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It's just complete speculation on my part, but it seems like years when the weather/water trends warmer, you see a lot more of those grubs in fish.
I'd have to agree with that. Notice that in Fall after a long hot Summer.
Both the black and white parasites come from birds. Kingfishers and herons. Low water levels might bring more birds. I notice more parasites in gills and pumpkinseeds. I don't even keep seeds anymore. Turns out the parasites go from the bird poop to the sediment and attach to snails, the small fish and gills feed heavily on the snails, the birds feed on the small fish and gills and poop in the water where they attach to the snails. The circle of life.