fish in season; seasonal fish
Refers to fish that is at its peak flavor and availability during a particular season.
春は旬の魚がたくさんある。
In spring, there are many fish in season.
この店ではいつも旬の魚を出している。
This shop always serves seasonal fish.
Compound of 旬 (season) + の (possessive particle) + 魚 (fish).