Reeves' shad, hilsa herring
鰣 refers specifically to a type of fish: Reeves' shad or hilsa herring. It is a narrow ichthyological term without a broader conceptual center.
鰣 combines 魚 (fish) with 時, which likely contributes the sound シ. The character was created to name a specific fish species.
The left side 魚 means fish, and the right side 時 suggests time. Picture a fish that appears only at a certain time of year, like the seasonal shad.
For シ, imagine a sheaf of herring: sheaf -> シ, and the fish 魚 on the left is that herring.