gizzard shad, (kokuji)
鮗 is a Japanese kokuji that names a specific fish, the gizzard shad. Its meaning is narrow and concrete, with no abstract extensions.
鮗 is a kokuji created in Japan by combining the fish radical 魚 with the phonetic component 冬 (ふゆ, winter), likely because the gizzard shad is associated with winter fishing.
The fish radical 魚 plus winter 冬 suggests a fish caught in winter: the gizzard shad.
For このしろ, imagine a winter fish that you know is white: この (this) しろ (white) fish is the gizzard shad. The phrase 'kono shiro' cues このしろ.