chin, which they need to scrape the algae from the ground or the rocks. Farmed ayu, on the other hand, have a darker color. Another important difference is the price: Wild caught ayu are significantly more expensive and less available than aquaculture products.
As opposed to the usual Japanese way of preparing fish, the best quality ayu specimens are not used for making Sushi or Sashimi, but ayu is most often grilled fresh with salt over charcoal. The fish is put on a skewer in such a way that their body forms a wave, making them look as if they are swimming (uneri gushi, うねり串). Furthermore, ayu can also be cooked wonderfully or prepared as tempura.
Ayu in Japan
Ayu are - because of their intensive smelling slime coating - also called “scented fish“ (香魚). The scent is said to be due to the fish's diet that consists of crustaceans, insects, sponges, worms, blue, pebble and gold algae that grow on the rocks of flowing streams. This unique scent only applies to wild caught ayu. With ayu from aquacultures, this smell is perceived only very weakly. Japanese people say: the fish tastes best where the algae are numerous and healthy. Furthermore, ayu living in rivers and streams typically maintain a feeding ground that they stubbornly defend. Accordingly, there is a saying in Japanese that “someone is as persistent as an ayu“.
Lake Biwa in Shiga Prefecture and the Nakagawa River, which flows through Tochigi and Ibaragi Prefectures, are known for the quality of their ayu fish. Traditionally ayu is mainly caught in pots and baskets. As a tourist attraction, tame cormorants are still used to catch ayu in some places. The Japanese cormorant, called umiu in Japanese (ウミウ), are tamed birds for this purpose. The bird catches the ayu, stores it in its crop and then brings it to its owner. The most important methods for obtaining ayu are fly fishing, using a fish trap and fishing with live bait (ayu no tomodzuri, アユの友釣り). The bait is a live ayu on a hook that swims when plunged into the water. The live bait provokes the territorial behavior of other ayu, which attack the “intruder“ and get caught on the hook of the angler.
The traditional names used to describe the fish are “fragrant fish“ (because of its unique scent), “annual fish“ (because it usually lives only one year), “silver-lipped fish“ (because its mouth glows silver when it swims), kisardashi (means sardine in a mountain stream) and “scaled fish“ (because of its small scales). In Japanese poems, the characters for ayu 鮎 and 鵜飼 are associated as a synonym for summer. In addition, waka ayu (若鮎) is found in spring, ochi-ayu (落ち鮎) in autumn and hio ayu (氷魚) in winter.
Characteristics & Ecology of Ayu (Sweetfish)
Although the fish is also called “annual fish“ (年魚) due to its typical life span of about one year, some individuals live two to three years. The color of the young fish is grey-green/silver-grey with a golden shimmer. It changes its body color during sexual maturity in autumn to orange and black (called sabi ayu).
Wild ayu spawns in autumn in the lower part of the rivers and lay their eggs in small pits, which they hide in the gravel. Some die after reproduction and then lived only one year. Others migrate into the ocean. They usually spawn up to three times, moving to the lower part of the rivers each time in autumn. In Japan, some populations live their whole life in fresh water and move only between lakes and the corresponding rivers where they breed. Most populations of this species are amphidromic. The populations whose access to fresh water is restricted usually reach an age of two or three years. Although there are reports of ayu up to 70 cm long, a maximum size typical for the species is about 30 cm. Ayu that spend their life in fresh water are much smaller than the amphidromic form. The ayu of Lake Biwa (freshwater), which migrate to their spawning streams in spring, can grow up to about 15 cm long. Those fishes that migrate later in the year (autumn) only become 10 cm long. This is due to the different availability of food. Usually, waka ayu, therefore young ayu, are only about the 6 cm long and very dainty.
Its distribution extends from the northwestern Pacific Ocean in Japan along the coast of Hokkaidō southwards to the Korean Peninsula, China, Hong Kong and North Vietnam. A few inland populations also exist in Japanese lakes, e.g. Lake Biwa (琵琶湖). It is an important fish for Japanese river fishing and for many local governments ayu even functions as a symbol of the city.
In aquaculture, adult fish are mainly bred for human consumption. In various parts of Japan, however, seedlings are also farmed for resource conservation and sport fishing. The main production areas are Shiga Prefecture, Tokushima Prefecture, Aichi Prefecture, Wakayama Prefecture, Gifu Prefecture, Aichi Prefecture and Shizuoka Prefecture. Near Tokushima, Aichi Prefecture and Wakayama Prefecture are the largest fish farms located.
Beast Season for Ayu
Video about Ayu
External video embedded from youTube.com: 【NIGIRU TV】鮨職人SUSHI太郎. 寿司職人によるアユの握り寿司と姿寿司と塩焼きの作り方！〜How to making sweetfish sushi〜
Species of Ayu
The following species are regarded as authentic. Either historically, according to the area of distribution or according to the common practice in today's gastronomy:
Common Names, Scientific Name
ayu, ko-ayu, ryukyu ayu-fish
References & Further Reading
- [Froese & Daniel, 2019]: Rainer Froese, Pauly Daniel. FishBase. The Leibniz Institute of Marine Sciences at the University of Kiel, FishBase.org. 2019. https://www.fishbase.org. Retrieved online on December 24, 2020.
- [Fscj, 2014]: 寄生虫による食中毒にご注意ください (engl. Beware of food poisoning caused by parasites). 食品安全委員会. 2014. https://www.fsc.go.jp/sonota/kiseichu_foodpoisoning2.html. Retrieved online on December 29, 2021.
- [Fujiwara, 2020]: 昌髙藤原. ぼうずコンニャクの市場魚貝類図鑑 (engl. Bozu Konyaku's Market Fish and Shellfish Book). Bozu Konnyaku Co., Ltd., Tokyo ぼうずコンニャク株式会社東京, zukan-bouz.com. 2020. https://www.zukan-bouz.com/. Retrieved online on December 27, 2021.