sardine
鰮 centers on a specific type of small, oily fish: the sardine. The meaning is narrow and concrete, referring directly to the fish itself.
鰮 is a phono-semantic compound: the left side 魚 (fish) indicates the meaning, while the right side 溫 (simplified to 囚 over 皿 in this form) likely contributes the sound オン. The character was created specifically to name the sardine.
The fish radical 魚 on the left tells you this is a fish. The right side looks like a person 囚 trapped over a dish 皿—imagine a sardine packed tightly in a tin, ready to be served.
For オン, picture turning on a can opener to open a tin of sardines: on -> オン, and the sound of the opener matches the reading.