Ayu
Fish · Last reviewed 2026-08-10
A territorial algae-grazing river fish of East Asia, fished by tomozuri — using a live decoy to provoke a territorial attack — and famous for a melon-like scent.
Ayu is one of the most distinctive angling propositions in the world, because the standard method for catching it does not involve trying to feed it. Adult ayu graze algae from river stones and defend feeding territories aggressively, and tomozuri exploits exactly that: an angler swims a live tethered ayu into a held territory, and the resident fish attacks the intruder and is hooked on trailing hooks.
The fish itself is slim, silver-green, with a distinctive comb-like set of teeth for scraping algae from rock and a yellow patch behind the gill. Fresh ayu carry a scent often described as melon or cucumber, which is where the name sweetfish comes from.
It is amphidromous and generally lives a single year, running to sea as a larva and returning to rivers in spring.
Identification
FishingHQ holds no photograph of this species whose identity and licensing it can vouch for, so it is not showing one. Misidentified fish photographs teach identification wrongly, and a picture with an unchecked caption would be worse than none. The identification points below are what to look at instead.
- Scientific name
- Plecoglossus altivelis
- Family
- Plecoglossidae
- Water
- Freshwater, Brackish, Moves between fresh and salt water
- Conservation status
- Data Deficient
- Regions
- East Asia
- Comb-like teeth set on movable bases, used to scrape algae from stones — unlike the teeth of any other river fish of the region.
- A yellow-orange patch on the flank behind the gill cover in feeding adults.
- Slim silver-green body with a large adipose fin and a deeply forked tail.
- A distinctive melon or cucumber scent in freshly caught fish.
Size
- Typical
- 60–120 g (2–4 oz)
- Large
- 150–250 g (5–9 oz)
Exceptional: FishBase records a maximum of 70 cm
Lifecycle and reproduction
Typically annual: eggs hatch in autumn, larvae drift to sea or to brackish water, juveniles return upstream in spring, adults grow and hold territory through summer, spawn in autumn and die. A one-year life cycle makes each year's run entirely dependent on the previous autumn's spawning.
Spawns in autumn over gravel in the lower reaches of rivers, and most adults die after spawning. Larvae drift downstream to sea or brackish water and return as juveniles in spring.
Feeding and senses
Grazes algae from the surfaces of river stones with comb-like teeth, defending a feeding territory against other ayu. Juveniles feed on invertebrates before switching to algae.
A daylight visual fish in clear fast water, with territorial behaviour driven by sight of intruders — the mechanism the tomozuri method depends on entirely.
Through the year
Runs upstream in spring, holds territory and feeds through summer, and drops downstream to spawn in autumn. The angling season is tightly defined around the territorial summer period.
Distribution
Northwest Pacific: western Hokkaido in Japan southward to the Korean Peninsula, Taiwan and China
This is a biological range — where the species occurs, not a prediction that you will catch one. To browse the fishing geography FishingHQ holds, start at fishing places.
Why anglers care
Fished principally by tomozuri, a highly specialised Japanese method using a live decoy ayu on a long rod to provoke territorial attacks, and also by fly and bait methods. Its significance is cultural as much as sporting: tomozuri is a distinct discipline with its own tackle, seasons and etiquette, and ayu fishing rights on Japanese rivers are managed by long-established local cooperatives.
Handling and returning
A small, delicate, short-lived fish that handles poorly. Keep it wet, handle minimally, and where decoy fish are used their welfare is a genuine consideration in how long they are worked and how they are kept.
Where to go next
How it is caught
- TomozuriTomozuri is built entirely on one behaviour of one fish. An ayu holds and defends a feeding territory on an algae-covered stone, so a live tethered ayu swum into that territory provokes the resident to attack it, and a hook trailing behind the decoy takes the attacker. No other quarry in the catalogue is fished by exploiting territoriality this directly.
- Float fishingTomozuri, using a live decoy to provoke a territorial attack rather than to feed the fish.
What these connections rest on (2)
Practitioner consensus — 2 connections
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Tomozuri
Established angling practice for this species, recorded as convention. The note states the property of the animal or its water that makes the method the answer.
- Fishers' Craft and Lettered Art: Tracts on Fishing from the End of the Middle Ages — University of Toronto Press, 1997 (Scientific literature)Relied on for: That fishing practices develop around the quarry and water available to them, and the historical method of reading such traditions.Background and mechanism onlyCitation checked — the work exists and is described correctly
- FAO fishing gear types and fishing technology information — Food and Agriculture Organization of the United Nations (Scientific institute)Relied on for: Classification and operation of the gear type described.States this directlyCitation checked — the work exists and is described correctly
The method-species association is convention; the territoriality it exploits is recorded biology, and the two are cited separately rather than blended.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Diet, habitat association and territorial feeding behaviour recorded for the species concerned.States this directlyPassage read and verified
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated angling practice and belief, recorded as practitioner consensus rather than as tested fact.Background and mechanism onlyCitation checked — the work exists and is described correctly
Float fishing
A practitioner claim about how Plecoglossus altivelis is commonly fished, following from where and how it feeds.
- General angling website restatements of established technique — Various (Credible secondary source)Relied on for: Widely restated practitioner accounts of fishing for ayu.Citation checked — the work exists and is described correctly
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Practitioner treatment of the methods used for ayu and comparable species.Citation checked — the work exists and is described correctly
Where it lives
- Chalk streamsHolds and defends a feeding territory on algae-covered stones in clear rivers.
- Lowland riversHolds and defends a feeding territory on algae-covered stones in clear rivers.
What these connections rest on (2)
Scientific evidence — 2 connections
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
Chalk streams
A habitat association for Plecoglossus altivelis taken from the environment and distribution data on its FishBase summary.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: The environment, depth range and habitat description recorded for Plecoglossus altivelis.States this directlyPassage read and verified
Lowland rivers
A habitat association for Plecoglossus altivelis taken from the environment and distribution data on its FishBase summary.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: The environment, depth range and habitat description recorded for Plecoglossus altivelis.States this directlyPassage read and verified
Where you will find it
What these connections rest on (1)
Scientific evidence — 1 connection
Supported by peer-reviewed research or fisheries science. The sources are listed and can be checked.
East Asian fishing
A distributional claim taken from reference sources on the species rather than from angling report. It states presence and asserts nothing about a fishery.
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Distribution, habitat association, depth range and life history recorded for the species concerned.States this directlyPassage read and verified
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: Global distribution, population trend and extinction-risk assessment for the species concerned.States this directlyCitation checked — the work exists and is described correctly
History and culture
What these connections rest on (1)
Practitioner consensus — 1 connection
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Ayu tomozuri: fishing with a live decoy
A record joined to the history behind it. Historical placement rests on the documentary and scholarly sources cited; where the tradition is living rather than documented, the claim is kept at the level those sources support.
- Words of the Lagoon: Fishing and Marine Lore in the Palau District of Micronesia — University of California Press, Berkeley, 1981 (Reference work)Relied on for: Systematic documentation of traditional fishing knowledge and customary marine management in Pacific island communities.States this directlyCitation checked — the work exists and is described correctly
- Code of Conduct for Responsible Fisheries — Food and Agriculture Organization of the United Nations, Rome, 1995 (Standards body)Relied on for: International principles for responsible fisheries including recognition of traditional and small-scale fisheries and their knowledge.States this directlyCitation checked — the work exists and is described correctly
Sources for this page
- FishBase: a global information system on fishes — Froese, R. and D. Pauly, eds. — FishBase Consortium (Scientific institute)Relied on for: Family, maximum size, depth and environment, distribution and the IUCN assessment for Plecoglossus altivelis.States this directlyPassage read and verified
- The IUCN Red List of Threatened Species — International Union for Conservation of Nature (Conservation authority)Relied on for: The Data Deficient assessment for Plecoglossus altivelis, assessed 03 September 2010.States this directlyCitation checked — the work exists and is described correctly
How FishingHQ labels evidence is explained in the evidence policy.