From Nookipedia, the Animal Crossing wiki
The Rainbow Trout (オオイワナ, Ooiwana) is an common fish that appears at different times of the year. From March to June, it can be found between 4am to 9am and 4pm to 9pm, but can be found again in Autumn from September to November, between 4am to 9pm. It sells for 800 Bells at Tom Nook's store.
Donating to the Museum
In Animal Crossing
"This is one of those fish that seem to mysteriously disappear from time to time. Drives fishermen batty, wot!"
In Wild World
In Animal Crossing: Wild World, Blathers will say the following when given the Rainbow Trout:
"Rainbow trout is best if cooked straightaway out of the water, eh wot? There are many recipes for this marvelous fish, too many to count! Sauteed trout almondine is a traditional preparation, for example. You can just tell by the name of that dish that it's pricey, can't you?"
After donation, the rainbow trout can be found in the second tank of the aquarium.
In City Folk
In Animal Crossing: City Folk, Blathers will say the following when given the Rainbow Trout:
"Did you know that the most popular bait used for rainbow-trout fishing is a few salmon eggs? Rather horrific when you remember that rainbow trout and salmon are related... Blech."
Once donated, it can be found in the middle-left tank in the aquarium.
In New Leaf
"When rainbow trout get bigger, they develop a pretty pink band from their gills to their tails. Some of them spend their entire lives in rivers, while others live in the ocean. Steelhead trout are a unique type of rainbow trout that live in the ocean and return to freshwater to spawn. Rainbow trout are a major species of fish, but finding them in the wild is actually fairly rare."
Once the player has caught a particular fish or bug, they can find information about it in the bug or fish menus. Once the rainbow trout has been caught, the following information can be found in the fish menu:
The rainbow fish or Oncorhynchus mykiss is a member of the salmon family native to tributaries of the Pacific Ocean in Asia and North America. Rainbow trout vary in color - lake varieties light and silvery while river variants are vibrant and colorful. It is a type of sport fish, which means a fish that has been introtuced to countries simply to be caught for the purpose of sport. Popular bait used for rainbow-trout fishing is salmon eggs, which causes confusion because they are related. Introducing this fish to parts of South America has caused problems for native fish, by either being eaten, outsported or catching disease, which has given them the label of pest by fisheries. Rainbow trout have a varied diet, mainly eating insects but can eat smaller fish starting from when they are born.
Names in other languages