small fish, small, minnow
鯫 centers on smallness in fish: a minnow or any small fish. The broader sense of 'small' is a natural extension from that core image.
鯫 combines 魚 (fish) with 取, which likely contributes the sound and may suggest the idea of taking or gathering small fish.
The fish radical 魚 on the left and 取 (take) on the right suggest taking small fish from a net: a minnow or any tiny fish.
For ソウ, imagine a small fish being taken and placed into a soulful little bowl: soul -> ソウ, and the minnow swims in its new home.