From Nookipedia, the Animal Crossing wiki
The Stringfish (イトウ, Itō) is an extremely rare fish that appears in the winter (December to February), from 4 p.m. to 9 a.m.. It is caught in the river. It sells for 15,000 bells, making it one of the most expensive fish in the game. Its scientific name is Hucho perryi and its real name is Sakhalin taimen. In City Folk, the 'Bug and Fish' window describes it as 'Rarely seen and treasured, these are known for having voracious appetites.'
Donating to the museum
In Wild World
"I've heard it said that this fish is actually very high in fat content... If such words are true, then whatever possessed someone to name it that? If you were to call a portly fellow "stringy", well... Madness would ensue!"
The Stringfish can then be seen swimming about in the first freshwater tank.
In City Folk
After donating the Stringfish, Blathers will impart his knowledge of the fish to the player:
The Stringfish can be found in the middle-left tank in the aquarium.
In New Leaf
The following information will be displayed by its tank:
"Stringfish live in cold rivers, can grow to over a yard long, and are stout and bulky like logs. They live up to 20 years but grow so slowly that it takes them 15 years to reach full maturity. They are called "phantom fish" because there are so few of them. Adults are particularly hard to find."
The Stringfish can be found in the larger of the two left-hand tanks in the freshwater fish room.
Further informationsalmon species which inhabits the lower to middle reaches of lakes and rivers. It is Critically Endangered. Females lay between 2,000 - 10,000 eggs each time on sandy/gravelly river bottoms, most of them do not hatch. The fish can be found in areas of the Northwest Pacific, namely the Sakhalin Island (Russia, where it gets the name Sakhalin taimen), the Island of Hokkaidō (Japan) and parts of the far eastern Russian mainland.