In The Herring: Its Effect on the History of Britain (1919) AM Samuel writes:

In Norfolk, according to Notes and Queries, October 7th, 1865, a queer legend existed that fleas and herrings came together. As an old Cromer fisherman said, “Times is as you may look in my shirt, and scarce see a flea, and then there won’t be but few herring. But when you see my shirt alive with fleas, then there is certain to be a good tidy lot of fish.”