Pond smelt

The pond smelt is a river fish in the that appears in all games. It can be found all day throughout winter.

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Upon donating it to the museum, it can be found in the middle/right tank in the aquarium.

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An information board in the aquarium will display this description of the pond smelt after donation:

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Once donated to the museum, the pond smelt can be found in the lower level of the waterfall tank with the salmon and king salmon.

Real-world information
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.