the young of fish
魫 refers specifically to the young of fish, such as fry or fingerlings. It is a specialized character with no broader abstract extensions.
魫 combines 魚 (fish) with 冘, which likely contributes the sound シン. The character was created to denote young fish specifically.
The left side 魚 means fish, and the right side 冘 looks like a small, immature form. Picture a tiny fish fry still developing its full shape, represented by the incomplete-looking right component.
For シン, imagine a thin, shimmering school of young fish: thin -> シン, and the fry are so small they look like a thin silver ribbon in the water.