Here's part of a survey on the spawning American Shad in the Delaware River, 2010....
"A total of 812 shad, 431 females and 381 males, were collected in 15 nights (May 9 – June 1, 2010) for an overall catch rate of 3.6 shad/net or 0.93 shad/hour. The observed catch rate of shad in 2010 at Smithfield beach is suggestive of a strong returning migration of shad in this pool. Relative to past years, the 2010 shad run was the 4th highest peak in CPUE since 1997. Sizes ranged from 425 - 585 mm total length or 16.7 – 23.0 inches (females: 18.3 – 23.0 inches; males: 16.7 – 22.2 inches) and weight ranged from 695 – 2,244 grams or 1.5 – 4.9 pounds (females: 1.6 – 4.9 pounds; males: 1.5 – 3.1 pounds). Scales and otoliths were also collected for estimation of age, but they have not yet been processed.
As with the Raubsville index, catches were initially predominately male, with nearly 100% of catch was males in the first three nights, but by May 16th the nightly catch was dominated by females ranging from 1 to 3 females captured for every one male." Looks like at the start of each run, males come first ...
So that's why fishermen will likely run into the males during that time..