dace (carp), (kokuji)
鮠 is a kokuji (Japanese-made kanji) for a specific freshwater fish, the dace, a type of carp. Its meaning is narrowly tied to this fish species.
鮠 is a kokuji formed by combining 魚 (fish) with 危, which likely contributes the sound ガイ/カイ. The exact historical reasoning behind the choice of 危 is uncertain.
The left side 魚 means fish, and the right side 危 suggests danger. Imagine a dace as a 'dangerous fish'—not because it's harmful, but because it's a rare catch that might slip away.
For カイ, picture a dace swimming in a kai (海, sea) that has somehow become freshwater: kai -> カイ, and the fish is the dace.