Spanish mackerel
鰆 centers on a specific fish: the Spanish mackerel. The meaning is narrow and concrete, with no broader conceptual cluster.
鰆 combines 魚 (fish) with 春 (spring), likely indicating a fish associated with spring. The exact historical origin is uncertain, but it is a typical phono-semantic or semantic compound for a fish name.
The fish radical 魚 on the left tells you it's a fish, and 春 (spring) on the right suggests a fish that is especially good or common in spring. Picture a Spanish mackerel leaping out of the water on a warm spring day.
For シュン, imagine a Spanish mackerel being served at a spring feast, and the chef says 'shun' as he presents it: shun -> シュン, tying the spring fish to its reading.