Swan Creek, 5-27-13

[Brian K. Shaffer]

what are those little round ball things attached to the fishes gills ??

[Dutch]

The look like the same kind of parasites that you see in the fins later in the summer. I have been told that they are transmitted by birds pooping in the water.

[bfishn]

Parasitic copepods, the larger microcrustacean cousins of the beneficial, pelagic zooplankton copepods. In numbers like your picture, they're of little concern. If the variety I've seen elsewhere, there should be two little trailing appendages if you look closely.