What is the allusion of Shaoxing proverb "When the pomfret crows under the bridge, no one else will eat it"?

It means that there are many pomfret (Shaoxing dialect homophonic: referring to some small fish) in the river under the bridge, which is very fresh, but you can't eat it without tools (referring to fishing nets and other fishing tools).

This is just Shaoxing dialect. If we must find an allusion, I think it is very similar to an idiom allusion: it is better to go home and make a net.

Personal guess may be the vulgar abbreviation of this idiom and allusion by Shaoxing literati in the old days, so that Shaoxing locals can better understand the meaning.