I dont know if they exist where you are, but there is a weevil down here that feeds on floating plants like WH and Salvinia. They obviously arent able to keep up with either plants' explosive nature, tho.

Could be something like these critters have found a new food source in your pond.

.10 surface acre pond, 10.5 foot deep. SW LA. The epitome of a mutt pond. BG, LMB, GSF, RES, BH, Warmouth, Longear Sunfish, Gambusia,Mud Minnows, Crappie, and now shiners!!...I subscribe!!