sillaginoid, (kokuji)
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鱚 is a Japanese kokuji for a specific fish, the sillaginoid (whiting). Its meaning is narrow and concrete, referring only to this fish.
sillago (any fish of genus Sillago, esp. the Japanese whiting, Sillago japonica)
deepsea smelt (Glossanodon semifasciatus)
Japanese whiting (Sillago japonica)
harlequin sandsmelt (Parapercis pulchella)
small-scale whiting (Sillago parvisquamis); blue whiting
black swallower (Chiasmodon niger)
鱚 is a Japanese kokuji, created by combining 魚 (fish) with 喜 (joy), likely to name the sillaginoid fish. The exact historical origin is uncertain, but it follows the common pattern of using 魚 as a semantic component for fish names.
A fish (魚) that brings joy (喜) when caught: the sillaginoid is a prized catch, so 鱚 is the joyful fish.
For きす, imagine a fish that you kiss (きす) because it's so joyful: kiss -> きす, and the sillaginoid is the fish you want to kiss.