From Nookipedia, the Animal Crossing wiki
The Pond Smelt (ワカサギ, Wakasagi), is a relatively common winter fish found in the river (River Pool in Animal Crossing: Wild World) at any time of the day between December and February. It cannot be found in holding ponds, despite its name and the characteristics it has in real life.
Donating to the museum
In Animal Crossing
"Even run-of-the-mill, common fish are valuable resources, you see. All part of the grand tapestry, wot!"
In City Folk
Blathers will give his usual short speech on the player's donation. His words are as follows:
"Pond smelt are a popular catch during winter ice-hole fishing, eh wot? It's a simple sport, really. You open a hole in the ice, drop your line in...and then wait for a nibbly-nibbly! Not that I've tried. I find cold weather rather unappealing, so ice fishing is hardly a possibility..."
Upon donating it to the Museum, it can be found in the middle/right tank in the Aquarium.
In New Leaf
An information board in the aquarium will display this description of the pond smelt after donation:
"Pond smelt are small but vigorous fish that can survive in water of relatively poor quality. Pond smelt are related to salmon and as such have a lot of the same migratory tendencies. Most of them swim from rivers to the ocean and then return to rivers and lakes. However, some of them actually live in lakes that have no connection to the ocean, so they don't migrate. They have small fins and are perfect for eating whole as tempura or grilled on barbecues."
Chip will say this when given a pond smelt:
"Ya know, with pond smelt, deep frying 'em is tasty. But grilling 'em up right is pretty good too! Not that I mind having this guy raw. Nope, not me! Happy to!"
The pond smelt is a type of harmless commercially-caught fish found in middle to surface waters in lakes, rivers, and ponds. It can be found across the world in the Arctic, Northwest to Northeast of the Pacific, Canada, Alaska, Korea, and Japan. They feed off plankton, small insects, and algae. Adults can be found spawning in in-shore areas between spring and summer.