I kind of agree with Rod that you may have had gizzard shad all along, but then again, I'm assuming you've owned your pond for more than 7 years (joined PB in 2017).

That would be surprising that you never saw them before.

I also agree that your LMB may be keeping the gizzard shad in check.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."