What you were catching were hickory shad. These run in the late summer and fall. The spring run usually consists of American or white shad. These are generally larger. I take most of my hickories in the Narrow River in Narragansett RI. My best fly on
one that beats all is a chatruse bunnicula. Drop a line and I'll share the pattern. I've also seen some hickories in the spring but chances are that any shad caught in May or June will be american.