This document provides information on the taxonomy, biology, farming, and aquaculture of ayu sweetfish (Plecoglossus altivelis). It discusses that ayu are an amphidromous fish found in rivers and lakes in Japan, Korea, Taiwan and China. They spawn in rivers in spring, with larvae entering the sea and returning to rivers as juveniles. It also outlines their natural life cycle and details of their intensive aquaculture in Japan, where they are farmed commercially in ponds at densities of 150-200 fish/m2. Feeding practices and common health issues in aquaculture are also summarized.

1. Sweet fish Ayu Taxonomy, Biology and
farming
Plecoglossus altivelis (Temminck & Schlegel, 1846)- Ayu sweetfish
By:
B. BHASKAR

2. Introduction
• Aquaculture production:
• 87.5 million tonnes of
aquatic animals mostly for
use as human food.
• 35.1 million tonnes of
seaweeds and other algae for
both food and non-food uses.
• 700 tonnes of shells and
pearls for ornamental use. Production volume of sweetfish (ayu) through
inland water aquacultures in Japan from 2014
to 2020(in 1,000 tons)

3. Taxonomic position and Morpho-metric Identification
• Actinopteri (ray-finned fishes)
> Osmeriformes (Freshwater smelts)
> Plecoglossidae(Ayu fish)
Etymology: Plecoglossus: Greek,
pleko, plekein = to fold + Greek,
glossa = tongue
authors: Temminck& Schlegel
• Environment and distribution:
Marine; freshwater; brackish;
demersal; amphidromous); depth
range 10 - ? m. Subtropical; 44°N -
23°N. Northwest Pacific: western
Hokkaido in Japan southward to the
Korean Peninsula, Taiwan and China.
• Maturity : Lm 27.5, range 30 - 40 cm
Max length : 70.0 cm TL
male/unsexed; common length : 15.0
cm SL male/unsexed;
• SOURCE: fishbase
Plecoglossus altivelis Picture by Islam, Md. S.
Morphology and Meristic identification
Dorsal spines (total): 0; Dorsal soft rays (total):
10-11; Anal soft rays: 14 - 15; Vertebrae: 60 - 63.
Pyloric caeca 350-400.
Availability and Utilization: Fisheries: highly
commercial; aquaculture: commercial; gamefish:
Nutrients: Calcium = 154 [63, 409] mg/100g ;
Iron = 1.98 [1.09, 3.77] mg/100g ; Protein = 15.1
[12.7, 17.4] % ; Omega3 = 0.657 [0.268, 1.741]
g/100g ; Selenium = 48.1 [20.9, 109.6] μg/100g ;
VitaminA = 3.69 [1.08, 12.60] μg/100g ; Zinc =
1.12 [0.73, 1.78] mg/100g (wet weight);

4. Biology of AYU
• Typical amphidromous fish; appears in near shore from late autumn to spring.
• Found in lakes and rivers, preferring clean river water and can be found the entire
river long, from the head to the mouth.
• Ascends the river during March when the temperature is around 10°C.
• Adults spawn in the spring, in the lower reaches of rivers. After spawning, some
adults die while others return to the sea.
• Larvae enter the sea immediately after hatching and remain there during winter,
feeding on plankton.
• In springtime, the young (5-7 cm TL) move upstream to the middle reaches of rivers
to feed on algae.
• Fish (about 6-9 cm) start schooling at the river mouth and are insectivores and eat
algae off small pebbles; this is assisted by small leaf-like teeth which are loosely
attached to the jaw with two ligaments
• Those that are ready to spawn (about 20 cm TL) move downstream to the lower
reaches of the river.
• Spawning adults from the sea migrate upstream to the lower reaches as well. Some
fish spawn two or three years in succession, others only once (Ref. 9987 & 559).
• River forms live usually only one year whereas lake forms can live two or three
years.
• Reaches maturity at 30-40 cm.
• Highly esteemed food fish. Marketed fresh and consumed fresh, fried and broiled

5. • During spawning some fish go up the river and some fish spawn on the river
shoreline above pebbles.
• Spawns at night and excavates a 10cm (diameter or depth?) pit.
• Eggs are around 1 mm in size and adhere to the sand or pebbles for 14 to 20
days before hatching. This fish is semelparous and releases eggs many times
over a short period each time releasing ~10,000 eggs.
• Sometimes females that want to spawn but cannot, save their strength, and
go to a deep stagnant pool.
• These females wait until spring before going up the river with young fish.
After hatching, the larvae are 6 mm and flow with the water current eating
plankton.
• Juvenile fish will come again to the river in spring.
• These live in schools. When schools of Plecoglossus altivelis return to their
river they guard their territory and eat food.
• Reproductive mode varies between semelparity and iteroparity. Large
females spawn once, while smaller females spawn twice during a two-week
interval

6. Natural history of Sweet fish: Ayu
• Downstream migration of matured Ayu for spawning begins in early autumn, following the
completion of spawning, spent males and feamles die.
• Eggs are adhesive& laid on sand in lower reaches of rivers, from sept to Nov, the eggs can
hatch after two weeks.
• Newly hatched larve 7mm (0.7cm) in length, are carried downstream to the sea, were
remain during winter.
• Larval stages of ayu were planktivorous, in following spring the individuals fishes(6-8cmin
total length& 3-5gm in weight, start anadromous migration back to rivers. After upstream
migration they change feeding habit to herbivorous.
• Main source of Ayu in rivers are adherent blue green algae, diatoms, which contains 45-48%
crude protein.
• Well known source of wild ayu fry for culture& stock enhancement are landlocked, captured
mainly in lake Biwa in Shiga prefecture & small ampunt of fry caught from seashore, very less
quantity of ayu seed are artificially produced.
• Seashore fry(wild ): 3.5-8cm in TL& 0.1-5g weight known to available&caught along seashore
by seine nets in Dec&April.
• Landlocked fry: of 0.3-5gm known to caught from Lake Biwa from Feb to June by using
pound net or scoop nets.
• Artificiallt produced Ayu fry: in hatcheries similar to trouts, ayu fish laid eggs range 20000 to
50000 (max 100000/female) per female, adhere to hatchery nets, favourable water
temprange 12-20 degree C.
• The hatch after 16-17 days at 15 degree. First foods rotifers( Brachionus plicatilis) for f of 7-
35mm fry in TL and brine shrimp(Artemia sp.) for fry of 13-35mm. Fry of > 10mm slowly
adjusted to formulated feed. Rearing density @10kg of fish/metric ton.

7. Aqua-farming of AYU
• Intensive farming:
• In Japan well established, the production range 20-70kg/ m2 with a water depth of 1m.
• During the 90 days of rearing mortality reported about 10% and grow about 4-6gm to a market
size of 50-80gm if there is no much disease problem.
• Culture system: Ayu are rared in built concrete ponds, measuring 100-400m2 and
average150m2 that are supplied with running water.
• octangular and round ponds are popular for cultivation.
• Optimum water flow rate is atleast 10L/sec in 100m2 ponds with depth of 1m.
• Round tanks are helpful in producing water currents in rearing ponds& for discharging solid
waste from a center outet.
• Most of the Ayu culture carried out in outdoor tanks but it can also done in indoor tanks.
• Rearing density 150-200/m2 and for culture density in farms about 500-600 fishes/m2. grow
about 70 to 100gms in 4 months.
• Feeding: fry stage- granular, crumble type formulated feedsin rearing period and extruded
type feed.
• Crude protein requirement: 50-60% at Starter feed, 47-55% Juvenile stage and 45-50% in
adult stage.fat 3-10%& 5-8%. Crude fiber 1-1.4%; 1-3% and 3-4%.
• Feed sise 0.07-0.45; 0.3-1.5 and 0.9-2.4mm. And feeding given 4times/day at 1gm fish size,
for 1-10gm fish 3-4times/day, and for 10-80gm fishes 3times/day and fishes of 180gm and
more weight given two times/day.
• Diseases reported Cytophaga psychrophila infection, Virbiro anguilalarium,Aeromonas
hydrophila, Glugea infection, Ichthyophonus, Phoma infection, Aphanomyces piscicida.

8. References:
• https://www.fishbase.de/summary/251
• Masuda, H., K. Amaoka, C. Araga, T. Uyeno and T.
Yoshino, 1984. The fishes of the Japanese Archipelago.
Vol. 1. Tokai University Press, Tokyo, Japan. 437 p.
• https://www.fao.org/state-of-fisheries-aquaculture/en/
• https://www.trueworldfoods.com/front/product/p020
4?page=&id=165
• https://www.statista.com/statistics/1177594/japan-
sweetfish-production-volume-inland-water-
aquacultures/
• https://www.google.com/search?q=best+Coral+beautif
ul+fishes&source=lnms&tbm=isch&sa=X&ved=2ahUKE
wjwg9DIt_P5AhWrZWwGHfuJCbEQ_AUoAXoECAEQAw
&biw=1366&bih=568&dpr=1#imgrc=pWTsq_RADk6yw
M&imgdii=Gt-hF59_n_a6QM

9. Thank you