Sweetfish

The Sweetfish (also known as the ayu) is a freshwater fish found in the river between 4 AM to 9 PM during the months of July through to the first half of September, but is most common between July to August. It sells for a decent price despite being abundant.

Donating to the Museum
Upon donation to the Museum, Blathers will say:

In City Folk
"You can always tell where a sweetfish has been by the trail of bite marks it leaves behind, eh wot? The fish will scrape away at rocks that have food stuck to them, leaving jagged teeth marks. ...Quite wild behavior for such a refined-looking little fish, I should say!" hootie hoo eh wot.

In can be found in the middle-right tank in the aquarium.

Wild World

 * Size: sAyu_Sweetfish_(Wild_World).pngAyu Sweetfish (Wild World)2.png
 * Habitat: Rivers
 * Season: Summer

City Folk

 * Size: Sweetfish_(City_Folk).png
 * Season: Summer
 * Location: Rivers

Further Information
The Sweetfish or Ayu is an amphidromous fish, the only species in the genus Plecoglossus and in family Plecoglossidae. It is a relative of the smelts and is placed in the order Osmeriformes. Native to the Palearctic ecozone, it is found in rivers, lakes, and coastal waters of western Hokkaidō in Japan southward to the Korean Peninsula, China, and Taiwan. The name "sweetfish" is due to the sweetness of its flesh. In reference to its typical one-year life span, it is also known as nen-gyo (year-fish). It is Gunma Prefecture's fish.

Decoys (plastic or similar material) of Sweetfish are often used to catch them, as referenced in-game by the Encyclopedia information. As they are an aggressive species they will charge and push out any intruders, and in the case of a decoy will get caught on the hook.